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SPDSS Memorandum <br />Final <br />To: Ray Bennett and Ray Alvarado <br />From: LRE -Erin Wilson and Greg Espegren <br />Subject: Task 71 -Estimate Historical Acreage <br />Date: October 31, 2006, revised March 21, 2007 <br />Introduction <br />Irrigated acreage and crop types are key data components used to estimate crop <br />consumptive use in the SPDSS study area. GIS coverages were developed for SPDSS <br />(RTi, 2006) that represent acreage and crop types by ditch or water source for 2001, <br />1987, and 1976 using thermal signatures from satellite imagery, aerial photos, and field <br />interviews. In addition, a coverage was developed that represents irrigated acreage by <br />ditch or water source for 1956 using aerial photography. Crop types were not able to be <br />assigned to irrigated acreage for 1956 using aerial photos. <br />Colorado Agricultural Statistics (CAS) and National Agricultural Statistics (NAS) report <br />user-provided acreage and yield by crop at the county level on an annual and a 5-year <br />basis, respectivelyi. This data was collected, digitized as necessary, and incorporated <br />into HydroBase during Phase 1. <br />This memorandum presents the general approach and results from the completion of the <br />following: <br />Determine an appropriate method for using agricultural statistics, water rights, <br />water availability, and other data to estimate historic irrigated acreage and crop <br />types by ditch or other water source for the entire SPDSS study period (e.g. time <br />periods before and between GIS irrigated acreage coverages). <br />Approach and Results -Agricultural Statistics (AgStats) Analysis <br />AgStats data was evaluated from 1950 through 2005 to determine whether observed <br />changes in crop type over time at the basin wide and county levels could be used to help <br />fill data gaps in the GIS irrigated acreage coverages at the ditch level. <br />A summary of AgStats over the entire South Platte River basin reveals that the total <br />number of irrigated acres declined from approximately 1.4 million acres in 1965 to <br />' CAS and NAS data are collectively referred to as "AgStats" data throughout this report. <br />