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<br />. OuiH9 <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />",. <br /> <br />. <br /> <br />6 inches, the factor is 0.93 and 1.06, respectively.) A 6-inch SMD is realistic for alfalfa, <br />short alfalfa, short pasture and orchards. The SMD at the time of irrigation on small <br />vegetables and beans is probably less than 3 inches because of their smaller root system <br />and their sensitivity to stress. A 3-inch SMD at the time of irrigation is probably only <br />typical for corn. With a review of soil surveys and using typical rooting depth (Jensen et <br />al.), better estimates of net depth of application can be made if the SCS TR-21 <br />methodology is used. A better alternative is local input from farmers, ranchers, ditch <br />company officials and county agents, who could provide a basis for estimating how often <br />crops are irrigated (frequency); then knowing the frequency and the crop ET, better <br />estimates of net depth of application could be made. <br /> <br />,} <br />"- <br />.'-. <br />.~ <br />~ <br />~. <br />,~ <br />~ <br /> <br />The effective precipitation used in SCS TR-21 was based upon a study by <br />Hershfield. Hershfield used a daily soil moisture budget at 22 U. S. Weather Bureau <br />stations throughout the United States. Only 8 of the 22 sites were in arid or semi-arid <br />climates, the remaining 14 were humid sites or sites of high rainfall. Therefore, the data <br />base used was biased to underestimate the effectiveness of precipitation because the <br />precipitation in humid sites is greater in quantity and more frequent. Furthermore, in his ' <br />determination of the methodology, Hershfield carried excess soil moisture over from one <br />month into another. The TR-21 procedure does not allow this. If crop ET is less than the <br />calculated effective precipitation, the excess precipitation is lost. Although, the amount <br />of precipitation lost in the calculation oversight is probably relatively small, the procedure <br />could be improved by including carryover into the next month. <br /> <br />ImDrove Weather Station to IrriGated Area Match. The Technical Appendix provides <br />a table which lists for each hydrologic unit the weather station used to calculate ET, the <br />weather station elevation and the average crop elevation. It is not reasonable to expect <br />a perfect match between the weather station and the crop location; however, a <br />comparison of the weather station elevation to the crop elevation found significant <br />differences that should be corrected by using the lapse rate. (Using the average May <br />through October monthly temperatures, it was found that the in the Colorado River basin <br />the lapse rate is about 4 degrees Fahrenheit per 1,000 feet.) Eleven p,;rcent (11 %) of the <br /> <br />13 <br />