Crops 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
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
Example
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 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
EVAPOTRANSPIRATION?

Definition
– Evaporation of water from the soil and plant
surfaces and transpiration from the stomatal
cavities of plants
What are some methods for
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
you would select?
Type, accuracy, and duration of available
climatic data
 Natural pattern of evapotranspiration
 Intended use of the evapotranspiration
estimates

Climatic Data

Type
– Temperature, radiation, wind, humidity
Quality
 Length of Record

Natural 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 Distributions
ET for an Averaging Period
Intended Use
Irrigation Scheduling
 System Design
 Reservoir operation

Various Methods

NRCS endorses four methods
–
–
–
–
Penman-Monteith
Radiation method
Temperature Method
Class A evaporation pan
Penman-Montieth Method
1


K1
ET0  ( )[(
)(Rn  G)  (
)(.622
)
   *
  *
BP
•More reliable for any length period
– daily, monthly, or seasonal
•If adequate data available
0
(ez  ez )
ra
]
ET0 = The evapotranspiration for grass
reference crop
 = heat of vaporization
Rn = net radiation
G = soil heat flux
 = slope of the vapor pressure curve
 = psychrometric constant
 = density of air
BP = mean barometric pressure
0
e z = average saturated vapor pressure
ez = actual vapor pressure
*
 = (1+rc/ra)
rc = surface resistance to vapor transport
ra = aerodynamic resistance to sensible heat and
vapor transport
K1 = the dimension coefficient
Radiation Method

Rs
ET0  0.012  (
)br
 

ET0 = The evapotranspiration for grass
reference crop
 = heat of vaporization
 = slope of the vapor pressure curve
 = psychrometric constant
 = density of air
br = adjustment factor for wind and humidity
Rs = incoming solar radiation
Not for daily ET, but for average daily ET over a
period of days ~ 5day period
Good for monthly and Seasonal
Temperature Method
ET0  Ce ( at  bt pT )
ET0 = evapotranspiration for grass reference crop
Ce = elevation adjustment factor
at = climate adjustment factor
bt = climate adjustment factor
p = mean daily percent of annual daytime hours
T = mean air temperature
Not for daily ET, but for average daily ET over a
period of days ~ 5day period
Good for monthly and Seasonal
Evaporation Pan Method
ET0=kpEpan
ET0 = evapotranspiration for grass reference crop
kp = pan coefficient
Epan = evaporation from pan
Good for monthly and Seasonal
Things to Consider before
changing ET Values
Legal ramifications
 Quantity of water supply
 System efficiency

SCS TR21
Where does TR21 fit in?
 Water Rights of many States Based on TR21
 Less Accurate

What is the difference between
ET0 and Consumptive Use?
CU = Crop coefficient*ET0
 Use reference ET for specific Crop

Crop 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
Calculations

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 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 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 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

Precipitation Pathways
Precipitation
from clouds
Evaporated in
atmosphere
Strikes soil
surface
Intercepted by
vegetation
Infiltrates
Retained by soil
in root zone
Useless or
harmful for
crop growth
Remains
unused
Retained and
evaporated
Evaporates on
soil surface
Deep percolation
below root zone
Utilized for
crop growth
Essential
for leaching
Return flow to
ground water or
streams
Drained to
soil surface
Surface runoff
Not needed
for leaching
Not reused
at site
Reused
elsewhere
Return flow
to streams or
ground water
Reused
at site
System 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
between Net and Gross?
Cg =
Cn
Ea(1-Dt )
100
Cg = gross system capacity
Cn = net system capacity
Ea = application efficiency
Dt = system downtime
Many efficiencies come in to play
Field efficiency
Farm efficiency
Conveyance efficiency
Project efficiency etc.
Things influencing Field efficiency
Deep percolation
Surface runoff
Spray, drift losses
Where to get more information

NRCS NEH 623 Chapter 2 “Irrigation
Water Requirements”