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CRDSS <br />TASK MEMORANDUM 1.14-19 <br />Consumptive Use Model <br />Improved Estimation of Irrigated Acreage in the Gunnison River Basin <br />1.0 ISSUE <br />This task memorandum documents the methods used to improve the estimation of crop-type acreage and <br />distribution by county for all irrigated crops in the Gunnison River basin. The method relies heavily on <br />data generated using irrigated acreage maps for 1993 and the Colorado Agricultural Statistics (CAS) for <br />the years 1985 through 1990 and 1993. <br />2.0 DISCUSSION/ANALYSIS <br />Introduction <br />There are many input data requirements for the CRDSS consumptive use (CU) model to accurately <br />estimate the consumptive water use by crops. Two of the most important components of these input <br />data are the irrigated crop types and acreage distribution in the counties and their relationship to <br />hydrologic units in the Gunnison River basin. The U.S. Bureau of Reclamation?s (USBR?s) Methods <br />Manual mentions the importance of this by stating that "The greatest error in calculating irrigation <br />consumptive use is determining the irrigated acreage for each hydrologic unit." The general procedure <br />for calculating irrigated acreage for each hydrologic unit involves subdividing the county irrigated <br />acreage, which is published each year in the CAS. <br />There is no actual field verification of data presented as irrigated acreage in the CAS. Also, there is no <br />generalized approach showing how to subdivide the total county acreage per hydrologic unit. A solution <br />that was discussed with the USBR is the use of the irrigated acreage maps stored in geographic <br />information systems (GIS). Using GIS, the crop type and area of irrigated acreage in the intersection of <br />each hydrologic unit and county can be determined. <br />The following sections present different approaches that were evaluated as part of the CRDSS project. <br />All approaches use data from the CAS and irrigated acreage maps for 1993 that were generated using <br />GIS. The work focused on the years 1985 through 1990. <br />The Method Used by the USBR to Subdivide the CAS Data <br />As mentioned earlier, the CAS provides a yearly tabulated county irrigated acreage that needs to be <br />subdivided into the hydrologic unit acreage per county. <br />In preparing the Colorado River System Consumptive Uses and Losses Report, 1985-1990 , the USBR <br />used CAS data to determine the irrigated acreage in each hydrologic unit per county. <br />The Integrated Decision Support (IDS) Group at Colorado State University (CSU) reviewed the USBR <br />and the CAS data files for the years 1985 to 1990 to determine if there is a observable approach that has <br />1 <br />A 275 01.09.95 1.14-19 Al-azzawe <br />