TOWARDS A COMPUTER MODEL TO SIMULATE DAILY WATER LEVELS IN SEBAGO LAKE

Sebago Lake (southern Maine) provides economically and intrinsically valuable recreational, commercial and environmental services. Different stakeholder groups for the 118 sq. km. Sebago Lake and its surrounding watershed advocate for different management strategies for the lake and its contributing waterways. Because limited hydrologic data is available to quantify processes that govern tributary flows and lake water levels, a 'simple' water-balance model was constructed (and continues to be modified) with the goal of creating a tool to guide lake management decisions. We present our efforts to collect data and construct a model that includes the following components: 1) watershed evapotranspiration using a McGuinness-Bordne style models, 2) river flow using the GR4J lumped parameter watershed models modified to include a degree-day snowmelt model, 3) a lake evaporation using Morton's deep lake evaporation model (CRLE), and 4) a lake outflow model based on stage discharge relation. Surface flow measurements in nine tributaries to Sebago Lake have been collected during the past four years. Precipitation, temperature and other weather data have been consolidated to create spatially averaged values for sub-basins draining to the lake. Simulations using the lumped parameter watershed model with estimates of watershed evaporation and precipitation produce results that are close to measured streamflow data (Nash-Sutcliffe = 0.4 to 0.9) in the monitored tributaries that do not contain hydraulic structures actively manipulated by humans. Preliminary results from the lake evaporation model indicate Sebago Lake typically annually loses 71 to 76 cm of water to evaporation.