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Section 3 <br />Agricultural Water Use <br />Figure 3-3 <br />The amount of ET and flze relative percent of <br />consumptive use evaporation vs. transpiration (E <br />vs. T) are dependent upon the following factors: <br />^ Crop type <br />^ Percent canopy cover (stage of developnlent <br />and plant population) <br />^ Irrigation system and frequency of application <br />^ Residue cover e.g., nlulch/tillage system <br />^ Soil nloisture status <br />ET is a driver for crop growth and yield. There is a <br />direct relationsllip l~etween fl1e anlount of ET and <br />crop biomass because plant stonlata nlust be open <br />for a crop plant to assimilate carbon. When plant <br />Relative Components of Crop Evapotranspiration (ET) stontata are open, water vapor is lost to the <br />atnlosphere. In this way, 99 percent of the water <br />that is taken up by the plant is returned to the atnlosphere in the fornl of water vapor <br />(Figure 3-3). <br />The ratio of evaporation to transpiration cllanges througllout the year. An illustration <br />of the changes in monthly E vs. T for a corn field is shown in Figure 3-4. In May and <br />June, as the corn is in the early growth stages, a greater percentage of ET is <br />attributable to evaporation and as fl1e corn increases in bionlass and closes t11e <br />canopy, evaporation as a percent of ET declines due to shading of the ground and the <br />increase ii1 bionlass and hence transpiration fronl fl1e plant. <br />Evaporation Decreases and Transpiration Increases <br />Figure 3-4 <br />Corn Crop and ET Changes During Growing Season <br />DRAFT 3-3 <br />