Estimating the sensitivity to climate change of the water budget of a mountain basin using a version of the Thornthwaite model

Estimating climate change at the sub-GCM-grid scale can be very difficult. Several downscaling methods, including both statistical and dynamical models, are currently being used. Prior to the employment of such models to estimate potential climate changes in a sub-grid scale region, however, it is important to estimate the sensitivity of the region to change. Here we present a version of the Thornthwaite water balance model, which includes the snow pack water reservoir of Willmott et al. (1985) as well as additional modifications including ground water storage and a method for running the model in a spatially distributed mode, to estimate the sensitivity of a mountain basin to temperature and precipitation changes. The model requires daily temperature, precipitation, and (if available) snowfall observations as input. The model estimates evapotranspiration, base flow, and runoff. The model is presented using as an example a basin in the Catskill Mountains of New York State, a region that provides water for ~10 million people.