Eugene L. Peck

Document Type


Publication Date

January 1967


The effects of exposure on pan evaporation rates were studied at the Davis County Experimental Watershed near Farmington, Utah, by operating a network of 12 class A evaporation stations on the watershed during the summer months of 1962 through 1966. Standard Weather Bureau observations on a daily basis were obtained from a total of 17 different sites representing widely idverse topography with a vertical range of 4, 630 feet. Deviations from mean relations with elevation on monthly values of observed meteorological factors were found to be related to the type of exposure. Dewpoint observations on different slopes were found to be related not only to the differences in station exposure but also to the stability of the air and direction of the upper air flow. Two commonly-used methods for estimating monthly pan evaporation were found to be within 3 and 6 percent ofobserved values from the mountain area. Estimates of daily pan evaporation using the mass transfer equations derived from the Lake Hefner and Lake Mean water-loss investigations were found to over-estimate and under-estimate for different types of exposures. The errors in the daily estimates were related to the type of exposure and stability indices. Revised mass transfer equestions were found to correlate well with daily and 2-hourly pan evaporation rates when type of exposure was considered. Further improvement was obtained in the reliability of the mass transfer equestions when the daily data were segregated on the basis of the diretion and speed of the 700-millibar lelvel wind. Pan evaporation for the network stations for open locatiosn on top of major ridges and along their southern slopes and on sites subject to strong night time drainage winds were found to have no discernable variation with elevation. For protected sites and those on northern slopes, pan evaporation showed a small decrease with increasin gelevation. The effect of elevation (atmosphere pressure) independently on evaporation rates was investigated through the use of data from stations where the other meteorological factors involved, other than pressure, were the same. The study indicated that pan evaporation increases with increase in pressure, all other factors considered being the same.