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Observations <br />The following observations can be made based on the evaluation of AgStats, water rights, <br />hydrology and GIS coverages: <br />• There is a general trend towards reduced crop acreage over time in most counties <br />(both AgStats and GIS). <br />• For most counties and basin-wide, total GIS acreage is less than AgStat acreage. <br />• CAS does not differentiate between irrigated and non-irrigated acres of corn and <br />therefore CAS estimates of corn appear to be higher than GIS coverage estimates <br />of corn in every county. <br />• AgStat surveys attempt to have larger users split their acreage by county, <br />however, it is recognized that most users report their acreage under one county <br />instead. <br />• Ditches may irrigate lands that span many counties. Irrigated acreage under a <br />single ditch may be represented in AgStats in more than one county making it <br />difficult to fill data at the ditch level with county-based AgStats data. <br />• There does not appear to be consistent crop type patterns between counties. <br />• CAS and NAS do not contain information on acres of alfalfa or pasture prior to <br />1967 or on irrigated corn prior to 1964. <br />• Neither senior nor junior ditches appear to follow general AgStat patterns. <br />• Unless transferred, acreage under both senior and junior ditches did not change <br />significantly in the four historical coverage years. <br />• Total irrigated acreage assigned to specific ditch systems in the GIS coverages <br />tends to be relatively consistent over time, even as wells were added. This <br />indicates that wells were added as a supplemental to surface water, not to increase <br />acreage. Lands served only by ground water generally were not irrigated prior to <br />well development, therefore as new sole-source wells were added, irrigated <br />acreage increased. Therefore, the recommended method to fill data gaps needs to <br />consider acreage under ditch systems separately from acreage irrigated only by <br />ground water. <br />Recommendation for Filling Data Gaps in GIS Irrigated Acreage Coverage <br />Opportunities for using agricultural statistics, diversion records, water rights, and <br />hydrology to fill data gaps in the GIS irrigated acreage coverage at the ditch level were <br />evaluated. <br />Agricultural statistics are most useful for evaluating general trends in crop rotation. The <br />county-specific statistical summaries provided in CAS and NAS are based on census <br />information that is provided voluntarily by farmers; consequently, the accuracy and <br />precision of variation in crop type and irrigated acreage may be somewhat masked by the <br />amount of user participation in a particular year. In addition, user-supplied estimates of <br />irrigated acreage are probably not as accurate as estimates based on GIS, aerial photos or <br />other scientific means. While the CAS and NAS data are useful for identifying general <br />trends at the county level, they do not seem to provide useful information for filling data <br />Page 9 of 11 <br />