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CRDSS <br />TASK MEMORANDUM 1.14-16 <br />Consumptive Use Model <br />Comparison of Blaney-Criddle and Penman-Monteith <br />Evapotranspiration Estimation Methods <br />1.0 ISSUE <br />The CRDSS consumptive use (CU) Model includes both aBlaney-Criddle (Soil Conservation Service <br />(SCS) Blaney-Criddle Original and Enhanced versions) and aPenman-Monteith Evapotranspiration (ET) <br />Estimation method. ET estimates using the latter method have been shown to be more accurate than those <br />of the former (Jensen et al. 1990). This task memorandum compares the two methods as implemented in <br />the Gunnison River basin. <br />2.0 DISCUSSION/ANALYSIS <br />The irrigation water requirement (IWR) calculation for the Gunnison River basin is done by subarea, <br />which is defined by county and hydrologic unit. For instance, subarea Delta2 includes areas that are in <br />Delta county and HU14020002. The whole Gunnison River basin is divided into 15 subareas. Several <br />weather stations that support the data requirements of the SCS Blaney-Criddle calculations can be <br />identified that allow the ET estimation for all subareas based on the SCS Blaney-Criddle method. <br />However, this is not true when using the Penman-Monteith method. Only a few weather stations collect <br />the set of daily weather data required by the Penman-Monteith method (see Task Memorandum 1.14-15). <br />For the purpose of this task memorandum, one weather station that has almost a complete daily weather <br />data set for 1 year is identified and used: the Jay weather station. The station is close to the Paonia <br />weather station that collects monthly weather data, and therefore it is applied to subareas that use the <br />Paonia weather station for the Blaney-Criddle calculation. Following the new recommendation for the <br />use of the weather stations (see Task Memorandum 1.14-18), these subareas will include Delta2, Delta4, <br />Gunnison4, and Montrose2. The calculation is done for 1987 only. <br />The irrigated acreages, including the crop cover, are obtained by using a geographic information system <br />(GIS) and a 1993 irrigated acreage map (Table 1). Following the procedure outlined in Task <br />Memorandum 1.14-22, the 1993 areas are set equal to the 1987 areas. The crops that are reported include <br />alfalfa, pasture, dry beans, corn grain, orchard, tree farms, small grains, and vegetables. Each crop is <br />associated with a particular soil type. This means that the CU of alfalfa in sandy-loam is calculated <br />separately from that of alfalfa in clay-loam. This refinement should not affect the computation based on <br />the SCS Blaney-Criddle (XCONS2). It should, however, affect the other ET estimation methods that use <br />a soil moisture budget. A total of 17 crop-soil combinations are identified. <br />The 1987 raw data for the daily weather station (Jay) is missing data for 62 days, 16 days of which are in <br />December (on the average, 4 days per month). These missing data are filled by using the monthly <br />average of the recorded data. This methodology was adopted for this weather station, but more formal <br />procedures for filling missing data are expected to be developed under the database development task in <br />Phase IIb. Required daily weather data are minimum temperature, maximum temperature, dew point <br />temperature, solar radiation, wind run, and daily rainfall. The dew point temperature is estimated from <br />relative humidity, minimum temperature, and maximum temperature. <br />A275 05.01.95 1 1.14-16 Manguerra <br />