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<br />. <br /> <br />0\31!~42 <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />< <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />published, better analytical tools and data for more precise estimates of water require- <br />ments have been developed. They reported that in a soon to be released Chapter 2, <br />Irrioation Water Reauirements of the USDA-SCS National Engineering Handbook, <br />Section 15, Irriaation the Penman Monteith, FAO Radiation and FAO Blaney-Criddle are the <br />ET methods to be used. Which method the SCS utilizes will be a function of the available <br />weather data. Martin et al. reports that SCS-BC is retained as an appendix in the new <br />Chapter 2 because it has been the basis for adjudicated water rights and may be needed <br />for comparison in future water right deliberations. <br /> <br />Several crop water models (Hill, 1992; Knisel, 1980; Saxton, 1989) developed in <br />recent years are available. Most of these models calculate ET using an alfalfa or grass <br />reference crop evapotranspiration times a crop coefficient. <br /> <br />Etp = Kc * (Etr or Eto) <br />where <br />Etr = Alfalfa reference crop evapotranspiration <br />Eto = , Grass reference crop evapotranspiration <br />Kc = Crop coefficient <br /> = Kcb Ka + Ks <br />Kcb - Basal crop coefficient <br />Ka = Coefficient dependent on available soil water <br /> . <br />Ks = Soil evaporation coefficient for increased evaporation from wet soil <br /> immediately after rain or irrigation <br /> <br />Part of CRDSS Task 1. 14a was to acquire current ET models. Monte McVey in the <br />USBR Boise office reported his office is calculating ET using a 1982 Kimberly Penman, an <br />Etr equation. Buchleiter et al. developed a compU1erized irrigation scheduling program <br />(SCHED) that use either a 1972 Penman, or Jensen-Haise equation to estimate alfalfa <br />reference crop evapotranspiration. In the report on the Animas-La Plata Project (USDA, <br />1979), the USBR reports using a computer program, ETM070, to calculate consumptive <br /> <br />6 <br />