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 

 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 11.1 inches 

 compared to an average rainfall of 1.45 inches 

 per month. Rainfall and PE are about equal in 

 September. Annual precipitation exceeds the 

 yearly PE by 41.5 to 16.6 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 0.22 inch per day, more than twice the 

 average PE of 0.10 inch per day. Total values 

 were 22.9 inches for the pan and 10.7 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 an adjacent 

 area from which the aspen had been removed. 

 ET was considered to be the sum of the soil 

 moisture depletion and rainfall for the period. 

 The estimated ET was 14.06 inches from the 

 aspen and 10.84 inches from the grass-forb 

 community. For the same period, calculated 

 PE was 14.24 inches, which is fairly close to 

 our estimated ET for the aspen. 



19 



