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”