Evaluation of Evaporative Drivers and Hydrologic Influences using Energy and Water Budget Techniques at a Shallow Saline Lake in the Western Sandhills of Nebraska, USA

The western Sandhills of Nebraska contain many shallow lakes and wetlands that interact strongly with groundwater and the overlying atmosphere. The region is semi-arid, and most of the lakes are saline, supporting a wide range of ecosystems. Water levels and salt concentrations are highly sensitive to variations in precipitation, evaporation, and groundwater fluxes, making the Sandhills an excellent laboratory for examining the effects of climate on the water balance of interdunal lakes. In this study, we investigate the atmospheric controls on evaporation rates, as well as the water balance of Alkali Lake, one of the more saline lakes in the western Sandhills. The Bowen ratio energy balance and mass-transfer methods are applied over a three-year period (2007-2009) to quantify summer evaporation rates. Daily evaporation is found to vary widely, but averages around 5-6 mm/day during the summer. Evaporation rates are largely controlled by solar radiation on a seasonal basis and by variations in wind and vapor pressure gradient at shorter timescales. Adjustments for salinity affected the mass transfer method more than the energy budget method, with a root mean squared error of 0.49 mm/day, but only 0.09 mm/day for the energy budget method. Evaporation dominated the water balance during the summer months, exceeding precipitation by a factor of 3.2, on average. The lake water balance also indicates that evaporation exceeds the sum of all water inputs during summer months, causing lake levels to decrease in summer (but rebound during the winter). Net groundwater inflow is the largest source of water into the lake and averages 2.5 mm/day. Occasional negative net groundwater values indicate complex interactions between the coupled lake and groundwater systems. Lake reactions to precipitation inputs suggest that short term groundwater flow reversals are possible. Adviser: Diego Riveros-Iregui