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reservoir did not exist. For this study the evapotranspiration that must be subtracted was determined in four parts. EVAPORATION FROM THE PRE-RESERVOIR WATER SURFACE The first part was the evaporation from the pre-reservoir river water sur- face. It is computed by multiplying the area of the river by the gross evaporation rate. The river water surface area was determined using the following criteria. (1) The aerial photographs from which the Lake Powell topography came were flown in September and October of 1958 and 1959 when the river was at a very low stage. (2) The average flow of the river over the period of record was about 18,000 c.f.s. at Lees Ferry. (3) Tail water curves taken in the vicinity of Glen Canyon Dam indicate a rise of only about 5 feet for an increase in flow of 15,000 c.f.s. (4) Since the water line shown on the topography represents a flow of about 6,000 c.f.s., it was assumed that the contour closest to this water line would represent average flow conditions. (5) Where a contour crossed the river on a given topography sheet, a line representing tne average f'cw 'ine was drawn in and the area planime- tered accordingly. (6) Sandbars that did not show vegetation were assumed to be included in the water surface area at 18,000 c.f.s. average flow. The river-elevation relationship tion rate from Table 4 was used. in Table 8 Column 2. is shown in Figure 4. The gross evapora- The resulting river evaporation is shown STREAMSIDE EVAPOTRANSPIRATION The second part was the evapotranspiration that would have occurred from the vegetation, which is mostly phreatophyte, and evaporation from the wetted soil on the streambank if the reservoir did not exist. It was com- puted by multiplying the evapotranspiration rate by tne streamside area. The evapotranspiration rate was computed by the Blaney - Criddle method 7/ as shown in Table 7 and described below. The streamside areas were deter~ mined under a contract between the University of Utah and the Bureau of Reclamation in the late 1950's before Lake Powell started to fill 9/. The vegetation density was measured along transects. These were used as an index to make ocular estimates of the remaining areas. The areas were measured by planimeter from maps which were prepared from overlay maps and tabular data from the field. The area-elevation relationship is shown in Figure 5. The main source of water for this part is from percolation with a small contribution from precipitation. The streamside evapotranspiration is shown in Table 8 Column 3. 14