CALIBRATING A LUMPED PARAMETER DRAINAGE BASIN MODEL TO ESTIMATE STREAM DISCHARGE TO SEBAGO LAKE

Sebago Lake supplies approximately 200,000 people in Maine with drinking water and is a multi-use recreational resource. Lake-level management has historically been an issue of debate and remains one today, yet there is limited knowledge of the environmental dynamics that interact to control lake level, especially with regard to the quantity of surface water flowing through the system. The natural dynamics in the system are further complicated by Eel Weir dam, which controls the release of water from the lake. A lake water budget model was developed to simulate lake-level fluctuations in the context of climate, land-use change, and evolving lake-level management plans. Stream discharge and stage data collected between April 2010 and December, 2012 was analyzed to quantify surface flows in nine of the largest in-flowing rivers. A USGS program entitled Streamflow Record Extension Facilitator (SREF) (Granato, 2009) was used to estimate stream flow where data was missing. Hydrographs for each of the nine rivers were created based on rating curves established for each river from stream stage and flow measurements over a range of flow conditions. The continuous discharge data for each river were used to calibrate a modified version of GR4J, a lumped parameter runoff model (Perrin, 2001). The model will serve as platform from which simulations of lake level fluctuations with respect to changes in precipitation, runoff, and temperature can be shown in conjunction with various discharge schedules from the dam. Thus, the intended utility of the model is as a tool in water resources decision making and planning, with specific regard for its application to lake level management issues.