CRDSS Task Memorandum 1.14-16 - Laserfiche WebLink

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