Date of this Version
Journal of Hydrology 553 (2017) 172–187
Despite potential evaporation rates in excess of the local precipitation, dry climates often support saline lakes through groundwater inputs of water and associated solutes. These groundwater-fed lakes are important indicators of environmental change, in part because their shallow water levels and salinity are very sensitive to weather and climatic variability. Some of this sensitivity arises from high rates of open-water evaporation, which is a dominant but poorly quantified process for saline lakes. This study used the Bowen ratio energy budget method to calculate open-water evaporation rates for Alkali Lake, a saline lake in the Nebraska Sandhills region (central United States), where numerous groundwaterfed lakes occupy the landscape. Evaporation rates were measured during the warm season (May – October) over three consecutive years (2007–2009) to gain insights into the climatic and limnological factors driving evaporation, as well as the partitioning of energy balance components at seasonal and interannual time scales. Results show a seasonal peak in evaporation rate in late June of 7.0 mm day–1 (on average), with a maximum daily rate of 10.5 mm day–1 and a 3-year mean July-September (JAS) rate of 5.1 mm day–1, which greatly exceeds the long-term JAS precipitation rate of 1.3 mm day–1. Seasonal variability in lake evaporation closely follows that of net radiation and lake surface temperature, with sensible heat flux and heat storage variations being relatively small, except in response to short-term, synoptic events. Interannual changes in the surface energy balance were weak, by comparison, although a 6-fold increase in mean lake level over the three years (0.05–0.30 m) led to greater heat storage within the lake, an enhanced JAS lake-air temperature gradient, and greater sensible heat loss. These large variations in water level were also associated with large changes in absolute salinity (from 28 to 118 g kg–1), with periods of high salinity characterized by reductions in mass transfer estimates of evaporation rate by up to 20%, depending on atmospheric conditions and absolute salinity. Energy balance estimates of evaporation, on the other hand, were found to be less sensitive to variations in salinity. These results provide regional insights for lakes in the Nebraska Sandhills region and implications for estimation of the energy and water balance of saline lakes in similar arid and semi-arid landscapes.