Spatial and temporal variability of prairie lake hydrology as revealed using stable isotopes of hydrogen and oxygen

Evaporation and groundwater fluxes are thought to regulate hydrologic variability in lakes of the northern Great Plains, but little is known of how the relative importance of these processes may vary in time or space. To address this issue, we measured the isotopic composition of water (delta O-18, delta H-2) from 70 closed-basin lakes in southern Saskatchewan, Canada. All lakes occupied endorheic basins along a long gradient of salinity (0.2-115 g total dissolved solids L-1). Lakes exhibited synchronous seasonal changes in salinity (synchrony, S = 0.78) and delta O-18 (S = 0.84) during the dry summer of 2003 (similar to 195 mm rain), whereas coherence was reduced to 0.56 and 0.22, respectively, during the wet summer of 2004 (similar to 295 mm rain). However, despite evaporative enrichment of isotopic ratios during dry summers, hydrologic balances were regulated mainly by changes in water inflow (I) rather than evaporation (E) in both wet and dry years, with particularly strong influence of inflow ( lowest E : I ratio) in dry southwestern regions. Analysis of isotopic composition also identified winter precipitation or groundwater as the most influential source of water to most lakes, despite only similar to 30% of annual precipitation being delivered during winter. Therefore, although seasonal variability in lake chemistry was influenced by evaporation during summer, long-term mean chemical characteristics of prairie lakes were regulated mainly by changes in winter precipitation or groundwater influx.