USING LUMPED PARAMETER DRAINAGE-BASIN MODELS TO ASSESS LAKE LEVEL IN A MANAGED LAKE SYSTEM

Sebago Lake, located in southern Maine, is a municipal drinking water source area for about 200,000 people in the greater Portland, Maine region. In addition to supplying drinking water, this 118 sq. km lake and the surrounding drainage basin, serve a wide variety of recreational, commercial and environmental services. Different lake-usage goals among interested groups drive advocacy for different lake management styles. Currently, a lake-level management plan adopted in 2000 is used to guide decisions at the only outflow, the Eel Weir Dam, where surface-water discharge is controlled. This plan specifies a 30 cm (1 ft.) range for lake water level throughout most of the year.

Several task are underway associated with the preparation of a simple computer model being prepared to assess factors that influence lake-water level. Anticipated use of this interactive modeling system include: 1) education of groups about the impacts of management options, 2) sensitivity analysis of different parameters used in the model, and 3) short-term forecasting of river discharge and associated lake level based on weather forecasts, potentially to assist in management decisions at the Eel Weir Dam. Completed activities include: 1) monitoring stream discharge to Sebago Lake through stream gaging and installation of water level data loggers, 2) creation of a simple lumped-parameter drainage-basin model based on GR4J (Perrin et al. 2003, J.Hydro. 279:275-289), and 3) development of a preliminary interface to this modeling system allowing interactive access to the model through the Internet. Currently, rating curves for the major streams flowing into Sebago Lake are being developed, allowing the use of continuously monitored stream stage to predict stream discharge rates. These data will be used to calibrate a model for each sub-basin, and stream discharge associated with each sub-basin will be used to estimate inflow into Sebago Lake and calculate lake level.