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Approach -Acreage Subset Filling <br />Two different methods were considered for filling acreage subsets between GIS coverages. Two <br />methods were also investigated for estimating well capacity over the SPDSS study period. <br />Representative diversion structures with irrigated land receiving both surface and ground water <br />use were chosen to evaluate both methods -Fort Morgan Canal, Riverside Canal, and Greeley- <br />Loveland Irrigation Company Canal (GLIC). One aggregated ground water grouping <br />representing Sedgwick County was also evaluated. These structures were chosen for evaluation <br />because of their varying levels of ground water and sprinkler acreage. <br />The acreage filling methods were based on the following premises: <br />^ Total irrigated acreage assigned to specific surface water ditch systems in the GIS <br />coverages tends to be relatively consistent over time, even as wells were added. In other <br />words, wells were added as supplemental ground water, not to increase acreage. <br />^ Conversely, lands served only by ground water generally were not irrigated prior to well <br />development, therefore as new sole-source wells were added, irrigated acreage increased. <br />As a consequence, the methods investigated to fill ground water and sprinkler acreage data gaps <br />were different for ditch systems and lands irrigated only by ground water. In both cases, the <br />filling occurs at the parcel level, then is summed and reported by structure or aggregation. <br />Acreage Filling A <br />The first approach, Acreage Filling A, uses ground water well decreed dates as a basis to <br />estimate acreage between coverages. <br />Lands Irrigated with Surface Water Only or Supplemented with Ground Water <br />Through interviews with water commissioners and users, it was determined that ditches in the <br />South Platte generally operate as mutual ditches. Based on review of the GIS coverages and <br />interviews with water users, it is generally believed that under mutual ditch systems, wells were <br />added to supplement surface water supplies, not to increase acreage. In addition, sprinklers were <br />added on previously flood irrigated lands to improve the use of supplies, not to increase acreage. <br />Based on this information, the procedure used for filling in the ground water and sprinkler <br />acreage subsets under lands irrigated with surface water only or supplemented with ground water <br />recognizes that total acreage does not change between GIS coverages as follows: <br />- As acreage receiving supplemental ground water increases between GIS coverages, <br />acreage receiving only surface water decreases by the same amount. <br />- As sprinkler acreage increases between coverages, flood acreage decreases by the same <br />amount. <br />The following serves as an example of how the acreage subsets under a specific ditch would <br />change for the year 1977, based on both the 1976 and 1987 GIS acreage, and well information: <br />1977 GF = 1976 total GF + 1987 GF parcels assigned to wells permitted in 1977 <br />