CRDSS_Task2-09-7_CropCUEstimates_YampaBasin

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2.2 Crop Distribution for each Diversion Structure The crop distribution of the irrigated areas served by each diversion structure is obtained using the DMI demandts as shown in Table 3. In cases in which the crop distribution for a particular structure cannot be identified, the crop distribution of the entire county-huc combination was adopted. 2.3 Irrigation Water Requirement Calculation The Irrigation Water Requirement for each structure being modeled is estimated using the SCS Blaney- Criddle method as implemented in the CRDSS-CU model. The CRDSS-CU Blaney-Criddle model has the capability of using a soil moisture budget. For this phase of CRDSS, the State decided to use the SCS Blaney-Criddle method without the soil moisture budget. In addition to the soil moisture budget, the CRDSS-CU model has the capability of computing CU using the Penman-Monteith equation. Some preliminary studies showed the potential for significant differences in the CU values computed using the SCS Blaney-Criddle and the Penman-Monteith. The State has a task force reviewing the results of the two equations. Based on extensive prior work by many researchers, there is a well established body of knowledge that shows that as meteorological data becomes available the Penman type equations can provide more accurate estimates of crop CU when used properly. However, since this area did not have sufficient weather data or the quality of these data was inconsistent, the SCS Blaney-Criddle was selected for this modeling effort. The weather stations that were recommended to be used in Yampa River Basin as part of the CRDSS project (Task Memorandum 2.09-15) were adopted. The weather stations and associated weights for the county-huc combinations in the Yampa River Basin are shown in Table 4. The weights of each weather station for each diversion structure is obtained by adopting the weights associated with the county-huc combination where the diversion structure is located. For the Yampa River Basin the county-huc assigned to each structure is based on the location of the diversion structures and not the areas served by the structure. 2.4 Water Supply results from STATEMOD The average water supplied for all structures being modeled is 62,563 acre-ft (Table 8), which is all usable (no excess water is supplied). While the average Irrigation Water Requirements (IWR) is 69,614 acre-ft (Table 6), representing an average shortage of 7,052 acre-ft. Although the annual water supply is close to the IWR, the years 1988, 1989, and 1990 have by far the largest shortages as can be seen in Table 7. 2.5 Pro-ration of CU of modeled Structures to CU of County-HUC At this time only a portion of the irrigated area is being modeled. Therefore, the CU for the area being modeled is pro-rated (using GIS data) to estimate the total CU for each county-HUC. This is done by generating a GIS report for all the structures in each county-huc and determining the percentage of irrigated acreage modeled for each county-huc in the basin. The demand, supply and shortages can then be pro-rated based on this percentage. The total irrigated lands, lands being modeled and percent of lands being modeled for each county-huc are shown in Table 5. This information can be used to evaluate the validity of the pro-rated estimates of CU and amount of water short in a county-huc (i.e. whether the county-huc combination is adequately 2 12/16/96 2.09-07 CSU/IDS