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TERRACE EVAPOTRANSPIRATION The third part is the evapotranspiration from the terrace areas under pre- reservoir conditions. It was computed by multiplying the evapotranspira- tion rate by the terrace area. The evapotranspiration rate was determined by the Blaney-Criddle method 7/ described below. The areas were deter- mined by the same survey referred to above. This water reaches the root zone mostly by capillary action with some by precipitation. The terrace area-elevation curve is shown in Figure 6. The resulting evapotranspira- tion is shown in Table a Column 4. BLANEY-CRIDDLE METHOD The Blaney-Criddle method of computing evapotranspiration (consumptive-use) was chosen because of the minimal data requirements and accuracy acceptable for this purpose. The basic equation is: u=fk Where u is the evapotranspiration rate in inches. k is an emperical coef- fecient dependent on the type of vegetation. Consumptive use coefficients have been determined mostly for irrigated crops so there has been little investigation of native vegetation. From values given for k by Blaney 10/ for native vegetation for several densities of vegetation 1.1 was selected for the streamside area and 0.9 was used for the terrace area. The variable f is calculated as (t)(p)/100. Where t is the mean monthly air temperature in degrees farenheit and p is the monthly percent of annual daylight hours and is dependent upon the latitude. 390 north latitude was used. Therefore u=(t)x(p)x(k)/100. Table 6 shows the computation of (p)x(k)/100 for the streamside area and the terrace area. This was multiplied by the average temperature for the month to obtain the evapotranspiration rates. EVAPOTRANSPIRATION OF REMAINING AREAS The fourth part is the evapotranspiration from the remaining area of the reservoir under pre-reservoir condition. Most of the precipitation falling on the area of Lake Powell would not have produced runoff because it would have evaporated from the vegetation and soil or it would have transpired from the vegetation. In consumptive use studies this is termed effective precipitation. It does not include preci- pitation that causes runoff. The method of determining the effective pre- cipitation used is described in a report by R. A. Schleusener ~/. The precipitation was computed from seven stations: Page, Wahweap, Bullfrog, Canyonlands, Hite, Mexican Hut, and Natural bridges. Since each of these stations were located above the maximum elevation of Lake Powell, and since precipitation increases with elevation, it was necessary to reduce the pre- cipitation to an equivalent for Lake Powell at a representative elevation of 3,600 feet. This was done by plotting the average annual precipitation for 1963 through 1980 against the elevation for the seven stations and drawing a curve throught the points. The resulting average annual precipi- tation from this curve is 6.33 inches. The ratio of 6.33 to the average annual precipitation at each station was multiplied by the monthly precipi- 18