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acreage is 1976 -parcels prior to that time have been assigned as flood. To reflect the change in <br />technology, SS and GS acreage is set to zero prior to 1969. <br />1955 GF = 1956 total GF - 1956 GF parcels assigned to wells permitted in 1956 <br />Acreage subsets for 1954 would follow the same algorithm, starting with the acreage subsets <br />determined for 1955, and so on back to 1950. <br />Ground water only acreage under each subset for years after 2005 was estimated to be the same <br />as in the 2005 coverage. <br />Acreage Filling B <br />The second approach, Acreage Filling B, was simplified to recognize that there is minimal <br />information available to infer acreage subset changes between GIS coverages. Therefore, a linear <br />interpolation approach was used to connect the known data points. <br />This method to fill ground water and sprinkler acreage data gaps considers the same filling <br />method for surface water structures and lands irrigated only by ground water with a minor <br />difference only for filling 1950 through 1955. <br />Again, to reflect the earliest use of sprinkler in the basin, SS and GS acreage is set to zero prior <br />to 1969. <br />1. Linear interpolation was used between GIS coverages for total acreage and acreage <br />subset data for both surface water structures and ground water only structures. <br />2. Fill forward for total acreage and acreage subset data points is recommended for both <br />surface water structures and ground water only structures when data is required <br />beyond the most recent GIS coverage. <br />3. For structures with a surface water supply (and potentially supplemental ground <br />water) a procedure similar to Acreage Filling A was used to create a 1950 data point. <br />Total acreage from the 1956 snapshot was held constant and acreage subsets were <br />allowed to adjust depending on well permit dates, as demonstrated in the following <br />example: <br />1950 GF = 1956 total GF - 1956 GF parcels assigned to wells permitted from <br />1951 through 1956 <br />1950 SF = 1956 total SF + 1956 GF parcels assigned to wells permitted from <br />1951 through 1956 <br />The years 1951 through 1955 were filled linearly between the known 1956 snapshot <br />and the developed 1950 data point. <br />4. For ground water only structures, a procedure similar to Acreage Filling A was used <br />to create a 1950 data point. Total acreage and acreage subsets were allowed to adjust <br />depending on well permit dates, as demonstrated in the following example: <br />