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001957 To estimate net depletions, the computer program applies a crop growth stage coefficient to the calculated monthly consumptive use factor to estimate the net monthly consumptive use rate, in inches/acre, for each crop grown. To determine the project consumptive use rate for each subbasin, crop consumptive use rates were weighted based on the Unit crop distribution. For example, if a Unit had a crop distribution of 50 percent alfalfa and 50 percent pasture, 50 percent of alfalfa's consumptive use rate would be added to 50 percent of pasture's consumptive use rate. Crop growth stage coefficient curves can be found in TR 21, Appendix A, "List of Crop Growth Stage Coefficient Curves. " Other variables required for XCONS are monthly percentage of daylight hours for each station evaluated which are found in TR 21, Table I, pg. 9. For areas not tabulated in TR 21, a linear interpolation is used to match the latitude which is multiplied by 100. The latitude is a weighted average of the latitudes of the weather stations used. For example, the January value for a site at 38 degrees, 31 minutes North latitude would be 684. All crop growth parameters are tabulated in Appendix A.IV. Irrigated Acres and Cropping Patterns The greatest error in calculating irrigation consumptive use is determining the irrigated acreage for each Unit and associated crop distribution on an annual basis. The methods used were dependent upon data contained in each state's agricultural statistical reports or on supplemental data supplied by the states. In Colorado, the GIS coverage was used to proportion crop types and area by Unit. That is, a Unit, being a combination of County and HUC, has a common cropping pattern and depletion rate. The cropping patterns for a given county were obtained from Colorado Agricultural Statistics (CAS). The State of New Mexico supplied Reclamation with a break down of crop acreage and distribution data by Units excluding NIIP lands (Memoranda dated 6/11/1997 and 9/10/1997 are found in Appendix H). Cropped acreage and distribution data for NIIP lands were obtained from Reclamation's crop summary statistics. Irrigated area in Utah and Arizona are CRSCU&L acres extended through the study period. Archuleta and La Plata counties were the only Colorado counties which lie entirely within the San Juan River Basin. Dolores, Hinsdale, Mineral, Montezuma, and San Miquel counties are Colorado counties which partially lie outside the San Juan River Basin. Because irrigated crop type acreage is reported on a county-wide basis and is not disaggregated by hydrologic boundaries, the use of the CAS total acreage was deemed inappropriate. GIS data were used to represent the average adjusted county irrigated crop acreage for these counties lying partially outside the San Juan River Basin. Rio Grande, San Miguel and San Juan counties in Colorado have lands which partially lie inside the San Juan River Basin but because the GIS data did not show any irrigated lands in the San Juan River Basin, these areas were excluded from the analysis. Annual Pasture acreage was assumed to be the same as the GIS data. This was determined to be the best approach after comparing the GIS data to the CAS data. Crops that displayed similar growth characteristics were combined as one crop. Com, as an example, is grown for grain and silage. Usually silage com is harvested earlier than grain com but in the San Juan basin silage com is not harvested until after a killing frost while grain com would not be harvested until after it had spent some time in the field drying. Since both crops ended their growth period at the same time, period of killing frost, they were combined as one crop called com. Most vegetable crops were combined and labeled field crops. DRAFT - SJRIP Hydrology Model Documentation June 18,1999 18