A network of automated weather stations is being installed in the Pacific Northwest to make this information available via computer. An example of crop water use over a season for alfalfa being grown at an elevation of about 3,000 feet is shown in Figure 2 . Note that water use is less during the cooler parts of the year and peaks in midsummer. Other crops have water use curves with different shapes. An irrigation system usually is designed to deliver a steady flow of water to an irrigated field at a rate sufficient to meet peak irrigation requirements.
Irrigation scheduling is a management tool that can help avoid such overirrigating.
Some water may be lost to deep percolation, runoff, wind drift, and evaporation. An estimate or measurement of the efficiency of application of the irrigation system is needed to determine the net application. Table 1 lists representative application efficiency factors to be used in calculating net irrigation. Multiply gross application by these factors to find net application to be used in scheduling. Make every effort to assure the most uniform irrigation possible.