HYDROLOGIC MODELLING OF A SNOWFED RIVER BASIN

Abstract

The snow that accumulates in the Himalayas is one of the principal sources of water in the Indo-Gangetic plains of north India. Reliable estimates of the volume of water contained in the snowpack and its release are required for the efficient management of the water resources. One of the general characteristics of the snowfed basins is the accumulation of snow in winter and its depletion during the spring and summer months. The magnitude of the snow storage and its melting rate are controlled by the climatic conditions over the basin. During the melt season, the basin acts like a reservoir draining out continuously through the melt stream. The melt water routes through different stages before reaching the outlet of the basin. The melt water first percolates through the existing snowpack very slowly and reaches the ground surface. On the ground surface it travels as overland flow and then joins the melt stream. The objective of the present study is to develop a snowmelt runoff model for the Beas basin. The study involves the development of a model, calibration and its use for simulation of a snowmelt runoff. The Beas basin upto Pandoh dam site, which covers an area of about 5278 km2 has been selected for carrying out the study. The elevation of the study area varies from 800 m to more than 5000 m. The basin is situated in the western Himalayas and has an extensive snow cover during winter. The snowmelt is estimated using the degree-day approach, which requires only few data for snowmelt computations. Remote sensing techniques have been used to estimate the aerial extent of the snow cover. IRS-1C, WiFS data has been digitally analyzed using the Integrated Land and Water Information System (ILWIS) software. A Digital Elevation Model (DEM) of the study area was also prepared. The aerial extent of the snow cover has been evaluated for each elevation zone. The snow cover depletion curves were derived and the daily snow coverage extracted. The daily runoff is computed separately for the snow covered area and the snow free area which is changing continuously owing to the melting snow, resulting in changes in the snowline elevation. The time lag between the generation of water on the catchment and its subsequent availability as runoff is accounted for by conceptualizing the catchment as two sets of linear reservoirs. The runoff produced from the snow covered area is routed through n, number of linear reservoirs with storage coefficient K,. Similarly, the runoff produced from bare area is routed separately through n, reservoirs with storage coefficient Kr. The routed runoffs are added to get the runoff at the outlet. The study was confined to the snowmelt period only i.e., for the months of April to June. The model has been calibrated for the year 1987 and the Nash-Sutcliffe goodness of fit value is 0.85 and the percentage volume difference is -5.42 %. The model was used for simulating the daily flows for the year 1996, using the same parameter values as obtained during the calibration. The percentage seasonal difference in the volume is - 1.46 % and the Nash-Sutcliffe goodness of fit value is 0.81. For comparison purposes the Snowmelt Runoff Model (SRM) developed by Martinec and Rango, was also applied on the same basin. The Nash-Sutcliffe goodness of fit is 0.77 and 0.72 during the calibration and validation respectively, and the percentage seasonal difference in volume is 7.90 % and 10.06 % during the calibration and validation respectively. It is seen that the performance of the proposed model is better in comparison to SRM. More hydro-meteorological data, regular satellite data, snow surveys for determining the degree-day factor, water equivalent, snow depth etc., are essential for a better. estimation of the snowmelt runoff. The model can be applied to the Beas basin as it gives fairly good results. iv