. Description and hydrologic analysis of two small watersheds in Utah's Wasatch Mountains. LEGEND:. ONDJ FMAMJ JASO MONTHS Figure 15. — Comparison of mean monthly precipitation and potential evapotranspira- tion for the period 1956-1959. mean monthly temperature. PE is an index of heat energy available to vaporize water and is an estimate of the amount of evapotranspira- tion (ET) that would occur if plant and soil water were not limiting. PE is assumed to be 0 when the mean monthly temperature is below 32° F. PE values are plotted along with mean monthly precipitation in figure 15. Precipita-
. Description and hydrologic analysis of two small watersheds in Utah's Wasatch Mountains. LEGEND:. ONDJ FMAMJ JASO MONTHS Figure 15. — Comparison of mean monthly precipitation and potential evapotranspira- tion for the period 1956-1959. mean monthly temperature. PE is an index of heat energy available to vaporize water and is an estimate of the amount of evapotranspira- tion (ET) that would occur if plant and soil water were not limiting. PE is assumed to be 0 when the mean monthly temperature is below 32° F. PE values are plotted along with mean monthly precipitation in figure 15. Precipita- tion exceeds ET for 8 months of the year, from October through May, followed by pre- cipitation deficit during June, July, and August, when ET exceeds precipitation. Aver- age PE during the summer was inches compared to an average rainfall of inches per month. Rainfall and PE are about equal in September. Annual precipitation exceeds the yearly PE by to inches, respective- ly. Our computations of annual PE are 2 to 5 inches lower than presented in the Hydrologic Atlas of Utah (Jeppson and others 1968), al- though the same method was used. It should be noted, however, that in the Hydrologic Atlas, PE for the entire State is based on tem- Figure 16. — Comparison of daily and average monthly values of pan evaporation and potential evapotranspiration (1962). perature records extrapolated from valley stations. Daily evaporation from a class "A" pan was compared with computed PE for the summer of 1962 (fig. 16). Pan evaporation fluctuated greatly in response to daily changes in climatic variables. Average pan evaporation was inch per day, more than twice the average PE of inch per day. Total values were inches for the pan and inches PE. It is generally accepted that pan evapora- tion characteristically overestimates ET. ET was estimated for the period May through September 1965 for two cover types, a mature aspen community and a
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