INNOVATIVE METHODS FOR SIMULATION OF TERMINAL LAKE SYSTEMS – THE CASE STUDY OF WALKER LAKE

The surface area and volume of terminal lakes are highly sensitive to rates of water inflows. Simulation of hydrologic interactions with terminal lake systems are often complicated by large changes in location and extent of the lake interface with stream and groundwater inflows and location where upstream river and groundwater interactions take place. Innovative methods were developed and incorporated in a model of the lower Walker River basin to address these complexities.

Walker Lake is a terminal lake in west-central Nevada that receives 83 percent of its inflow from the Walker River, and 17 percent of its inflow from other small tributaries, precipitation, and groundwater. Diversions from Walker River have supported irrigated agriculture upstream of Walker Lake over the past 140 years. Under this flow regime, Walker River only provides inflow to Walker Lake a few months during wet years when all upstream appropriated water rights are satisfied and upstream reservoir storage is full. As a result, Walker Lake has been receding at rate of 1.9 feet per year and salinity increases have decimated the native Lahontan Cutthroat Trout (Oncorhynchus clarki henshawi) fishery and the lake ecology is rapidly transitioning from a freshwater to saline water system. In October 2009, United States Congress established the Walker River Basin Restoration Program with the purpose of restoring Walker Lake for the recovery of the threatened Lahontan Cutthroat Trout and the efficacy of the lake as a stopover point for migratory waterfowl. Success of the restoration effort relies on understanding the system hydrology and impacts of water management decisions on lake inflows, levels, and salinity.

An integrated surface-water and groundwater model was developed to help guide restoration efforts. The model used innovative methods to simulate changes in surface area of Walker Lake up to 100 km² and extension of Walker River up to 15 km. The calibrated model has provided important insights into system response to water management decisions and has increased the confidence of water managers and other stakeholders with the potential outcomes. Consequently, negotiations between stakeholders has evolved from focusing on uncertainties of the response of the system to management decisions to establishing realistic target restoration goals.