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Other Data <br />Groundwater and irrigated acreage data are other data types that will be important for RGDSS <br />modeling efforts. The availability of these data types is discussed in detail in the feasibility study <br />To summarize the findings presented in that report: <br />Groundwater -The State Engineers Office maintains a well permit database and a well water <br />rights database. There is useful information in these databases regarding well characteristics, but <br />the quality of the information is inconsistent. Well pumping data "is inconsistent and generally <br />unavailable" except for records from the Closed Basin Project. Because groundwater data are so <br />sparse, they have little impact on the study period selection. <br />Irrigated acreage -The State and the RGWCD have compiled irrigated acreage and cropping data <br />from the 1980s to the present. According to Agro Engineering, irrigated acreage and cropping <br />data can be derived from aerial photographs that are available back to the 1950s, though <br />photographs were taken only sporadically until the 1980s. <br />Climate Considerations <br />It is important to consider climate when selecting a study period because both the supply and <br />demand for water are in large part determined by climate. Important planning decisions may be <br />based on results from the RGDSS modeling studies. If those results are based on a study period <br />that includes only average and wet years, it is likely that the results will paint a rosier picture of <br />water supply and demand than can reasonably be expected in the future. <br />The principal climate parameters influencing water supply and demand are precipitation and <br />temperature. An index that combines these parameters is the Palmer Drought Severity Index, or <br />PDSI. In 1965, Palmer developed a standardized method for describing moisture conditions that <br />uses precipitation and temperature data and soil moisture estimates. Complete descriptions of the <br />equations used in the PDSI can be found in the original study by Palmer (1965) and in a more <br />recent analysis by Alley (1984). <br />An advantage of the Palmer index is its independence from the influences of reservoir storage, <br />irrigation diversions or return flows, unlike an index based on streamflows. The Palmer index is <br />"the most commonly used drought index in the United States." (National Drought Mitigation <br />Center) <br />The Palmer index ranges from about 6.0 to -6.0. The classifications for each index value are <br />shown in Table 6. <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 10 of 18 <br />