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Water Resources of North Dakota |
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The purpose of this study is to provide a more accurate measure of the hydrologic components, especially evaporation, that affect the water balance of Devils Lake, N. Dak. Energy-budget and mass-transfer methods were used to compute evaporation from Devils Lake during the open-water periods of 1986-88. Mean daily energy-budget evaporation ranged from 0.038 inch to 0.253 inch. Analysis of terms in the energy-budget equation indicates that the same percentage of error in each term does not result in the same percentage of error in computed evaporation. For example, a 2-percent error in reflected shortwave radiation causes a 0.2-percent error in computed evaporation, whereas a 2-percent error in incoming shortwave radiation causes a 3-percent error in computed evaporation. In the mass-transfer method of computing evaporation, the mass-transfer coefficient was determined using energy-budget evaporation as the independent measure of evaporation. A linear regression equation was developed with energy-budget evaporation and the mass-transfer product. The coefficient of determination of the equation was 0.73, and the standard error of estimate was 0.03 inch per day. The greatest difference between energy-budget and mass-transfer evaporation during the open-water periods was 2.36 inches in 1987. Monthly estimates of evaporation from Devils Lake for 1986-88 ranged from 1,120 acre-feet in January 1986 and 1987 to 35,900 acre-feet in July 1988. The evaporation rate was greater in 1988 (40.0 inches) than in 1987 (37.9 inches); however, the total evaporation from the lake surface was greater in 1987 because the larger surface area in 1987 more than compensated for the lower evaporation rate. Annual mean precipitation for the precipitation-station network around Devils Lake ranged from 12.86 inches in 1988 to 23.06 inches in 1986. Precipitation during 1986 was 20 percent greater at the Devils Lake KDLR radio station than the annual mean precipitation for the precipitation-station network. Precipitation falling on Devils Lake ranged from 110 acre-feet in April 1988 to 31,300 acre-feet in July 1986. Water-balance estimates of Devils Lake for 1986-88 indicate that the computed inflow was greater than the gaged inflow during each year of the study. Although relatively minor errors in water-balance terms can account for the difference between computed and gaged inflow for 1986 and 1988, no reasonable combination of errors in water-balance terms can account for the difference of 61,800 acre-feet in 1987.
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