Comparison of Evaporation Rates from Feedyard Pond Effluent and Clear Water as Applied to Seepage Predictions

Evaporation estimates are often used in water balance calculations to determine seepage rates from feedyard holding ponds and lagoons. These estimates have been made using empirical equations derived for clear water, Class A Pan evaporation measurements using clear water, and rule-of-thumb estimates. However, feedyard effluent has different physical and chemical characteristics than clear water. The objectives of this research were to compare clear water and feedyard effluent evaporation rates and to determine how inaccuracies in evaporation estimates affect seepage predictions. Small evaporation pans were placed in a 4 × 4 Latin square design adjacent to a Class A Pan. Four experiments were conducted to compare evaporation rates at different concentrations of feedyard effluent, and a fifth experiment was conducted to compare clear water evaporation at different salt concentrations to test for potential vapor pressure 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 colored effluent, the feedyard effluent evaporated 8.3 and 10.7% more than the clear water (p = 0.001 and p = 0.0001). When week-old feedyard effluent was used, representing clearer effluent with minimal suspended sediment, the differences were reduced to 3.2 and 0.0% (p = 0.03 and p = 0.70). For clay liners with hydraulic conductivities of 1 × 10–7 to 1 × 10–8 cm/s, we show that underestimating evaporation by 10% when actual evaporation is 1.1 cm/day results in seepage rate predictions of 3 to 20 times higher than actual seepage rates. Similarly, underestimating evaporation by 10% when actual evaporation is 2.2 cm/day results in seepage rate predictions of 5 to 40 times higher than actual seepage rates. This corresponds 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 to 0.2 cm/day could have serious ramifications with environmental regulators, thus demonstrating the importance of accurate evaporation estimates when predicting seepage using the water balance method.