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-43- Water Balance A water balance for the North Fork of the Gunnison River has been performed for the watershed whose boundaries are illustrated in Figure I. The boundaries can be effectively divided into two sections; namely, North of the North Fork of the Gunnison River and South of the North Fork of the Gunnison River. Table 5 identifies how much area exists in each section, along with approximate average elevations. In order to perform a mss balance on this area, it is necessary to determine how much precipitation occurs annually. One technique that can be used is to utilize existing precipitation data. However, only two precipitation stations exist for the watershed; one at Paonia and one at Wilcox Ranch. Since these are .located near the North Fork of the Gunnison River, it was felt that precipitation values obtained would not adequately represent the total watershed. Therefore, isohyetal lines were used to determine average precipitation and total amount of water that falls on the watershed. Figure 4 represents mean annual precipitation and Figure 5 illustrates May to September precipitation of the years 1931 to 1960. All values were obtained from maps drawn by the U.S. Weather Bureau. Table 6 summarizes precipitation data for both the North and South sections of the watershed. Outflow from the system will occur by surface flows, ground water flows, evapo- transpiration, and diversions. Of these outflow components, evapotranspiration is the most difficult to quantify. Various techniques exist to determine how much evapotranspiration occurs. For this study, the technique proposed by Ramon (1961) was utilized to determine a value for potential evapotranspiration (PET). The advantage of utilizing Hamon's equation is that the value determined for 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-Raise method, where the values obtained for PET represent specific crops. Table 6 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 Paonia 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 Ramon method. It can be concluded that utilization of the Ramon 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 ~ inch of water equivalent per month. Table 6 illustrates that the loss of water during winter months, as predicted by the Ramon equation, is approximately ~ inch per month. Finally, the Ramon equation predicts PET for native vegetation when the temperature is greater than 32.0°F. This is in contrast to other techniques, which often require a minimum temperature greater than 40°F 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 5.4oF in temperature, the values of PET for the watershed were determined as illvstrated in Table 7.