Effect of DOC on evaporation from small Wisconsin lakes

Abstract

Summary Evaporation (E) dominates the loss of water from many small lakes, and the balance between precipitation and evaporation (P–E) often governs water levels. In this study, evaporation rates were estimated for three small Wisconsin lakes over several years using 30-min data from floating evaporation pans (E-pans). Measured E was then compared to the output of mass transfer models driven by local conditions over daily time scales. The three lakes were chosen to span a range of dissolved organic carbon (DOC) concentrations (3–20 mg L −1 ), a solute that imparts a dark, tea-stain color which absorbs solar energy and limits light penetration. Since the lakes were otherwise similar, we hypothesized that a DOC-mediated increase in surface water temperature would translate directly to higher rates of evaporation thereby informing climate response models. Our results confirmed a DOC effect on surface water temperature, but that effect did not translate to enhanced evaporation. Instead the opposite was observed: evaporation rates decreased as DOC increased. Ancillary data and prior studies suggest two explanatory mechanisms: (1) disproportionately greater radiant energy outflux from high DOC lakes, and (2) the combined effect of wind speed ( W ) and the vapor pressure gradient ( e s − e z ), whose product [ W ( e s − e z )] was lowest on the high DOC lake, despite very low wind speeds ( −1 ) and steep forested uplands surrounding all three lakes. Agreement between measured (E-pan) and modeled evaporation rates was reasonably good, based on linear regression results ( r 2 : 0.6–0.7; slope: 0.5–0.7, for the best model). Rankings based on E were similar whether determined by measured or modeled criteria (high DOC −1 (C.V. 9%), but statistically significant differences between lakes resulted in substantial differences in cumulative E that were consistent from year to year. Daily water budgets for these lakes show that inputs were dominated by P and outputs by E; and our findings indicate that subtle changes in the variables that drive E can have measurable effects on water levels by shifting the balance between P and E.