Surface IrrigationSurface Irrigation

522AE522AE

Evapotranspiration and drainage requirementsEvapotranspiration and drainage requirements

ET, is dependent upon:ET, is dependent upon:1) climatic conditions1) climatic conditions2) crop variety2) crop variety3) stage of growth3) stage of growth 4) soil moisture depletion4) soil moisture depletion5) various physical and chemical properties 5) various physical and chemical properties

of the soil.of the soil.

1) The seasonal distribution of reference 1) The seasonal distribution of reference crop crop "potential evapotranspiration", "potential evapotranspiration",

EtpEtp, which can be computed with , which can be computed with standard formulae. standard formulae.

2) The Etp is adjusted for crop variety 2) The Etp is adjusted for crop variety and stage of growth.and stage of growth.

Other factors like moisture stress can be Other factors like moisture stress can be ignored for the purposes of design ignored for the purposes of design

computations.computations.

Estimating ETEstimating ET

There are twenty commonly used methods There are twenty commonly used methods for calculating evapotranspiration, for calculating evapotranspiration, ranging ranging in complexity from the:in complexity from the:

Blaney-Criddle Method using primarily Blaney-Criddle Method using primarily mean monthly temperaturemean monthly temperature

Methods for calculating ETMethods for calculating ET

To more complete equations such as the To more complete equations such as the Penman Method requiring:Penman Method requiring:

1) Radiation1) Radiation

2) Temperature2) Temperature

3) Wind velocity3) Wind velocity

4) Humidity4) Humidity

5) Other factors comprising the net energy 5) Other factors comprising the net energy

balance at the crop canopy. balance at the crop canopy.

The actual crop water demand depends on The actual crop water demand depends on its stage of development and variety.its stage of development and variety.

It is estimated by multiplying ETr by a It is estimated by multiplying ETr by a crop growth stage coefficient, Kc.crop growth stage coefficient, Kc.

Some irrigation water should be applied in Some irrigation water should be applied in excess to leach salts from the rooting excess to leach salts from the rooting region. region.

Crop water demandCrop water demand

Soil moisture principlesSoil moisture principles Important soil characteristics in irrigated Important soil characteristics in irrigated

agriculture include: agriculture include:

1) the water-holding or storage capacity of the soil;1) the water-holding or storage capacity of the soil;

2) the permeability of the soil to the flow of water 2) the permeability of the soil to the flow of water and air; and air;

3) the physical features of the soil like the organic 3) the physical features of the soil like the organic matter content, depth, texture and structure; andmatter content, depth, texture and structure; and

4) the soil's chemical properties such as the 4) the soil's chemical properties such as the concentration of soluble salts, nutrients and trace concentration of soluble salts, nutrients and trace elements. elements.

The total available water, TAW, for plant use The total available water, TAW, for plant use in the root zone is commonly defined as:in the root zone is commonly defined as:

the range of soil moisture held at a negative the range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 barapparent pressure of 0.1 to 0.33 bar (a soil (a soil moisture level called 'field capacity') and 15 moisture level called 'field capacity') and 15 bars (called the 'permanent wilting point').bars (called the 'permanent wilting point').

The TAW will vary from: The TAW will vary from:

1.1. 25 cm/m for silty loams to25 cm/m for silty loams to

2.2. 6 cm/m for sandy soils.6 cm/m for sandy soils.

Relationships between soil types and total available soil moisture holding capacity, field capacity and wilting point

1) Porosity, 1) Porosity, ff 2) Volumetric moisture content, 2) Volumetric moisture content, qq3) Saturation, S3) Saturation, S 4) Dry weight moisture fraction, W4) Dry weight moisture fraction, W5) Bulk density, g b5) Bulk density, g b6) Specific weight, g s6) Specific weight, g s

Other Soil ParametersOther Soil Parameters

The relationships among these parameters The relationships among these parameters are as followsare as follows::

The porosity, The porosity, f f , of the soil is:, of the soil is: the ratio of the total volume of void or pore the ratio of the total volume of void or pore

space, Vp, to the total soil volume V:space, Vp, to the total soil volume V:

f f = = VpVp//VV

RelationshipsRelationships

The volumetric water content, The volumetric water content, q q , is the , is the ratio of water volume in the soil, VW, to the ratio of water volume in the soil, VW, to the total volume, V:total volume, V:

The saturation, S, is the portion of the pore The saturation, S, is the portion of the pore space filled with water:space filled with water:

These terms are further related as follows:These terms are further related as follows:

q q = Vb/V= Vb/V

S = VW/VpS = VW/Vp

q q = S * = S * ff

S = VW/VpS = VW/Vp

q q = S * = S * ff

When a sample of field soil is collected and When a sample of field soil is collected and oven-dried, the soil moisture is reported as oven-dried, the soil moisture is reported as a dry weight fraction, W:a dry weight fraction, W:

Convert a dry weight soil moisture Convert a dry weight soil moisture fraction into volumetric moisture content:fraction into volumetric moisture content:

The dry weight fraction is multiplied by The dry weight fraction is multiplied by the bulk density, g b; and divided by the bulk density, g b; and divided by specific weight of water, g w which can be specific weight of water, g w which can be assumed to have a value of unity. Thus: assumed to have a value of unity. Thus:

q q = g b W/g w= g b W/g w

The g b is defined as The g b is defined as the specific weight of the specific weight of the soil particles, g s, multiplied by the the soil particles, g s, multiplied by the particle volume or one-minus the porosity:particle volume or one-minus the porosity:

g b = g b * (1 - g b = g b * (1 - f f ))

Specific WeightSpecific Weight

The volumetric moisture contents at field The volumetric moisture contents at field capacity, capacity, q q fc, and permanent wilting point, fc, and permanent wilting point, q q wp, then are defined as follows:wp, then are defined as follows:

where Wfc and Wwp are the dry weight where Wfc and Wwp are the dry weight moisture fractions at each point.moisture fractions at each point.

q q fc = g b Wfc/g wfc = g b Wfc/g w q q wp = g b Wwp/g wwp = g b Wwp/g w

volumetric moisture contentsvolumetric moisture contents

The total available water, The total available water, TAWTAW is the is the difference between field capacity and wilting difference between field capacity and wilting point moisture contents multiplied by the point moisture contents multiplied by the depth of the root zone, RD:depth of the root zone, RD:

TAW = (TAW = (q q fc - fc - q q wp) RDwp) RD

Total Available WaterTotal Available Water

CropRoot Depth (metres)

Alfalfa1.5

Almonds1.8

Apricots1.8

Artichokes1.4

Asparagus1.5

Bananas0.9

Beans0.9

Beets0.8

Broccoli0.5

Cabbage0.5

The Soil Moisture Deficit, SMD, is a The Soil Moisture Deficit, SMD, is a measure of soil moisture between field measure of soil moisture between field capacity and existing moisture content, capacity and existing moisture content, q q i, i, multiplied by the root depth: multiplied by the root depth:

SMD = (SMD = (q q fc - fc - q q i) * RDi) * RD

Soil Moisture DeficitSoil Moisture Deficit

A similar term expressing the moisture that A similar term expressing the moisture that is allotted for depletion between irrigations is is allotted for depletion between irrigations is the 'Management Allowed Deficit', MAD.the 'Management Allowed Deficit', MAD.

This is the value of SMD where irrigation This is the value of SMD where irrigation should be scheduled and represents the should be scheduled and represents the depth of water the irrigation system should depth of water the irrigation system should apply.apply.

Management Allowed DeficitManagement Allowed Deficit

Soil Moisture MeasurementsSoil Moisture Measurements

The soil moisture status requires periodic The soil moisture status requires periodic measurements in the field, measurements in the field, from which one from which one can project when the next irrigation should can project when the next irrigation should occur and what depth of water should be occur and what depth of water should be applied.applied.

Such data can indicate Such data can indicate how much has been how much has been applied and its uniformity over the fieldapplied and its uniformity over the field. .

Techniques for Evaluating Soil MoistureTechniques for Evaluating Soil Moisture

i. i. Gravimetric samplingGravimetric sampling Collecting a soil sample from each 15-30 cm Collecting a soil sample from each 15-30 cm

of the soil profile to a depth at least that of the of the soil profile to a depth at least that of the root penetration.root penetration.

The soil sample of 100-200 gm is placed in an The soil sample of 100-200 gm is placed in an air tight container of known weight and then air tight container of known weight and then weighed. weighed.

The sample is then placed in an oven heated The sample is then placed in an oven heated to 105° C for 24 hours with the container to 105° C for 24 hours with the container cover removed.cover removed.

After drying, the soil and container are After drying, the soil and container are again weighed and the weight of water again weighed and the weight of water determined as the before and after readings.determined as the before and after readings.

The dry weight fraction of each sample can The dry weight fraction of each sample can be calculated using Eq. 5.be calculated using Eq. 5.

Knowing the bulk density, one can Knowing the bulk density, one can determine moisture contents from Eq. 6 and determine moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11the soil moisture depletion from Eq. 11

Sampling auger

Sampling tube

ii. ii. The neutron ProbeThe neutron Probe The neutron probe is inserted at various The neutron probe is inserted at various

depths into an access tube and the count depths into an access tube and the count rate is read from the scaler.rate is read from the scaler.

The manufacturers of neutron probe The manufacturers of neutron probe equipment furnish a calibration relating equipment furnish a calibration relating the count rate to volumetric soil moisture the count rate to volumetric soil moisture content.content.

Field experience suggests that these Field experience suggests that these calibrations are not always accurate under calibrations are not always accurate under a broad range of conditions so it is advisable a broad range of conditions so it is advisable for the investigator to develop an individual for the investigator to develop an individual calibration for each field or soil type.calibration for each field or soil type.

Most calibration curves are linear, best fit Most calibration curves are linear, best fit lines of gravimetric data and scaler lines of gravimetric data and scaler readings but may in some cases be slightly readings but may in some cases be slightly curvilinearcurvilinear

The volume of soil actually monitored in The volume of soil actually monitored in readings by the neutron probe depends on readings by the neutron probe depends on the moisture content of the soil, increasing the moisture content of the soil, increasing as the soil moisture decreases.as the soil moisture decreases.

The accuracy of soil moisture The accuracy of soil moisture determinations near the ground surface is determinations near the ground surface is affected by a loss of neutrons into the affected by a loss of neutrons into the atmosphere thereby influencing atmosphere thereby influencing measurements prior to an irrigation more measurements prior to an irrigation more than afterwards.than afterwards.

As a consequence, soil moisture As a consequence, soil moisture measurements with a neutron probe measurements with a neutron probe are usually unreliable within 10-30 cm are usually unreliable within 10-30 cm of the ground surface.of the ground surface.

iii. iii. Touch-and-feelTouch-and-feel As a means of developing a rough estimate As a means of developing a rough estimate

of soil moisture, the Touch-and-feel of soil moisture, the Touch-and-feel method can be used.method can be used.

A handful of soil is squeezed into a ball.A handful of soil is squeezed into a ball.

Then the appearance of the squeezed soil Then the appearance of the squeezed soil can be compared subjectively to the can be compared subjectively to the descriptions listed in Table 2 to arrive at descriptions listed in Table 2 to arrive at the estimated depletion level.the estimated depletion level.

Merriam (1960) has developed a similar Merriam (1960) has developed a similar table which gives the moisture deficiency in table which gives the moisture deficiency in depth of water per unit depth of soil.depth of water per unit depth of soil.

Over the years various investigators have Over the years various investigators have compared actual gravimetric sample results compared actual gravimetric sample results to the Touch-and-Feel estimates, finding a to the Touch-and-Feel estimates, finding a great deal of error depending on the great deal of error depending on the experience of the sampler.experience of the sampler.

iv. iv. Bulk densityBulk density Measurements of bulk density are Measurements of bulk density are

commonly made by carefully collecting a commonly made by carefully collecting a soil sample of known volume and then soil sample of known volume and then drying the sample in an oven to determine drying the sample in an oven to determine the dry weight fraction.the dry weight fraction.

Then the dry weight of the soil, Wb is Then the dry weight of the soil, Wb is divided by the known sample volume, V, to divided by the known sample volume, V, to determine bulk density, g b:determine bulk density, g b:

g b = Wb V (12) g b = Wb V (12)

Most methods developed for determining Most methods developed for determining bulk density use a metal cylinder sampler bulk density use a metal cylinder sampler that is driven into the soil at a desired that is driven into the soil at a desired depth in the profile.depth in the profile.

Bulk density varies considerably with Bulk density varies considerably with depth and over an irrigated field. depth and over an irrigated field.

Thus, it is generally necessary to repeat Thus, it is generally necessary to repeat the measurements in different places to the measurements in different places to develop reliable estimatesdevelop reliable estimates

Percent Depletion

Feel or Appearance of Soil

Loamy sands to fine sandy loams

Fine sandy loams to silt loams

Silt loams to clay loam

0 (field capacity)

no free water on ball* but wet outline on hand

same same

0-25 makes ball but breaks

easily and does not feel slick

makes tight ball, ribbons easily, slightly sticky and slick

easily ribbons slick feeling

25-50 balls with pressure but

easily breaks

pliable ball, not sticky or slick, ribbons and feels damp

pliable ball, ribbons easily slightly slick

50-75 will not ball, feels dry balls under pressure but

is powdery and easily breaks

slightly balls still pliable

75-100 dry, loose, flows

through fingers powdery, dry, crumbles

hard, baked, cracked, crust

v. v. Field capacityField capacity The most common method of determining The most common method of determining

field capacity in the laboratory uses a field capacity in the laboratory uses a pressure plate to apply a suction of -1/3 pressure plate to apply a suction of -1/3 atmosphere to a saturated soil sample. atmosphere to a saturated soil sample.

When water is no longer leaving the soil When water is no longer leaving the soil sample, the soil moisture in the sample is sample, the soil moisture in the sample is determined gravimetrically and equated to determined gravimetrically and equated to field capacity.field capacity.

A field technique for finding field capacity A field technique for finding field capacity involves irrigating a test plot until the soil involves irrigating a test plot until the soil profile is saturated to a depth of about one profile is saturated to a depth of about one metre. metre.

Then the plot is covered to prevent Then the plot is covered to prevent evaporation.evaporation.

The soil moisture is measured each 24 hours The soil moisture is measured each 24 hours until the changes are very small, at which until the changes are very small, at which point the soil moisture content is the point the soil moisture content is the estimate of field capacity.estimate of field capacity.

vi. vi. Permanent wilting pointPermanent wilting point Generally, at the permanent wilting point the Generally, at the permanent wilting point the

soil moisture coefficient is defined as the soil moisture coefficient is defined as the moisture content corresponding to a pressure moisture content corresponding to a pressure of -15 atmospheres from a pressure plate test.of -15 atmospheres from a pressure plate test.

Although actual wilting points can be Although actual wilting points can be somewhere between -10 and -20 atm, the soil somewhere between -10 and -20 atm, the soil moisture content varies little in this range.moisture content varies little in this range.

Thus, the -15 atm moisture content provides Thus, the -15 atm moisture content provides a reasonable estimate of the wilting point. a reasonable estimate of the wilting point.

Sandy Soils

Loamy/Silty Soils

Organic soil

Clay Soil

Objectives of evaluationObjectives of evaluation

The principal objective of evaluating surface The principal objective of evaluating surface irrigation systems is to identify management irrigation systems is to identify management practices and system configurations that can be practices and system configurations that can be feasibly and effectively implemented to improve feasibly and effectively implemented to improve the irrigation efficiency.the irrigation efficiency.

An evaluation may show that higher efficiencies An evaluation may show that higher efficiencies are possible by reducing the duration of the inflow are possible by reducing the duration of the inflow to an interval required to apply the depth that to an interval required to apply the depth that would refill the root zone soil moisture deficit.would refill the root zone soil moisture deficit.

The evaluation may also show opportunities The evaluation may also show opportunities for improving performance through for improving performance through changes in the field size and topography. changes in the field size and topography.

Evaluations are useful in a number of Evaluations are useful in a number of analyses and operations, particularly those analyses and operations, particularly those that are essential to improve management that are essential to improve management and control. and control.

The evaluation may also show opportunities The evaluation may also show opportunities for improving performance through changes for improving performance through changes in the field size and topography. in the field size and topography.

Evaluations are useful in a number of analyses Evaluations are useful in a number of analyses and operations, particularly those that are and operations, particularly those that are essential to improve management and control. essential to improve management and control.

Evaluation data can be collected periodically Evaluation data can be collected periodically from the system to refine management from the system to refine management practices and identify the changes in the practices and identify the changes in the field that occur over the irrigation season or field that occur over the irrigation season or from year to year. from year to year.

Evaluation the surface irrigation system is Evaluation the surface irrigation system is an important to optimize the use of water an important to optimize the use of water resources in this system.resources in this system.

A summary of the data arising from a field A summary of the data arising from a field evaluation is enumerated below. evaluation is enumerated below.

There are several publications describing the There are several publications describing the equipment and procedures for evaluating equipment and procedures for evaluating surface irrigation systems, but not all give a surface irrigation systems, but not all give a very correct methodology for interpreting the very correct methodology for interpreting the data once collected.data once collected.

The data analysis depends somewhat on the The data analysis depends somewhat on the data collected and the information to be data collected and the information to be derived. derived.

This section will deal with two aspects of an This section will deal with two aspects of an evaluation. evaluation.

The first is the definition of the typical field The first is the definition of the typical field infiltration relationship using the evaluation infiltration relationship using the evaluation data describing the surface flow. data describing the surface flow.

The second is the evaluation of the efficiency The second is the evaluation of the efficiency of the irrigation event studied. of the irrigation event studied.

Although many performance measures have Although many performance measures have been suggested, only four will be noted herein:been suggested, only four will be noted herein:

1) application efficiency1) application efficiency 2) storage efficiency2) storage efficiency 3) deep percolation ratio3) deep percolation ratio 4) runoff ratio. 4) runoff ratio. These will be defined here before detailing the These will be defined here before detailing the

analyses of infiltration and performance.analyses of infiltration and performance.