irrigation guide - usdairrigation guide acknowledgments part 652, irrigation guide, is an addition...

(210-vi-NEH, September 1997) v

Part 652Irrigation Guide

Irrigation Guide

United StatesDepartment ofAgriculture

NaturalResourcesConservationService

NationalEngineeringHandbook

(210-vi-NEH, September 1997)vi


Issued September 1997

The United States Department of Agriculture (USDA) prohibits discrimina-tion in its programs on the basis of race, color, national origin, sex, religion,age, disability, political beliefs, and marital or familial status. (Not all pro-hibited bases apply to all programs.) Persons with disabilities who requirealternative means for communication of program information (Braille, largeprint, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD).

To file a complaint, write the Secretary of Agriculture, U.S. Department ofAgriculture, Washington, DC 20250, or call 1-800-245-6340 (voice) or (202)720-1127 (TDD). USDA is an equal employment opportunity employer.

(210-vi-NEH, September 1997) i

Part 652Irrigation GuidePreface

Irrigation is vital to produce acceptable quality and yield of crops on aridclimate croplands. Supplemental irrigation is also vital to produce accept-able quality and yield of crops on croplands in semi-arid and subhumidclimates during seasonal droughty periods. The complete management ofirrigation water by the user is a necessary activity in our existence as asociety. Competition for a limited water supply for other uses by the publicrequire the irrigation water user to provide much closer control than everbefore. The importance of irrigated crops is extremely vital to the public'ssubsistence.

Today's management of irrigation water requires using the best informationand techniques that current technology can provide in the planning, design,evaluation, and management of irrigation systems. Support for many of thevalues included in this chapter come from field research, established designprocesses, and many system designs and evaluations over many years. Fieldevaluations must always be used to further refine the planning, design,evaluation, and management process. This design guide in the NaturalResources Conservation Service (NRCS), National Engineering Handbookseries provides that current technology.

Irrigation Guide, Part 652, is a guide. It describes the basics and process forplanning, designing, evaluating, and managing irrigation systems. It pro-vides the process for states to supplement the guide with local soils, crops,and irrigation water requirement information needed to plan, design, evalu-ate, and manage irrigation systems.

Irrigation Guide, Part 652, is a new handbook to the family of references inthe NRCS, National Engineering Handbook series. It is written for NRCSemployees who provide technical assistance to the water user with con-cerns for both water quantity and quality. Other technical personnel forFederal, State, private, and local agencies will also find the guide useful as abasic reference when providing technical assistance relating to planning,designing, evaluating, and managing irrigation systems. College and univer-sity instructors will also find the guide useful as a classroom reference.

In addition to the irrigation Guide (part 652), chapters in the NationalEngineering Handbook irrigation section (now part 623) describe:

• Soil-plant relationships and soil water properties that affect move-ment, retention, and release of water in soil

• Irrigation water requirements• Planning farm irrigation systems• Measurement of irrigation water• Design of pumping plants• Design criteria and design procedures for surface, sprinkler, and

micro irrigation methods and the variety of systems for each methodthat can be adaptable to meet local crop, water, and site conditionsand irrigation concerns

(210-vi-NEH, September 1997)ii

Part 652Irrigation GuideAcknowledgments

Part 652, Irrigation Guide, is an addition to the National Engineering Hand-book series. The document was initially prepared by Elwin A. Ross, irriga-tion engineer, NRCS, Engineering Division, Washington, DC, (retired) withprimary input and review from Leland A. Hardy, irrigation engineer,Midwest National Technical Center, NRCS, Lincoln, Nebraska, (retired).

Principal NRCS staff who contributed to the development and review ofthis publication were:John Brenner, irrigation engineer, North Platt, NebraskaJerry Walker, irrigation engineer, Temple, TexasCarl Gustafson, state conservation engineer, Amherst, MassachusettsDavid Nelson, assistant state conservation engineer, Amherst,MassachusettsJohn Dalton, agricultural engineer, Bozeman, Montana (retired)Jim Suit, state conservation engineer, Bozeman, Montana

State conservation engineers coordinated a multidiscipline (representingsoils, agronomy, water quality, and water management) review in all states.

William J. Carmack, national water management engineer, (retired),Ronald L. Marlow, water management engineer, NRCS, Washington, DC,and Thomas L. Spofford, irrigation engineer, NRCS, National Water andClimate Center, Portland, Oregon provided leadership and coordination.

Editing and publication production assistance were provided by the Techni-cal Publishing Team, NRCS, National Cartography and Geospatial Center,Fort Worth, Texas. Final technical review was provided by Elwin A. Rossand Leland A. Hardy.

(210-vi-NEH, September 1997) iii

Part 652Irrigation GuideIrrigation Guide

Contents: Chapter 1 Introduction

Chapter 2 Soils

Chapter 3 Crops

Chapter 4 Water Requirements

Chapter 5 Selecting an Irrigation Method

Chapter 6 Irrigation System Design

Chapter 7 Farm Distribution Components

Chapter 8 Project and Farm Irrigation Water Requirements

Chapter 9 Irrigation Water Management

Chapter 10 Conservation Management Systems and Irrigation Planning

Chapter 11 Economic Evaluations

Chapter 12 Energy Use and Conservation

Chapter 13 Quality of Water Supply

Chapter 14 Environmental Concerns

Chapter 15 Resource Planning and Evaluation Tools and Worksheets

Chapter 16 Special Use Tables, Charts, and Conversions

Chapter 17 Glossary and References

Index


IntroductionChapter 1

1–7(210-vi-NEH, September 1997)

Chapter 1 Introduction

Contents:

1–i

652.0100 Purpose and objective 1–1

652.0101 Water and energy conservation 1–1

652.0102 Soil conservation, water quality, and pollution abatement 1–2

652.0103 Using the guide 1–3

(a) Using irrigation procedures ......................................................................... 1–3

(b) Using worksheets .......................................................................................... 1–3

652.0104 Irrigation guide outline 1–4

(a) General ........................................................................................................... 1–4

(b) Soil-water-plant data .................................................................................... 1–4

(c) Irrigation and distribution systems ............................................................ 1–4

(d) Irrigation planning and management ......................................................... 1–5

(e) Special tools .................................................................................................. 1–5

652.0105 Use of computers 1–6

652.0106 State supplement 1–6




Chapter 1 Introduction

652.0101 Water and energyconservation

Conservation irrigation is an integral part of a com-plete farm management program of soil, water, air,plant, and animal resources. It is a principal consider-ation in the NRCS Conservation Management Systemapproach to conservation planning on irrigated crop-land, hayland, and pastureland. Irrigation must becomplemented with adequate management of nutri-ents and pesticides, tillage and residue, and water.Proper water management results in conservation ofwater quantities, maintenance of onsite and offsitewater quality, soil chemical management (salinity,acidity, applied fertilizers, and other toxic elements),and irrigation related erosion control.

For the farm manager, benefits must justify the costsof purchasing and operating the irrigation system andthe time required to adequately operate, manage, andmaintain the irrigation system while leaving a reason-able return on investment. For the groundskeeper,park or landscape superintendent, nursery grower, orhomeowner, irrigation must maintain the desiredgrowth of grass, ornamentals, flowers, and gardencrops while minimizing costs, labor, inefficient wateruse, and nutrient and chemical losses.

Escalating costs of energy used for pumping makesevery acre-inch of excess water a concern to manyirrigators. Improving and maintaining pumping plants,irrigation equipment, irrigation application efficien-cies, and following an irrigation scheduling programcan lead to significant reductions in pumping costs.

Escalating costs of farm equipment, fuel, seed, fertil-izer, pesticide, and irrigation equipment also makeevery irrigation and field operation a financial concernto the farmer. Field operations should be limited tothose necessary to grow a satisfactory crop. Conserva-tion irrigation typically reduces:

• Overall on-farm energy use• Soil compaction, which affects root develop-

ment and water movement• Water quantities used• Opportunity for ground water and surface

water pollution

652.0100 Purpose andobjective

The Irrigation Guide provides technical informationand procedures that can be used for successful plan-ning, design, and management of irrigation systems. Itis a guide only and does not imply or set NaturalResources Conservation Service (NRCS) policy.

Irrigation systems should apply the amount of waterneeded by the crop in a timely manner without wasteor damage to soil, water, air, plant, and animal re-sources. This includes, but is not limited to, offsitewater and air quality and desired impacts on plant andanimal (including fish and wildlife) diversity. Otherbeneficial uses of irrigation water are frost protection,crop quality, crop cooling, chemigation, desirablesaline and sodic balance maintenance, and leaching ofundesirable soil chemicals.

The Irrigation Guide includes current information andtechnical data on irrigation systems and hardware,automation, new techniques, soils, climate, watersupplies, crops, tillage practices, and farming condi-tions. Included are irrigation related technical data forsoils and irrigation water requirements for crops. Insome instances statements are based on field experi-ences of the primary authors.

The objective of this guide is to assist NRCS employ-ees in providing sound technical assistance for themaintenance of soil productivity, conservation ofwater and energy, and maintenance or improvement ofthe standard of living and the environment. Basic dataused will help ensure the planned irrigation system iscapable of supplying the amount of water needed byplants for planned production and quality during thegrowing season. Procedures for optimizing use oflimited water supplies are also included.

Planning for an irrigation system should take intoaccount physical conditions of the site, producerresources, cropping pattern, market availability, waterquantity and quality, and effects on local environment.Economics should provide the basis for sound conser-vation irrigation decisions, but may not be the ultimateconsideration. This is because many other factors mayinfluence final decisions.



1–2 (210-vi-NEH, September 1997)

Applying water too soon or in excess of crop needsresults in inefficient irrigation application. Too oftenirrigation decisionmakers subscribe to "when in doubtirrigate," rather than scheduling irrigations based onsoil moisture monitoring and measured crop need.

Another factor leading to inefficient water use is theuse-it-or-lose-it perception. Some irrigators and irriga-tion districts feel they must divert and use all thewater allocated to them whether they need it or not.This can result in less than desired crop yield andproduct quality. It also increases leaching of nutrients,toxic elements, and salts below the root zone andincreases the potential for erosion.

The direct cost of water to irrigators, when the wateris supplied by irrigation companies or irrigation dis-tricts, varies between $5 and $600 per acre per year. Inmany areas, however, water is relatively low in cost.Low cost water can lead to inefficient use if an irriga-tor uses a convenient application time rather thanproviding the labor to fully manage the water.

652.0102 Soil conserva-tion, water quality, andpollution abatement

Irrigation induced soil erosion is a problem on specificsoils in certain areas. Soil erosion can take the form ofwind erosion when smooth and bare ground occursbetween harvest and new crop growth periods. Soilerosion by water can result from high application ratesin the outer part of center pivot systems, excessivefurrow or border inflows, and uncontrolled tailwateror runoff. The use of surface irrigation on moderatelysteep to steep topography or leakage in the deliverysystem can also cause soil erosion by water.

Soil erosion can produce sediment loads in irrigationditches, drains, tailwater collection systems, roadsideditches, streams, and reservoirs. Sometimes it takescareful study of a site to realize that erosion is takingplace. Soil erosion on irrigated fields generally can becontrolled by careful planning, proper design, andadequate water, soil, and residue management. Offsitesediment damages are often a result of soil erosion fromcropland, tailwater ditches, and surface water drains.

Pollution of ground and surface water by agriculturalchemicals in irrigation water runoff or deep percola-tion is an increasing problem. Higher amounts offertilizers are being used today than in the past.Chemigation can improve the application of chemicalsthrough sprinkler systems, but can also create poten-tial environmental problems through spills and im-proper or careless application. Leached chemicals,including salts in irrigation water, can degrade groundand surface water qualities. All of these problems canbe minimized by proper planning, design, systemoperation, and water management.

Inefficient irrigation can have offsite benefits. Wetlandhabitat can be created from conveyance system leak-age and application of excess irrigation water. How-ever, excess irrigation water may contain undesirableor toxic organic or inorganic chemicals. In some partsof the United States, local, State, and Federal regula-tions are such that no irrigation runoff or subsurfacedrainage effluent from irrigation practices shall enterpublic water. In these areas irrigation runoff must becontained onsite, reused, or disposed of safely.




652.0103 Using the guide

The Irrigation Guide is prepared for local use; how-ever, it is recognized that this guide may not directlyapply to all areas. This guide contains sound water andirrigation system management concepts. It is a dy-namic document available in computer electronic filesor looseleaf form. As new, revised, or area-specificinformation becomes available, the guide can andshould be updated. Irrigation is a rapidly evolvingscience and industry. Frequent revisions and additionsare expected.

(a) Using irrigation procedures

The best available procedures and data should alwaysbe used, whether they are included in this irrigationguide or available elsewhere, for example, from Agri-cultural Research Service, Universities, CooperativeExtension Service, Bureau of Reclamation, or privateindustry.

Not all tables, charts, and procedures available inother readily available references are duplicated in theguide. Also, areas of the guide that describe proce-dures may not include all the processes and materialneeded to carry out the procedure. For instance, toperform a side roll sprinkler system design requiresthe use of National Engineering Handbook, Section 15,Chapter 11. However, most references referred to inthe guide are available for field office use.

A personal library or reference folder(s) containingspecific data and examples is recommended for tech-nicians performing procedures. This library can beused until computer software programs are availableand can then be used as a reference when the proce-dure is accomplished. Such a library or referencefolder(s) can contain the following types of material:

• Irrigation guide tables, charts, references,procedures, materials, and forms, includingexamples.

• Tables for local climate, soils, crops, and plantwater requirements.

• Available tables and figures from the NationalEngineering Handbook, Part 623, Irrigation.

• Information or aids from other sources forplanning, design, management, and systemevaluation.

• Previous jobs that have been designed, docu-mented and approved.

(b) Using worksheets

The use of worksheets in this guide is optional. Theyshould only be used if they are advantageous in savingplanning time and providing documentation. Onlythose parts of the worksheets that apply to the particu-lar job should be used. Blank master worksheets areincluded in chapter 15 of this guide.




652.0104 Irrigation guideoutline

(a) General

Chapter 1, Introduction—This chapter introducesthe irrigation guide, its purpose and contents. It alsodiscusses water and energy conservation needs andopportunities, soil conservation, water quality, andpollution abatement concerns and opportunities.

(b) Soil-water-plant data

Chapter 2, Soils—This chapter describes soil basics:soil surveys, physical soil characteristics, and therelation of soil characteristics to different irrigationmethods and systems. Several soil properties directlyinfluence the design, management, and operation of anirrigation system.

Basic soil-water irrigation related parameters includedin chapter 2 are variables and are to be used as a guideonly. The parameters include:

• Estimated available water capacity by horizonsor 1 foot (0.3 meter) increments

• Water intake characteristics for furrow andborder (basins) irrigation

• Intake rates or maximum application rates forsprinkle irrigation

• Up-flux or upward water movement in soil

Specific local soils and their characteristics pertainingto irrigation are included in the state supplementsection.

Chapter 3, Crops—This chapter describes the cropcharacteristics pertaining to irrigation; i.e., growthcharacteristics, rooting depth, and moisture extractionpatterns, Management Allowable Depletion (MAD)levels, and effects of temperature, sodicity, and salin-ity. Management, including critical irrigation andmoisture stress periods for plants and other specialirrigation considerations, is included as a primaryirrigation tool.

Crops respond to irrigation when rainfall does notmaintain favorable soil moisture levels. When rainfallevents are spaced too far apart for optimum plant-water conditions, plant biomass, yields, and quality areaffected. Knowledge of actual crop rooting depths,water requirements at different growth stages, criticalmoisture stress periods, crop temperature modifica-tion effect, seed germination, and pesticide control areall necessary in determining when and how muchwater to apply.

Chapter 4, Water Requirements—This chapterdescribes methods for determining crop evapotranspi-ration (ETc) and net irrigation water requirement.Water budget and balance analysis use are also de-scribed. Estimated evapotranspiration values for peakdaily, monthly, and seasonal periods for locally growncrops are included in the state supplement section.

(c) Irrigation and distributionsystems

Chapter 5, Selecting an Irrigation Method (Sur-face, Sprinkle, Micro, or Subsurface)—Thischapter includes factors that affect irrigation methodselection and system adaptation. The factors arelargely functions of crop selection and rotation, soils,topography, climate zone, tillage practices, laboravailability (including skills), economics, water avail-ability in quantity and quality, type of delivery sched-ule, and the irrigation decisionmaker’s personalpreference.

Chapter 6, Irrigation System Design—Criteriaand references for the implementation of the morecommonly used irrigation methods and applicablesystems are included in this chapter.

Chapter 7, Farm Distribution Components—Thischapter describes alternatives and various compo-nents of the farm distribution system. Water measure-ment should be a part of any distribution system as itis the key to proper water management.




(d) Irrigation planning andmanagement

Chapter 8, Project and Farm Irrigation WaterRequirements—Procedures for determining largescale water requirements are described in this chapter.It also includes the application of water budget analy-ses to group and project level water requirementversus availability.

Chapter 9, Irrigation Water Management—Goodirrigation water management should be practiced withall irrigation application systems. New techniques forirrigation scheduling and system automation areavailable and are a part of the information in thischapter. Field and climatic data should be accuratelycollected and an analysis of irrigation need, timing,and application amount made available to the irrigatorpromptly. Procedures for establishing soil intakecharacteristics and evaluation of existing irrigation aredescribed.

Chapter 10, Conservation Management Systemsand Irrigation Planning—This chapter containsthe basic steps for planning ecosystem-based resourcemanagement systems including irrigation systemplanning. The planning process as it pertains to irri-gated cropland is described.

Chapter 11, Economic Evaluations—This chapterincludes the criteria that can be used in evaluatingpumping plant operating costs. It also describes theprocedures for making economical pipe size determi-nations and other economic factors and processes thatcan be used in planning.

Chapter 12, Energy Use and Conservation—Thischapter reviews alternative energy sources and costsused in pumping and gives examples of irrigationsystem comparison and tillage and residue manage-ment that relate to overall on-farm energy require-ments. Improving water management almost alwaysdecreases water and energy use except where inad-equate irrigation has occurred and more water isneeded to meet yield and quality objectives.

Chapter 13, Quality of Water Supply—Quality ofwater to be used for irrigation of crops is briefly de-scribed in this chapter. To meet crop yield and qualityobjectives, a reliable supply of high quality water isdesired. However, with proper management, applying

saline water on salt tolerant crops, liquid waste fromagricultural related processing and products, treatedmunicipal sewage effluent, and other low quality watershould be considered as an irrigation water source.

Chapter 14, Environmental Concerns—A directrelationship can be established between downstreamwater quality and irrigation. This relationship is pre-sented in chapter 14. Improper selection of an irriga-tion method and system for a given site or the misman-agement of any system can result in poor water distri-bution uniformity, soil erosion, excessive runoff, andexcessive deep percolation. Runoff can carry agricul-tural chemicals and plant nutrients in solution orattached to soil particles (e.g., phosphates). Excessirrigation water moving below the plant root zone(deep percolation) can carry soluble salts, nutrients(nitrates), pesticides, and other toxic elements thatmay occur in the soil profile. Excess irrigation waterand whatever it contains in solution generally ends upeither as ground water recharge or returns to down-stream surface water.

(e) Special tools

Chapter 15, Resource Planning and EvaluationTools and Worksheets—Included in this chapter areaids, tools, and processes that can facilitate irrigationsystem planning, design, and evaluations. ExampleIrrigation Water Management or Irrigation WaterConservation Plans are also included. Master blankworksheets are included to help the technician orwater user.

Chapter 16, Special Use Tables, Charts, andConversions—This chapter contains special usetables, charts, and conversion factors that are useful inthe planning, design, and evaluation processes. En-glish units are used along with metric conversions asthey reasonably apply. A complete metric conversiontable relating to irrigation is included.

Chapter 17, Glossary and References—Thischapter contains a list and definition of the morecommonly used irrigation terms. Many terms are local,and some duplication is necessary. References avail-able and used in irrigation system planning, design,management, and evaluation are included.




652.0105 Use of computers

Only state approved computer software is available tothe field office for official use. These programs help tofacilitate planning, design, and evaluation of irrigationsystems and related components. The technician orengineer is fully responsible for plan or design integ-rity, adequate documentation, and obtaining necessaryreviews and engineering approval.

Information contained in this guide describes availabil-ity and use of computer software for performingcertain tasks. Additions or revisions to the guideincluding instructions or references to user manualswill be made as new software becomes available.

652.0106 State supplement

Chapter 1 Introduction Part 652 Irrigation Guide

WA652.0106 Introduction (a) General Information The Washington supplement to the National Engineering Handbook (NEH), Part 652, Irrigation Guide, has been adapted from the original Washington Irrigation Guide and has been prepared to cover all areas of Washington state. One set of guidelines is considered adequate for the entire state, with the exception of water requirements for crops. This information can be obtained from the attachment to Chapter 4, Water Requirements. Another planning tool for locations and crops not shown in the Chapter 4 attachment is the Irrigation Water Requirements (IWR) software. This software is explained and examples shown. The state supplements were developed to assist engineers, technicians, and others working with Washington irrigators to provide general planning criteria on various methods of irrigation commonly used in the state. When the irrigation system is installed and operated in accordance with basic data, the irrigator is assured first of all, that the system will be capable of supplying the amounts of water needed by plants for optimum production and, secondly, that with proper seasonal adjustments, irrigation water can be applied efficiently. The irrigation planner must be aware that every irrigation system should minimize erosion. Polymer is very useful for this. Alternatives include a tailwater recovery system with polymer or polymer alone for areas where waste water is recycled to lower irrigators. Tailwater systems and polymer is discussed further in the Washington Field

Office Technical Guide (FOTG) and NEH652, Chapter 7. Recommendations relating to economic evaluation of irrigation systems are included in NEH 652, Chapter 11. The economics of irrigation is usually an individual field or farm determination. A map of the annual precipitation in Washington can be found on the Washington state NRCS web page. This is located under the FOTG, Section 1, Maps, and Climatic data. The link to the NRCS is listed below: http://efotg.nrcs.usda.gov/treemenuFS.aspx?Fips=53025&MenuName=menuWA.zip (b) Irrigation Specifications Commonly used methods of water application are given in Parts WA652.0505 and WA652.0605 of this guide. The specific methods adapted for each soil group, each crop, and each slope group are listed. Applicable specifications are also given. The sprinkler irrigation specifications have not been shown in the irrigation design sheets because of the numerous sprinkler types available and their varied requirements. However, the suitability of sprinkler irrigation for use in each soil group, crop, and slope group is shown in the irrigation design sheets. Sprinkler criteria and installation guidance is given in Chapter 6 and Part WA652.0605 for drip (trickle) irrigation and subsurface drip irrigation (SDI), (c) Control of Erosion This guide contains the maximum slope recommendations for each crop and method along with companion practices such as terraces and contouring for the various soils

WA1-1 (210-VI-NEH 652, Amendment WA1, Jan 2005)



under normal conditions to prevent excessive water erosion whether by irrigation or by rainfall. The guide also designates irrigation water application rates, stream sizes, length of run, and time of irrigation to minimize erosion. The conservation treatment of irrigated land for wind and water erosion is listed in the Field Office Technical Guide, Section I-A. (d) Conservation Treatment Specifications Section IV of the Field Office Technical Guide contains specifications for conservation treatment practices. Standards and specifications for such irrigation component practices including: land leveling; underground irrigation pipeline; canal and ditch lining; farm irrigation structures; and tailwater recovery systems. Others are also found in Section IV of the Field Office Technical Guide. (e) Procedure Guide for Irrigation Land Development (1) Criteria for irrigation systems The design of all irrigation systems shall follow the criteria in Part WA652.0605 and conform to the requirements outlined in the applicable Washington standards and specifications. (2) Basic information The following basic information must be considered when planning and designing an irrigation system: (i) A water quality analysis

SDI systems will need a water quality analysis, including problems with sand or grit, prior to system design. A water quality analysis is provided by the irrigator. For information on a water quality analysis, refer to Agriculture Handbook No. 60, "Diagnosis and Improvement of Saline and Alkali Soils," Chapter 5, Quality of Irrigation Water, and/or "Determining Water Quality for Irrigation," C 396, Revised November 1975, issued by the Washington Cooperative Extension Service. Usual salinity limits of water are based on electrical conductivity (millimhos per centimeter) as follows:

< 0.75 mmhos - Safe water 0.75-2.25 mmhos - Safe where drainage is

adequate > 2.25 mmhos - Very limited use on high

salt tolerance crops in large quantities and very good subsoil drainage.

Note: Convert millimhos to micromhos by multiplying by 1,000: 0.75 mmhos x 1,000 = 750 micromhos

For detailed information, refer to Chapter 13, “Quality of Water Supply”. (ii) Quantity of water available Water right information shall be obtained from the Washington Department of Ecology for the authorized place of use. (iii) Topographic or grid map (iv) Soils information from Part WA652.0204



Soils not listed in the guide--The soil scientist will make the following determinations in the field: • Effective soil depth, texture, permeability,

slope, erosion, and fertility potential. • Intake rate with the effective rooting depth

and water holding capacity. Preparation of soils report--The soil scientist will then prepare a soils report and submit it to the state soil scientist for review. Upon the concurrence of the state soil scientist that the soil is suitable for irrigation, all field locations will be advised to list this information in the appropriate place in Part WA652.0204. (3) Guidelines In developing an overall irrigation system plan, the following guidelines should be observed and included: (i) Conservation cropping system--This will be developed in accordance with the Washington Standard and Specifications for Conservation Cropping System - 328. This may be listed on the irrigation development plan or in the case file with adequate cross references. (ii) Amount of land to be developed for irrigation using surface irrigation methods--This will depend on the planned cropping system, the intensity of irrigation desired by the irrigator, and the amount of irrigation water available. To determine the maximum irrigated acres, use the procedure outlined in Part WA652.0605. This indicates the crop acres that can be completely irrigated throughout

the growing season with no limitation of crop production due to moisture. In some cases, the irrigator may desire or need to adjust the system operation to something less than maximum as far as pumping time and water use are concerned. Factors including high energy costs, restrictions on water use, type of cropping system, and other items may dictate this choice. Usually irrigations will be limited to several pre-selected times during critical crop-growing periods. There may be other instances where the irrigator will use the same irrigation frequency, except he will use the maximum water available and adjust the irrigated acres accordingly. If supplemental irrigation is planned, the amount of irrigated acres will usually be determined by the irrigation intensity desired. For example, in western Washington where average annual rainfall is over 35 inches, the amount of irrigated acreage is usually based on providing 1 or 2 net applications from 2 to 4 inches. The primary purpose is to provide supplemental moisture during a dry period that usually occurs each summer at a critical stage of crop growth. (iii) Lands subject to flood scour and/or deposition more frequently than once every 10 years --These lands should not be developed for irrigation. (iv) Fields planned for land leveling--These fields shall meet the requirements of Washington Standard and Specifications for Irrigation Land Leveling - 464. (v) Irrigation Water Management –The IWM plan will be a part of the irrigation system design. This plan shall meet the requirements



of Washington Standard and Specification for Irrigation Water Management - 449. (vi) Tailwater recovery pits--These will be part of the irrigation plan if required under the conditions listed in the Washington Standard and Specifications for Irrigation System, Tailwater Recovery - 447.


SoilsChapter 2


Chapter 2 Soils

Contents:

2–i

652.0200 Introduction 2–1

652.0201 General 2–1

(a) Soil survey ...................................................................................................... 2–1

(b) Soil survey data base .................................................................................... 2–2

(c) Soil limitations for irrigation ....................................................................... 2–2

652.0202 Physical soil characteristics 2–4

(a) Soil properties and qualities ........................................................................ 2–4

(b) Soil-water holding capacity ......................................................................... 2–4

(c) Soil intake characteristics ......................................................................... 2–17

(d) Organic matter ............................................................................................ 2–28

(e) Soil depth ..................................................................................................... 2–28

(f) Slope ............................................................................................................. 2–29

(g) Water tables ................................................................................................. 2–29

(h) Soil erodibility ............................................................................................. 2–29

(i) Chemical properties ................................................................................... 2–31

(j) Saline and sodic soil effects ...................................................................... 2–31

(k) Soil reaction/acidity .................................................................................... 2–32

652.0203 Explanation of tables and data bases 2–34

652.0204 State supplement 2–34

(a) Soil surveys .................................................................................................. 2–34

(b) Soil properties ............................................................................................. 2–34

Tables Table 2–1 Available water capacity (AWC) by texture 2–5

Table 2–2 Correction of available water capacity for rock 2–6

fragment content

Table 2–3 Available water capacity adjustment factors 2–7

Table 2–4 Available water capacity adjustments because of salinity 2–10


Soils


Chapter 2

Table 2–5 General terms, symbols, and size of soil separates 2–11

for basic soil texture classes

Table 2–6 Soil intake ranges by surface texture 2–17

Table 2–7 Maximum net application amounts with zero potential 2–21

runoff for center pivot systems

Table 2–8 Maximum sprinkler application rate—periodic move 2–22

and fixed set sprinkler (for alfalfa-grass, grass, or

clean tilled with residue > 4,000 lb/ac)

Table 2–9 Soil intake family adjustment factors 2–23

Table 2–10a Amount of surface storage needed for no runoff— 2–25

Silty clay

Table 2–10b Amount of surface storage needed for no runoff— 2–25

Silty clay loam

Table 2–10c Amount of surface storage needed for no runoff— 2–26

Silt loam

Table 2–10d Amount of surface storage needed for no runoff—Loam 2–26

Table 2–10e Amount of surface storage needed for no runoff— 2–26

Fine sandy loam

Table 2–10f Amount of surface storage needed for no runoff— 2–26

Loamy fine sand

Table 2–10g Amount of surface storage needed for no runoff— 2–27

Fine sand

Table 2–11 Estimated effective basin surface storage 2–28

Table 2–12 Surface storage available for rough and cloddy 2–28

bare ground

Table 2–13 Surface storage available with residue 2–28

Table 2–14 Soil erodibility hazard (S K values) for surface irrigation 2–30

Table 2–15 Soil properties and design values for irrigation 2–34

2–ii


SoilsChapter 2

2–37(210-vi-NEH, September 1997) 2–iii

Figures Figure 2–1 Typical water release curves for sand, loam, and clay 2–9

Figure 2–2 Examples of soil structure 2–13

Figure 2–3 Intake families for border and basin irrigation design 2–18

Figure 2–4 Intake families for furrow irrigation design 2–19

Figure 2–5a Nomenclature—dike spacing and height; furrow 2–27

width and ridge height and spacing

Figure 2–5b Dike spacing, height, and surface storage capacity 2–27

Figure 2–6 Water table contribution to irrigation requirement, as 2–29

a function of soil type (texture) and water table depth

Figure 2–7 Example soil-water retention curves for clay loam soil 2–31

at varying levels of soil salinity—ECe

Figure 2–8 Threshold values of sodium adsorption ratio of 2–32

topsoil and electrical conductivity of infiltrating water

associated with the likelihood of substantial losses in

permeability

Figure 2–9 Effect of pH on nutrient availability in soils 2–33

Exhibits Exhibit 2–1 Soil properties, limits, and restrictive features 2–3

for irrigation

Exhibit 2–2 Process to determine total volume of water held 2–16

in a soil


SoilsChapter 2


Chapter 2 Soils

652.0200 Introduction

Plant response to irrigation is influenced by the physi-cal condition, fertility, and biological status of the soil.Soil condition, texture, structure, depth, organicmatter, bulk density, salinity, sodicity, acidity, drain-age, topography, fertility, and chemical characteristicsall determine the extent to which a plant root systemgrows into and uses available moisture and nutrientsin the soil. Many of these factors directly influence thesoil’s ability to store, infiltrate, or upflux water deliv-ered by precipitation or irrigation (including watertable control). The irrigation system(s) used shouldmatch all or most of these conditions.

Many conditions influence the value of these factors.The estimated values for available water capacity andintake are shown as rather broad ranges. Workingwith ranges is a different concept than used in previ-ous irrigation guides. In the field, ranges are normalbecause of many factors. The values in local soils databases need to be refined to fit closer to actual fieldconditions. The actual value may vary from site to siteon the same soil, season to season, and even throughoutthe season. It varies throughout the season dependingon the type of farm and tillage equipment, number oftillage operations, residue management, type of crop,water quality, and even water temperature.

Soils to be irrigated must have adequate surface andsubsurface drainage. Internal drainage within the croproot zone can be either natural or from an installedsubsurface drainage system.

This guide describes ways to interpret site conditionsfor planning and design decisions. Where necessary,actual field tests should be run to determine specificplanning and design values for a specific field. Evalua-tion results can also be used to fine tune individualirrigation system operations and management. When aparticular soil is encountered frequently in an area,efforts should be made to gather field data to verifythe site conditions or to use in refining values in theguide. These field derived values should be added assupport for data presented in the guide.

652.0201 General

Soil consists of mineral and organic materials, cover-ing much of the Earth’s surface. It contains livingmatter, air, and water, and can support vegetation.People have altered the soil in many places. Soil is oneof the resources of major concern to USDA and theNatural Resources Conservation Service. The soilfunctions as a storehouse for plant nutrients, as habi-tat for soil organisms and plant roots, and as a reser-voir for water to meet evapotranspiration (ET) de-mands of plants. It contains and supplies water, oxy-gen, nutrients, and mechanical support for plantgrowth.

Soil is a basic irrigation resource that determines howirrigation water should be managed. The amount ofwater the soil can hold for plant use is determined byits physical and chemical properties. This amountdetermines the length of time that a plant can besustained adequately between irrigation or rainfallevents, the frequency of irrigation, and the amount andrate to be applied. Along with plant ET, it also deter-mines the irrigation system capacity system neededfor desired crop yield and product quality.

(a) Soil survey

NRCS is responsible for leadership of the NationalCooperative Soil Survey. Partners include other Fed-eral, State, and local agencies and institutions. Soilsurvey data and interpretations have information thatcan be used for planning, design, and managementdecisions for irrigation.

Soil map units represent an area on the landscape andconsist of one or more soils for which the unit isnamed. Single fields are rarely a single map unit or asingle soil. Many soil map units include contrastingsoil inclusions considered too minor to be a separatemap unit. Because of variations in soil properties thatexist in map units, additional onsite soils investiga-tions are often needed.


Soils


Chapter 2

Soil properties within a profile can be modified byland grading, deep plowing, subsoiling, or other deeptillage practices. Shallow tillage practices can affectwater infiltration and soil permeability rates. Theseproperty changes may not be reflected in the map unitdescription. Personnel doing irrigation planning areexpected to obtain accurate onsite soil information tomake recommendations. Adjacent farms may needdifferent recommendations for the same soil seriesbecause different equipment, tillage practices, andnumber of tillage operations are used.

(b) Soil survey data base

Soil survey data are available from the local NationalSoil Information System (NASIS) Map Unit Interpreta-tion Record (MUIR) soil data base on the Field OfficeComputing System (FOCS). Irrigation related softwareapplications access this data base through a soil char-acteristics editor to create point data located within afield or operating unit. Where maximum and minimumranges of soil attribute data are contained in the database (for example, percent rock or available watercapacity), the editor can be used to select or input theappropriate value. If a soil profile has been examinedin the field, then data for the profile are entered in-stead of using the data base. Soil data points created inthis way can be used to create summary soil reports,or the data can be used directly either manually or inirrigation related software applications.

(c) Soil limitations for irrigation

Exhibit 2–1 displays soil limitations when determiningthe potential irrigability of a soil. It displays specificlimits and restrictive features for various soil proper-ties; however, it does not necessarily mean the soilshould not be irrigated. A restriction indicates thereare limitations for selection of crops or irrigationmethod and will require a high level of management.Some restrictions may require such an excessive highlevel of management that it may not be feasible toirrigate that soil. Likewise, a deep well drained loamysoil with minor restrictions can become nonirrigabledue to poor water management decisions and culturalpractices.


SoilsChapter 2


Exhibit 2–1 Soil properties, limits, and restrictive features for irrigation1/

Property Limits Restrictive features

USDA surface texture COS, S, FS, VFS, High intake for surface irrigationLCOS, LS, LFS, LVFS systems.

USDA surface texture SIC, C, CS Low intake for level basin andcenter pivot irrigation systems.

Slope surface >3% Water runoff.

Weight percent of stone particles >25% Large stones, reduced plant root>3" (weighted avg. to 40" depth) zone AWC.

Ponding + Soil air is removed.

Depth to high water table 13 Excess sodium ions.

Calcium carbonate equivalent >40 Excess lime.(% in thickest layer, 10-60" depth)

Sulfidic materials, Great Group Sulfaquents, sulfihemists Excess sulfur.

Soil reaction, pH, at any depth 0-60" 8.0 Too acid or too alkaline.

1/ Part 620, NRCS, National Soil Survey Handbook, 1993.


Soils


Chapter 2

652.0202 Physical soilcharacteristics

(a) Soil properties and qualities

Soil properties and qualities are important in design,operation, and management of irrigation systems.These properties include water holding capacity, soilintake characteristics, permeability, soil condition,organic matter, slope, water table depth, soil erodibil-ity, chemical properties, salinity, sodicity, and soilreaction (pH).

(b) Soil-water holding capacity

The potential for a soil to hold water is important indesigning and managing an irrigation system. Totalwater held by a soil is called water holding capacity.However, not all soil-water is available for extractionby plant roots. The volume of water available to plantsthat a soil can store is referred to as available watercapacity.

(1) Available Water Capacity (AWC)This is the traditional term used to express the amountof water held in the soil available for use by mostplants. It is dependent on crop rooting depth andseveral soil characteristics. Units of measure areexpressed in various terms:

• Volume unit as inches of water per inch or perfoot of soil depth

• Gravimetric percent by weight• Percent on a volume basis

In fine textured soils and soils affected by salinity,sodicity, or other chemicals, a considerable volume ofsoil water may not be available for plant use.

(2) Soil-water potentialSoil-water potential is a more correct way to definewater available to plants. It is the amount of workrequired per unit quantity of water to transport waterin soil. In the soil, water moves continuously in thedirection of decreasing potential energy or from higherwater content to lower water content. The concept ofsoil-water potential replaces arbitrary gravitational,capillary, and hygroscopic terms. Total water potential

consists of several components. It is the sum ofmatric, solute, gravitational, and pressure potential.Refer to the National Engineering Handbook (NEH),Section 15, Chapter 1, Soil-Plant-Water Relationshipsfor a detailed explanation of this concept.

The soil-water potential concept will become moreintegrated into field procedures as new proceduresevolve. For practical reasons, the terms and conceptsof field capacity and permanent wilting point aremaintained. Units of bars and atmospheres are gener-ally used to express suction, tension, stress, or poten-tial of soil water.

(i) Field capacity—This is the amount of water awell-drained soil holds after free water has drainedbecause of gravity. For coarse textured soil, drainageoccurs soon after irrigation because of relatively largepores and low soil particle surface tension. In finetextured soil, drainage takes much longer because ofsmaller pores and their horizontal shape. Major soilproperties that affect field capacity are texture, struc-ture, bulk density, and strata within the profile thatrestrict water movement. Generally, fine textured soilholds more water than coarse textured soil. Somesoils, such as some volcanic and organic soils, areunique in that they can retain significant volumes ofwater at tensions less than one-tenth bar, therebygiving them a larger available water capacity.

An approximation of field capacity soil-water contentlevel can be identified in the laboratory. It is the waterretained in a soil when subjected to a tension of one-tenth atmosphere (bar) for sandy soils and one-thirdatmosphere for other finer textured soils.

Field capacity water content level can be estimated inthe field immediately following a rain or irrigation,after free water has drained through the soil profile.Some judgment is necessary to determine when freewater has drained and field capacity has been reached.Free water in coarse textured soils (sandy) can drainin a few hours. Medium textured (loamy) soils takeapproximately 24 hours, while fine textured (clayey)soils may take several days.

(ii) Permanent wilting point—This is the soil-water content at which most plants cannot obtainsufficient water to prevent permanent tissue damage.The lower limit to the available water capacity hasbeen reached for a given plant when it has so ex-


SoilsChapter 2


hausted the soil moisture around its roots as to haveirrecoverable tissue damage, thus yield and biomassare severely and permanently affected. The watercontent in the soil is then said to be the permanentwilting percentage for the plant concerned.

Experimental evidence shows that this water contentpoint does not correspond to a unique tension of 15atmospheres for all plants and soils. The quantity ofwater a plant can extract at tensions greater than thisfigure appears to vary considerably with plant species,root distribution, and soil characteristics. Some plantsshow temporary plant moisture stress during hotdaytime periods and yet have adequate soil moisture.In the laboratory, permanent wilting point is deter-mined at 15 atmospheres tension. Unless plant specificdata are known, any water remaining in a soil atgreater than 15 atmosphere tension is consideredunavailable for plant use.

Major soil characteristics affecting the available watercapacity are texture, structure, bulk density, salinity,sodicity, mineralogy, soil chemistry, and organicmatter content. Of these, texture is the predominantfactor in mineral soils. Because of the particle configu-ration in certain volcanic ash soils, these soils cancontain very high water content at field capacitylevels. This provides a high available water capacityvalue. Table 2–1 displays average available watercapacity based on soil texture. Table 2–2 providesadjustments to the available water capacity based onpercent rock fragments. Generally, rock fragmentsreduce available water capacity.

The available water capacity value shown on the SoilInterpretation Record (SOI-5) accounts for the esti-mated volume of coarse fragments for the specific soilseries. However, any additional coarse fragmentsfound upon field checking must be accounted for.Coarse fragments of volcanic material, such as pumiceand cinders, can contain water within the fragmentsthemselves, but this water may not be available forplant use because of the restricted root penetrationand limited capillary water movement. A process toadjust the available water capacity based on additionalfield information is displayed in table 2–3.

Table 2–1 Available water capacity (AWC) by texture

Texture Texture AWC AWC Est.symbol range range typical

AWC(in/in) (in/ft) (in/ft)

COS Coarse sand .01 – .03 .1 – .4 .25S Sand .01 – .03 .1 – .4 .25

FS Fine Sand .05 – .07 .6 – .8 .75VFS Very fine sand .05 – .07 .6 – .8 .75

LCOS Loamy coarse sand .06 – .08 .7 – 1.0 .85LS Loamy sand .06 – .08 .7 – 1.0 .85

LFS Loamy fine sand .09 – .11 1.1 – 1.3 1.25LVFS Loamy very fine sand .10 – .12 1.0 – 1.4 1.25

COSL Coarse sandy loam .10 – .12 1.2 – 1.4 1.3SL Sandy loam .11 – .13 1.3 – 1.6 1.45

FSL Fine Sandy Loam .13 – .15 1.6 – 1.8 1.7VFSL Very fine sandy loam .15 – .17 1.8 – 2.0 1.9

L Loam .16 – .18 1.9 – 2.2 2.0SIL Silt loam .19 – .21 2.3 – 2.5 2.4

SI Silt .16 – .18 1.9 – 2.2 2.0SCL Sandy clay loam .14 – .16 1.7 – 1.9 1.8

CL Clay loam .19 – .21 2.3 – 2.5 2.4SICL Silty clay loam .19 – .21 2.3 – 2.5 2.4

SC Sandy clay .15 – .17 1.8 – 2.0 1.9SIC Silty clay .15 – .17 1.8 – 2.0 1.9

C Clay .14 – .16 1.7 – 1.9 1.8


Soils


Chapter 2

Table 2–2 Correction of available water capacity for rock fragment content 1/

- - - - - - - - - - - - - - - - - - - - - - - - - - - - % coarse fragments (by volume) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Soil 0 10 20 30 40 50 60 65 70

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - % passing #10 sieve (by weight)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -100 85 70 55 45 35 25 20 20

- - - - - - - - - - - - - - - - - - - - - Available water capacity (in/in)- - - - - - - - - - - - - - - - - - - - - -

Clay .14-.16 .12-.14 .11-.12 .09-.10 .08-.09 .06-.07 .05-.06 .04-.05 .03-.04

Silty clay .15-.17 .13-.15 .11-.13 .10-.11 .08-.10 ..07-.08 .06-.07 .05-.06 .04-.05

Sandy clay .15-.17 .13-.15 .12-.14 .10-.11 .08-.09 .07-.08 .06-.07 .04-.05 .04

Silty clay loam .19-.21 .17-.19 .15-.17 .13-.15 .11-.13 .09-.11 .08-.09 .06-.07 .06

Clay loam .19-.21 .17-.19 .15-.17 .13-.15 .11-.13 .09-.11 .08-.09 .06-.07 .06

Sandy clay loam .14-.16 .12-.14 .11-.13 .10-.11 .08-.10 .07-.08 .06-.07 .05-.06 .04-.05

Silt loam .19-.21 .17-.19 .15-.17 .13-.15 .11-.13 .09-.11 .08-.09 .06-.07 .06

Loam .16-.18 .14-.16 .13-.14 .11-.13 .10-.11 .08-.09 .07-.08 .05-.06 .05

Very fine sandy loam .15-.17 .13-.15 .12-.14 .10-.12 .09-.10 .07-.09 .07-.08 .05-.06 .04-.05

Fine sandy loam .13-.15 .12-.14 .10-.12 .09-.11 .08-.09 .06-.08 .06-.07 .04-.05 .04-.05

Sandy loam .11-.13 .10-.12 .09-.10 .07-.09 .07-.08 .05-.07 .05-.06 .04-.05 .03-.04

Loamy very fine sand .10-.12 .09-.11 .08-.10 .07-.08 .06-.07 .05-.06 .04-.05 .03-.04 .03-.04

Loamy fine sand .09-.11 .08-.10 .07-.09 .06-.07 .05-.07 .04-.06 .04-.05 .03-.04 .03

Loamy sand .06-.08 .05-.07 .05-.06 .04-.06 .04-.05 .03-.04 .03-.04 .02-.03 .02

Fine sand .05-.07 .04-.06 .04-.06 .03-.05 .03-.04 .03-.04 .02-.03 .02-.03 .01-.02

1/ Use this chart only when NASIS or more site specific information is not available. Compiled by NRCS, National Soil Survey Laboratory,Lincoln, Nebraska.


SoilsChapter 2


Table 2–3 Available water capacity adjustment factors 1/

" + " " – "Modifying factor Increased Decreased(%) AWC AWC

Rock content Rocks decrease soil and pore space volume

Sodicity Sodium salts disperse clays, decreases soilaggregation and destroys structure increasingsoil density.

Salinity Increased salt concentration makes it moredifficult for the plant to take in water byosmosis. The tension required to extractwater from the soil is increased.

Organic matter In general, OM increases aggregation (0 to +10%) and improves soil structure, decreases

soil density, and increases AWC. Insandy soils, OM provides fine particles,which effectively reduces averageparticle size.

Soil structure Granular, blocky, columnar and Single grain (sand - large sized pores release(–10% to +10%) Prismatic (low density) large proportion of gravitational water).

Massive or platy (usually high density).

Compaction (–20% to 0) Compaction increases soil density, reducespore space and decreases permeability.

Restrictive layers Restrictive layers in the subsoil can Restrictive layers can restrict root develop (0 to +10%) effectively increase AWC of upper ment and water movement lower in the soil

layers after an irrigation or rain. Water, profile.held up by the restrictive layer, has thepotential to be all or partially used bythe plants.

See footnote at end of table.


Soils


Chapter 2

Table 2–3 Available water capacity adjustment factors 1/—Continued

" + " " – "Modifying factor Increased Decreased(%) AWC AWC

Soil condition—the soil's Good soil condition results in decreased Poor soil condition results in increased soilphysical condition related soil density, increased soil micro- density, a more massive soil structure,to tillage, micro-organism organism activity, increased pore space. decreased pore space, decreased soilactivity, erosion. micro-organism activity.(–10% to +10% )

Depth within the soil In general, with increased depth, soils be-comeprofile (–5% per foot) more consolidated or dense, are affected by

mineralization, have less structure and or-ganic matter.

Vegetative cover Root penetration improves soil(0 to +5%) structure and condition, and

decreases soil density.

1/ Density can make AWC differences of –50% to +30% compared to average densities. Dense soils have low available water capacity becauseof the decreased pore space.


SoilsChapter 2


Different soils hold water and release it differently.When soil-water content is high, very little effort isrequired by plant roots to extract moisture. As eachunit of moisture is extracted, the next unit requiresmore energy. This relationship is referred to as a soilmoisture release characteristic. Figure 2–1 showswater release curves for typical sand, loam, and claysoils. The tension in the plant root must be greaterthan that in the soil at any water content to extract thesoil water. Typically with most field crops, crop yieldis not affected if adequate soil water is available to theplant at less than 5 atmospheres for medium to finetextured soils.

At soil-water tensions of more than about 5 atmo-spheres, plant yield or biomass is reduced in mediumto fine textured soils.

Salts in the soil-water solution decrease the amount ofwater available for plant uptake. Maintaining a highersoil-water content with more frequent irrigationsrelieves the effect of salt on plant moisture stress.Table 2–4 displays AWC values adjusting for effect ofsalinity versus texture. ECe is defined as the electricalconductivity of the soil-water extract corrected to77 °F (25 °C). Units are expressed in millimhos percentimeter (mmho/cm) or deci Siemen per meter (dS/m). 1 mmho/cm = 1dS/m. See section 652.0202(i) foradditional information.

Tension levels for field capacity and wilting point intable 2–4 are assumed.

AWC is the major soil factor in irrigation scheduling.Only a partial depletion of the AWC should be allowed.For most field crops and loamy soils, 50 percent isallowed to be depleted to limit undue plant moisturestress. For most vegetables, 30 percent depletion isdesirable. As an example, data from figure 2–1 pro-vides the following approximate potential (tension)levels for three general soil types:

Soil Tension at Tension at Depletion at 550% depletion 30% depletion bars tension

clay 4.5 bars 2.5 bars 55%loam 2 bars 1.2 bars 70%sand < 1 bars < 1 bars 84%

Allowed soil-water depletion is a management deci-sion based on the type of crop grown, stage of cropgrowth, total AWC of the soil profile, rainfall patterns,and the availability of the pumped or delivered water.It is referred to as the Management Allowed Depletion,or MAD level. See Chapter 3, Crops, for MAD levels foroptimum yield and quality of most crops. The conven-tional concepts of total soil volume AWC and MAD donot apply to microirrigation where root volumes andwetted volumes are restricted.

NEH, Section 15, Chapter 1, Soil-Plant-Water Relation-ships provides an excellent and thorough descriptionof soil-water relationships; therefore, the informationincluded here is quite limited.

Figure 2–1 Typical water release curves for sand, loam,and clay

100

30% level

50% level

Field capacity

80

60

40

20

00 2 4 6 8 10 12 14 165

Per

man

ent

wilt

ing

poin

t - 1

5 ba

rs

Soil water potential (-bars)

Ava

ilab

le w

ater

(%

)

Sand

ClayLoam

Texture

CourseMedium & fine

@ field capacity

0.10.33

@ Perm. wilting point

15.015.0

Tension level (atmospheres or bars)


Soils


Chapter 2

Table 2–4 Available water capacity adjustments because of salinity 1/

Soil texture - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Electrical conductivity (ICe x 103)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -0 2 4 6 8 10 12 14

- - - - - - - - - - - - - - - - - - - Available water capacity (inch/inch) 2/ - - - - - - - - - - - - - - - - - - -

clay .14-.16 .13-.15 .12-.14 .11-.13 .10-.12 .09-.11 .07-.08 .04-.05

silty clay .15-.17 .14-.16 .13-.15 .12-.14 .11-.12 .09-.11 .07-.08 .05-.06

sandy clay .15-.17 .14-.16 .13-.15 .12-.14 .11-.12 .09-.11 .07-.08 .05-.06

silty clay loam .19-.21 .18-.20 .17-.18 .15-.17 .14-.15 .12-.13 .09-.10 .06-.07

clay loam .19-.21 .18-.20 .17-.18 .15-.17 .14-.15 .12-.13 .09-.10 .06-.07

sandy clay loam .14-.16 .13-.15 .12-.14 .11-.12 .09-.11 .08-.09 .06-.07 .03-.04

silt loam .19-.21 .18-.20 .17-.18 .15-.17 .14-.15 .12-.13 .09-.10 .06-.07

loam .16-.18 .15-.17 .14-.16 .13-.15 .12-.13 .10-.11 .08-.09 .05-.06

very fine sandy loam .15-.17 .14-.16 .13-.15 .12-.14 .11-.12 .09-.11 .07-.08 .05-.06

fine sandy loam .13-.15 .12-.14 .11-.13 .11-.12 .09-.11 .08-.09 .06-.07 .04-.05

sandy loam .11-.13 .10-.12 .10-.11 .09-.11 .08-.09 .07-.08 .05-.06 .03-.04

loamy very fine sand .10-.12 .10-.11 .09-.11 .08-.09 .07-.08 .06-.07 .04-.05 .02-.03

loamy fine sand .09-.11 .09-.10 .08-.10 .07-.09 .06-.08 .06-.07 .04-.05 .03-.04

loamy sand .06-.08 .06-.08 .05-.07 .05-.06 .04-.06 .04-.05 .03-.04 .02-.03

fine sand .05-.07 .05-.07 .04-.06 .04-.06 .04-.05 .03-.04 .02-.03 .02

1/ Compiled by NRCS National Soil Survey Laboratory, Lincoln, Nebraska.2/ 15 mmhos conductivity results in 75 to 95 percent reduction in available water capacity.


SoilsChapter 2


Table 2–5 General terms, symbols, and size of soilseparates for basic soil texture classes(USDA, SCS 1993)

Texture Soil Symbol

Sandy soils:Coarse Sands

Coarse Sand COSSand SFine sand FSVery fine sand VFS

Loamy sandsLoamy coarse sand LCOSLoamy sand LSLoamy fine sand LFSLoamy very fine sand LVFS

Loamy soils:Moderately coarse Coarse sandy loam COSL

Sandy loam SLFine sandy loam FSL

Medium Very fine sandy loam VFSLLoam LSilt loam SILSilt SI

Moderately fine Clay loam CLSandy clay loam SCLSilty clay loam SICL

Clayey soils:Fine Sandy clay SC

Silty clay SICClay C

Size of soil separates:

Texture Size (mm) Texture Size (mm)

GR > 2.0 FS 0.25 – 0.10VCOS 2.01.0 VFS 0.10 – 0.05COS 1.0 – 0.5 SI 0.05 – 0.002MS 0.5 – 0.25 C < 0.002

(3) Soil textureSoil texture refers to the weight proportion of the soilseparates (sand, silt, and clay) for the less than 2 mmfraction, as determined from a laboratory particle sizedistribution analysis. It defines the fineness or coarse-ness of a soil. Particle sizes larger than 2 mm areclassed as rock or coarse fragments and are not usedto define texture. Table 2–5 shows terms and symbolsused in describing soil textures.

Fine textured soils generally hold more water thancoarse textured soils. Medium textured soils actuallyhave more available water for plant use than someclay soils. Water in clay soils can be held at a greatertension that reduces its availability to plants.

Figure 1–2, of NEH, Part 623, Chapter 1, Soil-Plant-Water Relationship, displays what is commonly re-ferred to as the USDA textural triangle. It describesthe proportions of sand, silt, and clay in the basictextural classes. Texture determines the amount ofsurface area on soil particles within the soil mass. Clayand humus both exist in colloidal state and have anextremely large surface area per unit weight. Theycarry surface electrical charges to which ions andwater are attracted.

The USDA Soils Manual includes the following generaldefinitions of soil textural classes in terms of fieldexperience. These definitions are also specificallyused in estimating soil-water content by the feel andappearance method. See Chapter 9, Irrigation WaterManagement and Chapter 15, Irrigation Water Manage-ment Plan.

Sand—Sand is loose and single-grained. The indi-vidual grains can be readily seen and felt. Squeezed inthe hand when dry, sand falls apart when pressure isreleased. Squeezed when moist, it forms a cast, butcrumbles when touched.

Sandy loam—A sandy loam is soil containing a highpercentage of sand, but having enough silt and clay tomake it somewhat coherent. The individual sandgrains can be readily seen and felt. Squeezed whendry, a sandy loam forms a cast that falls apart readily.If squeezed when moist, a cast can be formed thatbears careful handling without breaking.


Soils


Chapter 2

Loam—A loam is soil having a relatively even mix-ture of different grades of sand, silt, and clay. It isfriable with a somewhat gritty feel, but is fairlysmooth and slightly plastic. Squeezed when dry, itforms a cast that bears careful handling, and the castformed by squeezing the moist soil can be handledfreely without breaking.

Silt loam—A silt loam is soil having a moderateamount of fine sand with a small amount of clay. Overhalf of the particles are silt size particles. When dry, asilt loam appears cloddy, but the lumps can be readilybroken. When pulverized, it feels soft and floury. Whenwet, the soil runs together readily and puddles. Eitherdry or moist, silt loam forms a cast that can be handledfreely without breaking. When moist and squeezedbetween thumb and finger, it does not ribbon, but hasa broken appearance.

Clay loam—A clay loam is moderately fine-texturedsoil that generally breaks into clods or lumps that arehard when dry. When the moist soil is pinched be-tween the thumb and finger, it forms a thin ribbon thatbreaks readily, barely sustaining its own weight. Themoist soil is plastic and forms a cast that bears muchhandling. When kneaded in the hand, clay loam doesnot crumble readily, but works into a heavy compactmass.

Clay—A clay is fine-textured soil that usually formsvery hard lumps or clods when dry and is very stickyand plastic when wet. When moist soil is pinchedbetween thumb and finger, it forms a long flexibleribbon. Some clays that are very high in colloids arefriable and lack plasticity at all moisture levels.

Organic—Organic soils vary in organic matter con-tent from 20 to 95 percent. They generally are classi-fied on the degree of decomposition of the organicdeposits. The terms muck, peat, and mucky peat arecommonly used. Muck is well-decomposed organicmaterial. Peat is raw, undecomposed, very fibrousorganic material in which the original fibers constituteall the material.

(4) Soil structureSoil structure is the arrangement and organization ofsoil particles into natural units of aggregation. Theseunits are separated from one another by weaknessplanes that persist through cycles of wetting anddrying and cycles of freezing and thawing. Structureinfluences air and water movement, root development,and nutrient supply.

Structure type refers to the particular kind of groupingthat predominates in a soil horizon. Single-grained andmassive soils are structureless. In single-grained soils,such as loose sand, water percolates rapidly. Watermoves very slowly through most clay soils. A morefavorable water relationship occurs in soils that haveprismatic, blocky and granular structure. Platy struc-ture in fine and medium soils impedes the downwardmovement of water. See figure 2–2. Structure can beimproved with cultural practices, such as conservationtillage, improving internal drainage, liming or addingsulfur to soil, using grasses in crop rotation, incorpo-rating crop residue, and adding organic material or soilamendments. Structure can be destroyed by heavytillage equipment or excess operations.

Texture, root activity, percent clay, percent organicmatter, microbial activity, and the freeze-thaw cycle allplay a part in aggregate formation and stability. Someaggregates are quite stable upon wetting, and othersdisperse readily. Soil aggregation helps maintainstability when wet, resist dispersion caused by theimpact from sprinkler droplets, maintain soil intakerate, and resist surface water and wind erosion. Irriga-tion water containing sodium can cause dispersing ofsoil aggregates. See discussion of SAR in Section652.0202(i). Clay mineralogy has a major influence onsoil aggregation and shrink-swell characteristics. SeeNEH, part 623, chapter 1, for additional discussion.


SoilsChapter 2


Platy—The units are flat and plate-like. They are generally orientedhorizontal. (Soil Survey Manual,fig. 3-26, p. 159)

Prismatic—The individual units arebounded by flat to rounded verticalfaces. Units are distinctly longervertically, and the faces are typicallycasts or molds of adjoining units.Vertices are angular or subrounded;the tops of the prisms are somewhatindistinct and normally flat. (SoilSurvey Manual, fig. 3-27, p. 159)

Figure 2–2 Examples of soil structure


Soils


Chapter 2

Columnar—The units are similar toprisms and are bounded by flat orslightly rounded vertical faces. Thetops of columns, in contrast to thoseof prisms, are very distinct andnormally rounded. (Soil SurveyManual, fig. 3-28, p. 160)

Blocky—The units are block like orpolyhedral. They are bounded byflat or slightly rounded surfaces thatare casts of the faces of surroundingpeds. Typically, blocky structureunits are nearly equi-dimensional,but grade to prisms and plates. Thestructure is further described asangular blocky (with sharp corners)and subangular blocky (withrounded corners). (Soil SurveyManual, fig. 3-29, p. 161)

Figure 2–2 Examples of soil structure—Continued


SoilsChapter 2


Figure 2–2 Examples of soil structure—Continued

Granular—The units are approxi-mately spherical or polyhedral andare bounded by curves or irregularfaces. (Soil Survey Manual, fig. 3-30,p. 161)


Soils


Chapter 2

(5) Soil bulk densityRefers to the weight of a unit volume of dry soil, whichincludes the volume of solids and pore space. Unitsare expressed as the weight at oven-dry and volume atfield capacity water content, expressed as grams percubic centimeter (g/cc) or pounds per cubic foot(lb/ft3). Soil is composed of soil particles, organicmatter, water, and air.

(6) Soil pore spaceBulk density is used to convert water measurementsfrom a weight basis to a volume basis that can be usedfor irrigation related calculations. Many tools are avail-able to measure bulk density in the field as well as in thelaboratory. They are described in Chapter 9, IrrigationWater Management. Exhibit 2–2 displays the process todetermine the total volume of water held in a soil.

Pore space allows the movement of water, air, androots. Dense soils have low available water capacitybecause of decreased pore space. Density can makeAWC differences of –50 percent to +30 percent com-pared to average densities. Sandy soils generally havebulk densities greater than clayey soils. Sandy soils

have less total pore space than silt and clay soils.Gravitational water flows through sandy soils muchfaster because the pores are much larger. Clayey soilshold more water than sandy soils because clay soilshave a larger volume of small, flat-shaped pore spacesthat hold more capillary water. Clay soil particles areflattened or platelike in shape, thus, soil-water tensionis also higher for a given volume of water. When thepercent clay in a soil increases over about 40 percent,AWC is reduced even though total soil-water contentmay be greater. Permeability and drainability of soilare directly related to the volume and size and shapeof pore space.

Uniform plant root development and water movementin soil occur when soil profile bulk density is uniform,a condition that seldom exists in the field. Generally,soil compaction occurs in all soils where tillage imple-ments and wheel traffic are used. Compaction de-creases pore space, decreasing root development,oxygen content, and water movement and availability.Other factors affecting soil bulk density include freeze/thaw process, plant root growth and decay, worm-holes, and organic matter.

Exhibit 2–2 Process to determine total volume of water held in a soil

Let: Db = bulk density Schematic:Dp = particle density (specific gravity)Ws = weight of soil solids (oven dry)Ww = weight of soil waterVs = volume of solidsVp = volume of pores (both air & water)Vw = volume of waterVs + Vp = total soil volume

DW

V Vbs

s p=

+D

W

Vps

s= D V D V Vb s b s p× = +( )

V

V V

D

Ds

s p

b

p+= % Solids

V

V V

D

Ds

s p

b

p=

+× = ×100 100

% pore space = − ×

100 100

D

Db

p% water

W

Ww

s= × 100

% volume of water =+

× = × ×V

V V

W

WDw

s p

w

sb100 100

Volume of water (in/ft) volume of water 12 in/ft= ×%

100

Air

Water

Solids

W

Ww

Ws

Va

Vw

Vs

V

Vp


SoilsChapter 2


(c) Soil intake characteristics

Soil intake/water infiltration is the process of waterentering the soil at the soil/air interface. NEH, Part623, Chapter 1, Soil-Plant-Water Relationship providesdetailed discussion of the water infiltration process.

Infiltration rates change during the time water isapplied, typically becoming slower with elapsed time.They typically decrease as the irrigation seasonprogresses because of cultivation and harvest equip-ment. This is especially true if operations are done athigher soil-water content levels. Preferential flowpaths, such as cracks and wormholes, influence infil-tration and permeability. Infiltration rates are alsoaffected by water quality; for example, suspendedsediment, temperature, sodicity, and SAR, affectwater surface tension.

Soil intake characteristics affect design, operation, andmanagement of surface irrigation systems.

(1) Surface irrigation systemsThe water infiltration capability of a soil is referred toas soil intake characteristic. For surface irrigationsystems, intake characteristic is expressed by theequation:

F aT cob= +

where:

F = Cumulative intake for an opportunity timeperiod (inches)

a = Intercept along the cumulative intake axisTo = Opportunity time (minutes)b = Slope of cumulative intake vs. time curvec = Constant (commonly 0.275)

(See NEH, Part 623, Chapter 4, Border Irrigation, andChapter 5, Furrow Irrigation.)

Soil intake characteristics directly influence length ofrun, required inflow rate, and time of set that provide auniform and efficient irrigation without excessive deeppercolation and runoff. Table 2–6 displays estimatedsoil infiltration characteristics for border, furrow andfixed set or periodic move sprinkler irrigation systemsbased on surface soil texture.

For surface systems, water is considered pondedwhere it is 2 to 8 inches deep. Water infiltration forborders and basins is vertically downward. For fur-rows, infiltration is vertically downward, horizontal,and upward into furrow ridges. More field testing hasbeen done for borders than for furrows; therefore,intake estimates for borders are more readily avail-able. These intake characteristics can be converted foruse with furrows, but the intake process differencesmust be accounted for in the conversion.

Figure 2–3 displays intake groupings used for design-ing border and basin and contour surface irrigationsystems. Figure 2–4 displays intake groupings used fordesigning furrow irrigation systems. Furrow intakecharacteristics differ from border and basin intakecharacteristics because of the direction of watermovement near the soil surface and the percent of soilsurface covered by water.

Table 2–6 Soil intake ranges by surface texture 1/

Soil - - - - - - - - - - - Intake characteristics - - - - - - - - - - - -texture Sprinkle Furrow Border & basin

C, SIC .1 – .2 .1 – .5 .1 – .3

SC, SICL .1 – .4 .2 – .8 .25 – .75

CL, SCL .1 – .5 .2 – 1.0 .3 – 1.0

SIL, L .5 – .7 .3 – 1.2 .5 – 1.5

VFSL, FSL .3 – 1.0 .4 – 1.9 1.0 – 3.0

SL, LVFS .3 – 1.25 .5 – 2.4 1.5 – 4.0

LFS, LS .4 – 1.5 .6 – 3.0 2.0 – 4.0

FS, S .5 + 1.0 + 3.0 +

CS 1.0 + 4.0 + 4.0 +

1/ These are estimates based on soil texture. They should be usedonly where local data are not available.


Soils


Chapter 2

Figure 2–3 Intake families for border and basin irrigation design

8

6

4

2

1

10 100 1000

10 20 30 50 70 100 200 500 700 1000300 2000 3000 5000

Cu

mu

late

d i

nta

ke,

F (

inch

es)

4.0 3.0 2.0 1.5 1.0 0.5 0.3 0.1

Time, T (minutes)

F = cumulative intake for an opportunity time T period (inches)a = intercept along the cumulative intake axisTo = opportunity time (minutes)b = slope of cumulative intake vs. time curvec = constant (commonly 0.275)

F = aT + cbo


SoilsChapter 2


Figure 2–4 Intake families for furrow irrigation design 1/

10

8

4

2

1

Cu

mu

lati

ve i

nta

ke,

F (

inch

es)

30 40 50 60 80 100 200 300 400 500 600 800 1000 1500 2000

Time, T (minutes)

2.0

1.5

1.00.9

0

0.80 0.7

0

0.60 0.5

0

0.45 0

.40

0.35 0.

30

0.25

0.20

0.10

0.05

0.15

F = cumulative intake for an opportunity time T period (inches)a = intercept along the cumulative intake axisTo = opportunity time (minutes)b = slope of cumulative intake vs. time curvec = constant 2/ (commonly 0.275)

F = aT + cbo

1/ Source: NEH, Section 15, Chapter 5, Furrow Irrigation.2/ Constant can be adjusted based on local information.


Soils


Chapter 2

rate significantly. This condition is most likely tooccur on soils that are

• sodic,• poorly graded,• bare,• contain low organic matter,• have little or no surface residue, and• have limited vegetation canopy.

Table 2–7 displays the estimated maximum net appli-cation amounts and rates for center pivot systems. Thetable displays the sprinkler intake group and theamount of soil surface storage needed to apply anallowable irrigation amount. All systems are consid-ered to be 1,320 feet in length. The following systemsare compared in the table:

• High pressure impact heads with a peak rate of1.0 in/hr.

• Medium pressure impact heads with a peakrate of 1.5 in/hr.

• Low pressure impact heads with a peak rate of2.5 in/hr.

• Low pressure spray, two direction system withpeak rate of 3.5 in/hr.

• Low pressure spray, one direction system withpeak rate of 6.0 in/hr.

Values for various slopes for the maximum allowablenet application amount without additional storagecreated by special practices, are:

Field Approximate soilslope surface storage(%) (in)

0 - 1 0.51 - 3 0.33 - 5 0.1> 5 0.0

The infiltration process is different when using sprin-kler and border (or furrow) irrigation. With borderirrigation, a small head or depth of water (pressure) isplaced on the soil surface. With sprinkler and microir-rigation, the soil surface remains mostly unsaturated.The association with sprinkle application rate andborder intake family is through surface texture.

(2) Sprinkler irrigation systemsFor sprinkle irrigation, infiltration is referred to aseither an intake rate or maximum application rate,expressed as inches per hour (in/hr). Application ratesand timing vary according to type of sprinkler or sprayhead. With impact heads, water on the ground surfaceis at a single point only with each head rotation. Withspray heads, water is on the ground surface continu-ously, but at very shallow depth. Soil surface storage isimportant where water is applied in short time peri-ods; i.e., the outer end of low pressure center pivotlaterals.

Caution should be used when comparing averagesprinkler application rates with published soil infiltra-tion values. Some of the problems include:

• Low angle nozzles apply proportionally morewater in the area nearest the nozzle.

• Peak instantaneous application rates undercontinuously self-moving sprinkler laterals canbe very high. However, when expressed as anaverage hourly rate over the total irrigatedarea, these rates may appear quite low. Forexample: A 1-inch irrigation application beingmade at the outer end of a quarter mile longLow Pressure In Canopy center pivot lateralcan apply water at instantaneous rates exceed-ing 50 inches per hour for 2 to 10 minutes, butthe average hourly rate is considerably less. Inmedium and fine textured soils, the amountinfiltrated during the application period can bevery low.

Adequate soil surface storage is required to limittranslocation of water within the field and perhapsfield runoff during the infiltration process. Sprinklersystems should be designed with application rates thatdo not exceed the soil intake rate unless soil surfacestorage or other considerations are made.

Water droplet impact on a bare soil surface fromsprinkler systems can cause dispersion of some soils.The bigger the droplets, the more the potential disper-sion and micr

irrigation guide - usdairrigation guide acknowledgments part 652, irrigation guide, is an addition...

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