Sebago Lake Watershed Model

Two avenues of computer simulation are being pursued by faculty in the Environmental Geoscience group within the School of Earth and Climate. Sean Smith and his graduate students are applying distributed computer models to individual basins (eg. Northwest River) in the Sebago Lake Watershed. This work has only recently been started, with the intent of assessing the influence of changes to the watershed at different spatial scales.

In contrast, Andrew Reeve has written a computer program based using a lumped parameter approach, where all the variability across a watershed are averaged spatial change is not directly dealt with in the computer model. The lumped parameter models are based on GR4J, a daily four parameter watershed model developed at Cemagref (France). This page will focus on this modeling approach.

A computer model to simulate lake level has been developed by coupling lumped parameter watershed models, a reservoir (lake) model, and lake outflow models. The lumped parameter models are based on GR4J, a daily four parameter watershed model developed at Cemagref (France). The basic model has been re-coded into the python scripting language and modified to include reservoir for snow accumulation. In this python implementation, potential evapotranspiration is calculated from average daily temperature and the latitude. Daily precipitation (entered) and potential evapotranspiration (calculated from temperature) are used to calculate a moisture surplus used to add or remove water from a snow reservoir, soil reservoir, routing reservoir, and finally to river flow (discharge).

Flow chart for modified GR4J model.

Flow chart for modified GR4J model used to simulate watershed hydrology.

In the modified model, a snow storage reservoir has been added to GR4J. Two threshold temperatures are assigned to the model: a freezing temperature and a melting temperature. When the temperature drops below the freezing temperature, all precipitation is assumed to be frozen and accumulates in the model's snow reservoir. When average air temperature is greater than the melting temperature, water stored in the snow reservoir is released based on the following equation.

\begin{equation*} Q=K_{melt} \cdot (T_{air} - T_{melt}) \end{equation*}

The Watershed model can be accessed here.

In this equation, 'K(melt)' is a melting rate constant that will vary from watershed to watershed depending on tree cover, basin topography, slope orientation, and other factors. The discharge (Q) is the amount of water released over a day, and this is subtracted from the snow reservoir until no frozen water is left.