CRDSS_Task1_14-22_IrrigationCULosses_GunnisonBasin

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Four different data sets are prepared. The first data set involves the most detail wherein crops in every subarea are further identified by soil type. This means that the consumptive use of alfalfa in sandy loam is calculated separately from that of alfalfa in clay-loam. This refinement should not affect the computation based on the Soil Conservation Service (SCS) modified Blaney-Criddle (XCONS2). However, it should affect the other ET estimation methods that use a soil moisture budget. The second data set is created by assigning each type of crop an aggregate soil type. All the soil types associated with a crop in a subarea are combined and re-named using the soil type with the largest area in the combination. The consumptive use is estimated using the first two data sets with the assumption that all crops complete their growing season normally with an adequate supply of water. The third data set incorporates the effects of water short areas. The effect of water shortage is taken into account by identifying alfalfa-short and pasture-short areas. The first three data sets use the set of weather station weights recommended by the USBR as shown in Table 2. Eleven weather stations are identified. The fourth data set is prepared exactly the same as the third data set except for a modification to the weights of the weather stations and the addition of a twelfth weather station (Table 3) as presented in Task Memorandum 1.14-18 for use in the CRDSS project. The data sets are prepared to support the input requirements for estimating ET by SCS Blaney-Criddle. This gives two consumptive use estimates: (1) by using the SCS Blaney-Criddle (XCONS2), and (2) by adopting the CRDSS enhancements to the SCS Blaney-Criddle methodology. The data set for the Penman-Monteith method cannot be prepared for the whole Gunnison River basin mainly because of the lack of representative daily weather data for the whole basin. A few weather stations that collect daily weather information have been identified for comparing the Penman-Monteith ET estimation method against the SCS Blaney-Criddle method. However, at the present time, only one covers the period of this study (1985-1990). This is detailed in Task Memorandum 1.14-16. Data Set 1 - Detailed Combinations of Crop and Soil Types For each subarea, several unique combinations of crop and soil types are identified using the GIS data. A total of 140 combinations are considered for the whole Gunnison River basin. It should be noted that the same crop and soil combination in a subarea (i.e. alfalfa in clay-loam) could have different crop (i.e. planting/harvesting dates), soil (i.e. water holding capacity, AWC), and management (i.e. management allowed depletion, MAD) characteristics (terms defined in Task Memorandum 1.14-8). Permutations of these other characteristics would dramatically increase the number of combinations. In this data set, the 140 combinations only include the permutations of AWC. Therefore, alfalfa-clay loam in Delta2 could have a different AWC from the alfalfa-clay loam in Gunnison1. A constant planting/harvesting date is specified for each annual crop regardless of its spatial position in the basin. The dates for perennial crops are automatically specified by the model based on earliest moisture use in spring and latest moisture use in fall. The MAD varies according to the type of crop. Figure 1 shows the annual irrigation water requirement (IWR) for 1985-90 for the whole Gunnison River basin as estimated by using the two forms of the SCS Blaney-Criddle ET estimation method (with and without enhancements). The 6-year average annual use is 355,710 and 400,730 acre-ft for the SCS Blaney-Criddle with and without enhancements, respectively. The peak use was in 1989 (108, 107 percent of the average annual use), while the lowest water use occurred in 1986 (84, 83 percent of the 2 A275 01.0995 1.14-22 Manguerra