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3.0 APPROACH <br />3.1 Ditch Service Area Coverage <br />The first major GIS product developed as part of this study was the ditch service area coverage <br />for the ground water model area of the Rio Grande Basin (figure 3). This theme describes which land <br />is served by a ditch and enables the assignment of surface water sources to irrigated parcels. The <br />service area boundaries represent the area of land that the ditch has the physical ability or legal right <br />to irrigate. The entire service area of each ditch system is not necessarily indicative of the amount of <br />lands that were irrigated during the 1998 season because ditch service areas typically include fallow <br />fields, farm houses, and non irrigated corners. <br />3.1.1 Data Acquisition <br />Initially, a meeting was held with all of the water commissioners to assess data availability and <br />obtain information. Available ditch service area maps were acquired and digitized into ArcView. <br />These maps include water commissioner maps, maps previously prepared for the state=s AWDI case <br />within the San Luis Valley ground water model area, and maps of several major ditch systems <br />prepared by the Rio Grande Water Conservation District. Some of these service areas were found to <br />be erroneous and were later corrected during the interview process. The service areas for many <br />important structures were not delineated from these initial sources, thus warranting further research. <br />3.1.2 Boundary Delineation <br />To refine the service areas described in Section 3.1.1 and add service area coverages for unmapped <br />ditches, thirty-five personal interviews were conducted with each district water commissioner, many <br />ditch company representatives, and knowledgeable water users. Each ditch service area was hand <br />digitized during these personal interviews to ensure inclusion and accuracy. Appendix A details the <br />individuals who provided information for each water district. An emphasis was placed upon the <br />identification of ditch service areas within the ground water model area, although ditches outside <br />this area with easily available information were also mapped. Structures were mapped, regardless of <br />their importance as related to the surface water model, for all ditches for which information was <br />available. Each ditch coverage is meant to represent the service area for which ditch water can be <br />legally and physically used rather than the areas actually irrigated by the ditch during a particular <br />year. A ditch service area varies from the amount of land actually irrigated by the ditch during a <br />particular year based on available water supplies and land-owner intentions. Approximately 155 <br />new ditch service areas were added through this process resulting in a total of 633 structures <br />mapped. <br />There are some lands where more than one structure serves the same land. These fell into two <br />categories: 1) Where surface water is commingled from several sources and used to irrigate fields <br />7 <br />