Crops to be IrrigatedCrops to be Irrigated

Factors for consideration– Rooting depth of crop ( Irr. Guide pp.. 3-8,3-9)

Are there any soil barriers rock, hard pan, etc. establishment and shallow root depths require more

frequent irrigations

– Crop Height wheel or riser height above canopy or within canopy application

– Water tolerance water sensitive crops develop diseases (beans)

What is Management What is Management Allowable Depletion (MAD)?Allowable Depletion (MAD)?

Mad is defined as the percentage of the available soil water that can be depleted between irrigations without serious plant moisture stress. MAD is expressed as:– a percentage of the total Available Water Content

(AWC) the soil will hold in the root zone– a soil-water deficit (SWD) in inches, or– an allowable soil-water tension level

More information Irrigation Guide pg.. 3-7

ExampleExample

Given: Silt Loam soil (AWC= 2.1”/ft), growing potatoes with rooting depth of 2.5’and a MAD of 35%.

Find : The maximum amount of water depletion before irrigation is necessary?

2.1*2.5*.35 = 1.84”

Evapotranspiration OverviewEvapotranspiration Overview

Recently with more weather station and a greater demand for the available water there has been a great push to update ET values. The new ET values are needed to provide a more accurate picture of what is actually happening in the field.

What is What is EVAPOTRANSPIRATION?EVAPOTRANSPIRATION?

Definition– Evaporation of water from the soil and plant

surfaces and transpiration from the stomatal cavities of plants

What are some methods for What are some methods for determining ET?determining ET?

Estimated crop evapotranspiration ETc

– Blaney-Criddle, etc. Direct measurement

– aerodynamic method– detailed soil moisture monitoring– lysimetry– plant porometers– regional inflow-outflow measurements

What influences the method What influences the method you would select?you would select?

Type, accuracy, and duration of available climatic data

Natural pattern of evapotranspiration Intended use of the evapotranspiration

estimates

Climatic DataClimatic Data

Type– Temperature, radiation, wind, humidity

Quality Length of Record

Natural Pattern of Crop Water UseNatural Pattern of Crop Water Use

Crop ET varies from day to day – Fluctuating climatic– Plant growth

Daily average Vs. average for a period– 1 day Vs 5 day

Frequency DistributionsFrequency Distributions

ET for an Averaging PeriodET for an Averaging Period

Intended UseIntended Use

Irrigation Scheduling System Design Reservoir operation

Various MethodsVarious Methods

NRCS endorses four methods– Penman-Monteith– Radiation method– Temperature Method– Class A evaporation pan

Penman-Montieth MethodPenman-Montieth Method

ET R GK

BP

e e

rnz z

a0

101

622

( )[(

*)( ) (

*)(. )

( )]

•More reliable for any length period– daily, monthly, or seasonal

•If adequate data available

ET0 = The evapotranspiration for grassreference crop = heat of vaporizationRn = net radiationG = soil heat flux = slope of the vapor pressure curve = psychrometric constant = density of airBP = mean barometric pressuree0

z = average saturated vapor pressureez = actual vapor pressure* = (1+rc/ra)rc = surface resistance to vapor transportra = aerodynamic resistance to sensible heat andvapor transportK1 = the dimension coefficient

Radiation MethodRadiation Method

ET bR

rs

0 0 012

. ( )

ET0 = The evapotranspiration for grassreference crop = heat of vaporization = slope of the vapor pressure curve = psychrometric constant = density of airbr = adjustment factor for wind and humidityRs = incoming solar radiation

Not for daily ET, but for average daily ET over a period of days ~ 5day periodGood for monthly and Seasonal

Temperature MethodTemperature Method

ET C a b pTe t t0 ( )

Not for daily ET, but for average daily ET over a period of days ~ 5day periodGood for monthly and Seasonal

ET0 = evapotranspiration for grass reference cropCe = elevation adjustment factorat = climate adjustment factorbt = climate adjustment factorp = mean daily percent of annual daytime hoursT = mean air temperature

Evaporation Pan MethodEvaporation Pan Method

ET0=kpEpan

ET0 = evapotranspiration for grass reference cropkp = pan coefficientEpan = evaporation from pan

Good for monthly and Seasonal

Things to Consider before Things to Consider before changing ET Valueschanging ET Values

Legal ramifications Quantity of water supply System efficiency

SCS TR21SCS TR21

Where does TR21 fit in? Water Rights of many States Based on TR21 Less Accurate

What is the difference between What is the difference between ETET00 and Consumptive Use? and Consumptive Use?

CU = Crop coefficient*ET0

Use reference ET for specific Crop

Crop CurvesCrop Curves

NRCS has switched from an Alfalfa based crop reference to a Grass crop reference

To convert use a multiplier factor , usually 1.15

New ones and procedures found in NEH part 623 chapter 2

Consumptive Use Consumptive Use CalculationsCalculations

Field by field– CU = crop coefficient * ET

Farm CU - multiple fields, multiple crops– weighted CU based on percentage of crops

Project CU– gpm/acre weighted by percent

Weighted Consumptive UseWeighted Consumptive Use

CROP ACRES CU %

alfalfa 80 .31 35

wheat 100 .21 43

beets 50 .25 22

Total 230 .25 100

CUw= 80/230*.31+100/230*.21+50/230*.25

Net Irrigation RequirementsNet Irrigation Requirements

Fn = ETc + Aw - Pe - GW -SW

Fn = net irrigation requirement for season ETc = crop evapotranspiration Aw = auxiliary water - leaching, temperature modification, crop quality Pe = effective precipitation GW = ground water contribution SW= soil water depleted during season

Effective precipitationEffective precipitation

The part of rainfall that can be used to meet the evapotranspiration of growing crops.

Does not include surface runoff or percolation below the crop root zone

Precip ita tion Pathw ays

E va p o ra te d ina tm o sp h e re

U se le ss o rh a rm fu l fo r

c ro p g ro w th

R e m a in su n u se d

U tilize d fo rc ro p g ro w th

R e ta in e d b y so ilin ro o t zo n e

E sse n tia lfo r le a ch in g

R e tu rn flo w toground water or

s tre a m s

N o t n e e d e dfo r le a ch in g

D eep perco la tionb e lo w ro o t zo n e

In filtra te s

S trike s so ilsu rfa ce

R e ta in e d a n de va p o ra te d

E va p o ra te s o nso il su rfa ce

R e u se de lse w h e re

R e tu rn flo wto stream s orground water

N o t re u se da t s ite

R e u se da t s ite

S u rfa ce ru n o ff

D ra in e d toso il su rfa ce

In te rce p te d b yve g e ta tio n

P re c ip ita tio nfro m c lo u d s

System SizingSystem Sizing Simply put

Q = Fg*A/t

Q = to system flowrate

Fg = Gross irrigation requirement

A = irrigated area

t = time to irrigate the field

What is the difference What is the difference between Net and Gross?between Net and Gross?

Cg = Cn Ea(1-Dt ) 100

Cg = gross system capacityCn = net system capacityEa = application efficiencyDt = system downtime

Many efficiencies come in to play Field efficiencyFarm efficiencyConveyance efficiencyProject efficiency etc. Things influencing Field efficiency

Deep percolationSurface runoffSpray, drift losses

Where to get more informationWhere to get more information

NRCS NEH 623 Chapter 2 “Irrigation Water Requirements”