A SIMPLE ENERGY BUDGET ALGORITHM FOR THE SNOWMELT RUNOFF MODEL

The snowmelt runoff model (SRM) uses a degree-day approach for melting snow in a basin. A simple radiation component was combined with the d egree-day approach (restricted degree-day method) in an effort to impr ove estimates of snowmelt and reduce the need to adjust the melt facto r over the ablation season. A daily energy balance model was formulate d that requires not only the input of radiation but also measurements of daily wind speed, air temperature, and relative humidity. The three approaches for computing snowmelt, namely, the degree-day, restricted degree-day, and daily energy balance model were tested at the local s cale by comparing melt rates with lysimeter outflow measurements. Beca use radiation measurements are not often available, a simple model for simulating shortwave and longwave components of the radiation balance that requires minimal information (i.e., daily cloud cover estimates, air temperature, and relative humidity) was developed It was found th at clouds and their effects on daily insolation at the surface can pro duce significant differences between measured and model estimates. In the comparisons of snowmelt estimates with the lysimeter outflow, the restricted degree-day method yielded melt rates that were in better ag reement with the observed outflow than the degree-day method and were practically the same as estimates given by the energy balance model. A sensitivity analysis of runoff generated with SRM using as input the local snowmelt computations given by the three models and measured out flow from the lysimeter was performed for a basin. A comparison of the synthetic hydrographs for the basin suggests that a radiation-based s nowmelt factor may improve runoff predictions at the basin scale.