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The daily effective rainfall is assumed equal to 70 percent of the total daily rainfall. The SCS Blaney- <br />Criddle method is used to calculate the effective rainfall by month. The monthly data of the Jay weather <br />station is calculated from the daily weather data. With this approach, both ET methods use the same <br />weather station. The annual carry-over soil moisture coefficient is assumed equal to 0.5. The <br />management allowed depletion varies for each crop (terms defined in Task Memorandum 1.14-8). <br />Variation in the root length of the crop during the growing season is assumed to be insignificant, and <br />therefore the root length is kept constant. The planting harvesting dates of a particular crop are kept the <br />same regardless of their location. <br />Figure 1 shows the IWR by subarea as estimated by SCS Blaney-Griddle and Penman-Monteith ET <br />estimation methods. The total IWR for the whole project is 130,270 acre-ft, 116,740 acre-ft, and 136,660 <br />acre-ft for SCS Blaney-Griddle without the enhancements, with the enhancements, and Penman-Monteith, <br />respectively. It is interesting to note that the Penman-Monteith estimates are consistently higher than the <br />estimates of the SCS Blaney-Griddle methods. Figure 2 shows the monthly variation in the IWR for the <br />whole project. The peak requirement occurred in July. It should also be noted that the two forms of the <br />SCS Blaney-Griddle method gave exactly the same values at the falling limb of the curve. This is not true <br />in the rising limb of the curve because SCS Blaney-Griddle method with enhancements assumes that the <br />soil moisture storage is initially at full capacity. This suggests that the main difference between the <br />results of the two forms of the SCS Blaney-Griddle is in the initial part of the growing season. <br />Figures 3, 4, and 5 show the monthly potential ET of selected crops as computed by the SCS Blaney- <br />Criddle and Penman-Monteith methods. The Penman-Monteith estimates are, for the most part, higher <br />than the SCS Blaney-Griddle estimates. <br />3.0 CONCLUSIONS AND RECOMMENDATIONS <br />A data set is prepared to support the ET computations by SCS Blaney-Griddle and Penman-Monteith. A <br />single weather station that collects daily weather data is identified and used for subareas Delta2, Delta4, <br />Gunnison4, and Montrose2. The computation is performed for 1987 only. <br />The annual IWR for the total area is 130,270 acre-ft, 116,740 acre-ft, and 136,660 acre-ft for SCS <br />Blaney-Griddle without the enhancements, with the enhancements, and Penman-Monteith, respectively. <br />The Penman-Monteith estimates are consistently higher than the estimates of the SCS Blaney-Griddle <br />without the enhancements, but lower than those of the SCS Blaney-Griddle with enhancements. <br />At this time, the period of record for which the comparison between the SCS Blaney-Griddle and the <br />Penman-Monteith is possible is only 1 year; therefore, it is recommended that this information not be <br />used as the basis for the local calibration. When and if additional information is developed, this <br />recommendation should be re-evaluated. <br />4.0 REFERENCE <br />Jensen, M.E., R.D. Burman, and R.G. Allen (1990). "Evapotranspiration and Irrigation Water <br />Requirements." ASCE Manuals and Reports on Engineering Practice No. 70. American Society of Civil <br />Engineers, New York, New York. <br />A275 05.01.95 G 1.14-16 Manguerra <br />