Quantity and quality of the available water supply must be considered. The objective of irrigation management is to enhance growing conditions by providing the water needed to optimize plant growth and yield. Water use characteristics Curve A in Figure 1 illustrates an average daily water use pattern for corn. Curve B illustrates the daily fluctuation possible in actual ET values. Evapotranspiration, or crop water use, is the water removed from the soil by evaporation from the soil surface and transpiration by the plant.
Transpiration includes water evaporated from leaf and other plant surfaces.
Transpiration is the last step in a continuous water pathway from the soil, into the plant roots, through the plant stems and leaves and into the atmosphere. High air temperatures, low humidity, clear skies and high wind exert a large evapotranspiration demand. Seasonal water use is also affected by growth stage, length of growing season, soil fertility, water availability and the interaction of these factors. Although the total amount of water used by a crop will vary from season to season and location to location, it will generally follow the pattern shown by Curve A in Figure 1 . In Nebraska, total crop water use for corn ranges from 21 inches per year in the west to 28 inches in the east.