GROUNDWATER CONTROL OF CLOSED-BASIN LAKE LEVELS UNDER STEADY-STATE CONDITIONS

Groundwater exerts both local and regional controls on closed-basin lake-level changes. Locally, the equifer surrounding a lake can modify how lake-level changes result from a shift in surficial water balance. "Lake pumping" is defined as the net steady-state removal of lake water via surficial processes, namely evaporation minus direct precipitation and minus any input from overland runoff that reaches the lake. A simple analytic groundwater model of a circular lake next to an infinitely long river shows that the sensitivity of the lake level to a change in lake pumping is proportional to the distance from the lake to the river. Further, lakes with large surface areas are more sensitive than small lakes to a shift in lake pumping. Regionally, lake levels change from a shift in the surrounding water-table elevation. A review of a simple analytic groundwater model of a water table between two infinitely long canals demonstrates that a shift in groundwater recharge changes the water-table elevation most near the middle of the interfluve. Three types of lake-level sensitivities (two local, one regional) are defined as the partial derivative of lake level with respect to: (1) lake size (radius); (2) lake pumping rate; and (3) regional groundwater recharge. All three sensitivities may be further categorized as "positional sensitivities" because their values are a function of the lake's position in the watershed. Specifically, lakes far from rivers are more susceptible to lake-level change than are lakes close to rivers. Both simple models further demonstrate that lake-level sensitivity to the above factors is inversely proportional to the permeability of the surficial aquifer. When compared to a system of a more realistic geometric arrangement of lakes and rivers, the simple models underestimate lake-level change in response to both local and regional factors. Knowledge of the sensitivities of lakes can be important in identifying those lakes on the modern landscape that are susceptible to lake-level change, and in interpreting the causes of past lake-level changes as revealed by geologic investigations. © 1990.