A Simple Monthly Runoff Model for Snow Dominated Catchments in Western Himalayas

The rivers originating from middle and greater Himalayas have a significant part of their catchments under permanent snow cover and glaciers. Modeling runoff becomes difficult with almost no data from these parts. Even in the seasonal snow covered zones, the network is generally inadequate. However precipitation characteristics show repetitiveness and snowline movement elevation wise by and large occurs the same pattern each year. The snowline movement is distinct on a monthly basis and the location of permanent snowline is also more or less constant at about 4,500 m. A simple monthly snowmelt runoff model with relatively few parameters is proposed to take advantage of above mentioned characteristics, using the degree day method. The model uses monthly rain, snow (snow water equivalent), mean air temperature and snowline elevation as primary inputs. Model conceptualisation has been made in view of the data constraints. All parameters are estimated through few trial simulations, except the storage coefficient, which is optimised using Rosenbrock technique. The model was applied on two sub-catchments of Chenab basin (of Indus river system) to evaluate the model capability. The results are encouraging. There is further scope for model improvement, generalisation and for application to other catchments in the Western Himalayas.