COMPARISON OF EVAPORATION RATES FROM FEEDYARD POND EFFLUENT AND CLEAR WATER AS APPLIED TO SEEPAGE PREDICTIONS

Abstract

Evaporation estimates are often used in water balance calculations to determine seepage rates from feedyardholding ponds and lagoons. These estimates have been made using empirical equations derived for clear water, Class APan evaporation measurements using clear water, and rule-of-thumb estimates. However, feedyard effluent has differentphysical and chemical characteristics than clear water. The objectives of this research were to compare clear water andfeedyard effluent evaporation rates and to determine how inaccuracies in evaporation estimates affect seepagepredictions. Small evaporation pans were placed in a 4 4 Latin square design adjacent to a Class A Pan. Fourexperiments were conducted to compare evaporation rates at different concentrations of feedyard effluent, and a fifthexperiment was conducted to compare clear water evaporation at different salt concentrations to test for potential vaporpressure effects. For the two experiments when freshly collected feedyard effluent from a holding pond was used,representing typical feedlot holding pond conditions with visible suspended sediment concentrations and dark coloredeffluent, the feedyard effluent evaporated 8.3 and 10.7% more than the clear water (p = 0.001 and p = 0.0001). Whenweek-old feedyard effluent was used, representing clearer effluent with minimal suspended sediment, the differences werereduced to 3.2 and 0.0% (p = 0.03 and p = 0.70). For clay liners with hydraulic conductivities of 1 107 to 1 108cm/s, we show that underestimating evaporation by 10% when actual evaporation is 1.1 cm/day results in seepage ratepredictions of 3 to 20 times higher than actual seepage rates. Similarly, underestimating evaporation by 10% when actualevaporation is 2.2 cm/day results in seepage rate predictions of 5 to 40 times higher than actual seepage rates. Thiscorresponds to 0.10 and 0.20 cm/day higher seepage rates for actual evaporation of 1.1 and 2.2 cm/day, respectively.Considering that some states have allowable seepage rates ranging from 0.08 to 0.63 cm/day, an overestimation of 0.1 to0.2 cm/day could have serious ramifications with environmental regulators, thus demonstrating the importance ofaccurate evaporation estimates when predicting seepage using the water balance method.