1981-08-20_GENERAL DOCUMENTS - C1981038

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-18- PET will represent a net value for all vegetative cover. This is in contrast to the values obtained by other methods, such as the Blaney-Criddle method or the Jensen-Haise method, where the values obtained for PET represent specific crops. Table 2 illustrates a PET comparison for the area around Paonia, Colorado. It was assumed for the Blaney-Criddle method that the entire area consisted of deciduous orchards. In reality, there are many acres around Peonia that grow other crops. As a result, there will be a difference between the total amount of water required for PET as predicted by the Blaney-Criddle method when compared to that calculated by the Hamon method. It can be con- cluded that utilization of Hamon technique for the entire watershed will yield values which are representative for conditions of the area. An additional advantage of utilization of the Hamon technique is its ability to predict the amount of water lost by sublimation. The U.S. Army Corps"of Engineers (1960) found that for the middle latitudes, in the winter and early spring before snowmelt runoff begins, sublimation loss from the snow surface cover can be assumed, for hydrologic computation purposes, to be about 112 inch of water equivalent per month. Table 3 illustrates that the loss of water during winter months, as predicted by the Hamon equation, is approximately 112 inch per month. Finally, the Hamon equation predicts PET for native vegetation when the temperature is greater than 32.OoF. This is in contrast to other techniques, which often require a minimum temperature greater than 40OF before accurate results can be obtained. Assuming adiabatic conditions exist so that for every 1,000 feet increase in elevation, there is a decrease of 5.40F in temperature, the values of PET for the watershed were determined as illustrated in Table 3. r r From Table 1, it becomes evident that during the time period of May through September, the amount of precipitation which falls upon the watershed is less than the amount of water required for PET. Therefore, for this time period (May - September) , the actual evapotranspiration (AET) will be less than PET. In other words, the native vegetation will use most of the precipitation and also will remove water from the soil .profile. For those areas where cash crops (such as apples, peaches, and so forth) are grown, irrigation water is removed from the North Fork of the Gunnison River and applied on such lands. It is expected that those areas would have an AET which is nearly equivalent to the value predicted by PET. However, since the majority of the watershed has native vegetation, it will be assumed that the value for AET for the May to September period will be equivalent to the May through September precipitation value. Therefore, for the entire year, the value of AET will be represented by approximately 159,000 acre-feet (AF) . Another inflow parameter to a mass balance study involves how much water is entering the watershed via the North Fork of the Gunnison River. Ideally, it would be desirable to have a gaging station at the downstream location of the watershed illusted in Figure 1. However, such is not the case. A station presently exists near Somerset, Colorado, as illustrated in Table 4. By use of this particular station, we are able to approximate the inflow into the basin via the North Fork of the Gunnison River and releases from Paonia Reservoir. Outflow from the basin is not as easily determined. For example, the nearest station to Paonia is located on the Gunnison River near Lazear, Colorado. Such a station is not acceptable for this analysis since many inflows and outflows occur between Paonia and Lazear. However, a review of past U.S.