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<br />001358 <br /> <br />Distribution of County Crop Data <br />In determining the Unit crop acreage, it was decided that the GIS coverage of irrigated acreage would <br />serve as the base for determining acres of irrigated area since it more accurately represents the true <br />irrigated area. It was felt that the GIS data was more accurate than the CAS data because of the <br />method used to obtain the data. But, because the data only represented a single point in time, 1993, <br />it was decided to use a combination GIS data and CAS data to preserve fluctuations in irrigated area <br />and changes in cropping patterns. Cropped area and cropping patterns change as weather patterns and <br />economic conditions change. <br /> <br />CAS data was tabulated by county on a annual basis for the period 1970 to 1993 showing total <br />irrigated acreage and acreage by crop. Since expansion of irrigated lands occurred within both <br />Dolores and Montezuma counties (Dolores Project) in Colorado during discrete periods, the GIS total <br />irrigated acreage was reduced by the amount of expansion for each of these discrete periods for both <br />county and Units back to 1970. Acres expanded and periods in which acres were expanded was <br />obtained from Reclamation files. In order to preserve the acreage established by the GIS coverage, <br />the Unit annual CAS acreage was readjusted by the ratio of 1993 GIS county irrigated acreage to <br />1993 CAS county irrigated acreage to reflect a more accurate irrigated acreage as shown in following <br />equation: <br /> <br />= <br /> <br />County CAS <br />Acreage <br />(Year) <br /> <br />Unit GIS Acres (Year) <br /> <br />County GIS Acres(I993) <br /> <br />Unit <br />Acreage <br />(Year) <br /> <br />* <br /> <br />* <br /> <br />County GIS Acres (Year) <br /> <br />County CAS Acres (1993) <br /> <br />For example: <br /> <br />1970 Colorado Unit 1 Acres = 16,149 = 20,140.2 * 16,590 / 21,661 * 21,661/20,690 <br /> <br />Note: the only areas where Colorado GIS acres vary are units involving the Dolores Project. <br />The next step in the process was to estimate the crop distribution by Unit for each year in the study <br />period. Annual Unit acreage was multiplied by the ratio of county crop acreage to county total <br />acreage for the time period of 1970 to 1993 to obtain a yearly crop distribution by Unit as shown in <br />following equation: <br /> <br />= <br /> <br />Unit <br />Acreage <br /> <br />County CAS Crop N Acreage <br /> <br />Unit <br />Crop N <br />Acreage <br /> <br />* <br /> <br />County CAS Acreage <br /> <br />For example: <br /> <br />1970 Colorado Unit 1 Alfalfa Acres = 738 = 16149 * 920 / 20140.2 <br /> <br />Pasture acres were not varied annually by agreement with the state of Colorado. New Mexico <br />cropping patterns except for NIIP were provided by New Mexico Interstate Stream Commission, <br />NllP cropping patterns were provided by Keller-Bliesner Engineering. Yearly acreage distribution by <br />Unit is presented Appendix A.I. Appendix A.II tabulates CAS values by county and year. Appendix <br />A.III. presents annual cropping patterns by Unit. <br /> <br />DRAFT - SJRIP Hydrology Model Documentation <br />June 18, 1999 <br /> <br />19 <br />