1. Irrigation Water Requirement

2. Evapotranspiration Terminology Evaporation Transpiration
Process of water movement, in the vapor form, into the atmosphere from soil, water, or plant surfaces
Transpiration
Evaporation of water from plant stomata into the atmosphere
Evapotranspiration
Sum of evaporation and transpiration (abbreviated “ET”)
Consumptive use
Sum of ET and the water taken up the plant and retained in the plant tissue (magnitude approximately equal to ET, and often used interchangeably)

3. Magnitude of ET
Generally tenths of an inch per day, or tens of inches per growing season
Varies with type of plant, growth stage, weather, soil water content, etc.
Transpiration ratio
Ratio of the mass of water transpired to the mass of plant dry matter produced (g H2O/g dry matter)
Typical values: 250 for sorghum for wheat for alfalfa

4. Plant Water Use Patterns
Daily Water Use: peaks late in afternoon; very little water use at night
Alfalfa: Ft. Cobb, OK
June 26, 1986

5. Plant Water Use Patterns
Seasonal Use Pattern: Peak period affects design
Corn Water Use Pattern
Irrigation system must be able to meet peak water use rate or the crop may be lost.

6. Evaporation Rate and Time Since Irrigation Energy or Water Availability as the Limiting Factor in ET Rate

7. Evapotranspiration Modeling
Estimation based on:
climate
crop
soil factors
ETc = Kc ETo
ETc = actual crop evapotranspiration rate
ETo = the evapotranspiration rate for a reference crop
Kc = the crop coefficient

8. Evapotranspiration Modeling
Reference Crop ET (ETo)
ET rate of actively growing, well-watered, “reference” crop
Grass or alfalfa used as the reference crop (alfalfa is higher)
A measure of the amount of energy available for ET
Many weather-based methods available for estimating ETo
(FAO Blaney-Criddle; Jensen-Haise; Modified Penman; Penman-Montieth)
Crop Coefficient (Kc)
Empirical coefficient which incorporates type of crop & stage of growth (Kcb); and soil water status-- a dry soil (Ka) can limit ET; a wet soil surface (Ks) can increase soil evaporation
Kc = (Kcb x Ka) + Ks
Kc values generally less than 1.0, but not always

9. One way to track real-time water use of your lawn and garden is to use the evapotranspiration model on the Mesonet Agweather page. Agweather is a free site with lots of management information for farmers and homeowners. Agweather is organized according to interest areas, such as Weather, Livestock, Crops, and Horticulture. Clicking on Horticulture . . .

10. . . . takes you to a page where you can select among Fruit & Nut, Ornamental, Turf and Vegetable options. Selecting Turf takes you to several more options, including an OSU Pest Diagnostics link, and one titled Evapotranspiration.

11. Selecting the Mesonet station nearest your location can be done either from the alphabetized, drop-down menu of stations or from the map. The dots representing the station locations on the map are active and will select the station if you click on them. Select your type of turf, either warm-season (Bermuda) or cool-season (fescue, ryegrass, etc.). Estimate the date that began actively growing (probably around mid-March for Bermuda grass). Then click “Get Turf Grass Data”.

12. Using the ET Table to Schedule Lawn Irrigation
This brings us a table which shows the daily estimate of ET for the type of turf selected, showing the most recent days at the top of the table. The “ET” column is the water use for the day shown. The “ET_ACC” column show the cumulative ET for all days from the day at the top of the table (yesterday) going back successive days. For the example shown, yesterday’s ET was 0.19 inch of water, and the cumulative ET is also 0.19 inch. Going back to September 21 (day before yesterday) the ET is 0.20 inch and the cumulative ET is 0.19 (for yesterday) plus 0.20 (for day before yesterday) which equals 0.39 inch. The value of this information is– suppose it is the morning of September 23 and you last irrigated your lawn on September 17. Looking down the table you see the estimated cumulative water use from September 17 through yesterday is 1.23 inch. If you have turf rooted 12 inches deep in a Vanoss silt loam with a total of 1.20 inches of available water stored in the root zone after it is well irrigated– you need to water today. The water balance is meant to show accumulated water use, less any rainfall received, which should be the amount of irrigation required. However, in Oklahoma, if you are more than a few hundred yards away from a rain gauge the measured rainfall is of little value in judging your water balance. I would urge you to have one or more rain gauges on your property and use your local rainfall measurements rather that the Mesonet rainfall in any water balance irrigation scheduling work.

13. Efficiencies and Uniformities
Application efficiency (Ea)
dn = net irrigation depth
dg = gross irrigation depth
fraction or percentage
Water losses
Evaporation
Drift
Runoff
Deep percolation

14. Water Losses

15. Application Uniformity
Distribution uniformity (DU)
dLQ = average low-quarter depth of water received
dz = average depth applied
Popular parameter for surface irrigation systems in particular

16. Application Uniformity Cont’d…
Christiansen’s Coefficient of Uniformity (CU)
n = number of observations (each representing the same size area)
dz = average depth for all observations
di = depth for observation i
Popular parameter for sprinkler and microirrigation systems in particular
For relatively high uniformities (CU > 70%), Eq. 5.4 and 5.5 relate CU to DU

17. Turf Sprinkler Uniformity Test
(catch cans placed on a 5 ft x 5 ft grid)

20. Adequacy
Because of nonuniformity, there is a tradeoff between excessive deep percolation and plant water stress
Adequacy: the percent of the irrigated area that receives the desired depth of water or more
Figure 5.3
Plotting the percentage of area in the field that receives a given depth of irrigation water or more gives a distribution uniformity curve
Irrigating for a longer or shorter time moves the curve up or down
System modifications may be required to change the shape of the curve

21. Figure 5.3a
SWD

22. Fig 5.3b
SWD

23. Figure 5.3c
SWD

24. Figure 5.3d
SWD

25. Same adequacy but different uniformities and Ea’s

26. Same uniformity but different adequacies and Ea’s

27. Conveyance Losses

28. Application Efficiency of The Low Quarter (AELQ)
Ratio of the average low-quarter depth of water that infiltrates and is stored in the crop root zone relative to the average depth of water applied (x 100 for %)
AELQ = DU when all applied water infiltrates
Also AELH (low-half)
Accurate rules of thumb
for 90% adequacy, apply a gross depth = (desired net depth)/AELQ (acceptable for higher-valued crops)
For 80% adequacy, apply a gross depth = (desired net depth)/AELH (acceptable for lower-valued crops)

29. System Capacity Net system capacity (Qn) Peak ET method:
Function of plant needs (keep soil water balance above some specified level)
The rate at which water must be stored in the root zone
Peak ET method:
Provide enough capacity to meet peak ET over a given time period
Less conservative method:
Recognize that rainfall and/or soil water can allow a reduced capacity
Water stored in the soil can provide a buffer over short time periods
Also, over longer time periods, concept of an allowable depletion (AD) -- amount of water that can be depleted from the soil before plant stress occurs

30. System Capacity Gross system capacity (Qg)
The rate at which water must be supplied by the water source
A function of:
the net system capacity, Qn
the efficiency of the irrigation system
the system downtime

31. System Capacity Definition
Required system capacity is the water supply rate that must be provided to prevent plant water stress (may or may not = actual system capacity)
Units could be inches per day or gpm per acre or gpm over a given area (Qn & Qg must be in consistent units)
Qg = gross system capacity, in/day or gpm/A
Qn = net system capacity, in/day or gpm/A
AELQ = application efficiency of low quarter, (%)
Dt = irrigation system downtime (%)

32. Operational Terminology
Set or zone:
Smallest portion of the total area that can be irrigated separately
Application time :
Length of time that water is applied to a set/zone
Set time :
Time between starting successive sets in a field
Application time = set time if system is not stopped to change sets (automated vs. manual systems

33. Operational Terminology
Cycle time or irrigation interval:
Length of time between successive irrigations
Idle time:
Time during the irrigation interval that the system is not operated
Duration:
Time that water is provided to the farm by an irrigation district
Rotation:
Time between times when the water is provided by the district