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• Board Meeting Date: 3/18/2008
• Description: IWMD Section - Presentation of Agricultural Water Conservation Paper
• Board Meetings - Doc Type: Memo

Section 3
Agricultural Water Use
E = 20-30%
The amount of ET and the relative percent of
consumptive use evaporation vs. transpiration (E
vs. T) are dependent upon the following factors:
~ ~ ~ ~ ^ Crop type
T = 70-80% ^ Percent canopy cover (stage of development
~' /~~ and plant population}
^ Irrigation system and frequency of application
^ Residue cover e.g., mulch/ tillage system
^ Soil moisture status
Figure 3-3
ET is a driver for crop growth and yield. There is a
direct relationship between the amount of ET and
crop biomass because plant stomata must he open
for a crop plant to assimilate carbon. When plant
Relative Components of Crop Evapotranspiration (ET) stomata are open, water vapor is lost to the
atmosphere. In this way, 99 percent of the water
that is taken up by the plant is returned to the atmosphere in the form of water vapor
(Figure 3-3).
The ratio of evaporation to transpiration changes throughout the year. An illustration
of the changes in monthly E vs. T for a corn field is shown in Figure 3-4. In May and
June, as the corn is in the early growth stages, a greater percentage of ET is
attributable to evaporation and as the corn increases in biomass and closes the
canopy, evaporation as a percent of ET declines due to shading of the ground and the
increase in biomass and hence transpiration from the plant.
Evaporation Decreases and Transpiration Increases
Figure 3-4
Corn Crop and ET Changes During Growing Season
DRAFT 3-3