Characterizing Snowmelt Regime of the River Swat - A Case Study

Abstract

Snowmelt generates 70 to 80% of runoff of Indus River and its tributaries. Forecasting snowmelt generated flow is important for water management, reservoir operation and channel diversion. River Swat being not direct contributor to the existing reservoirs remained out of focus for characterizing its snowmelt regime. Thirty years (1971-2000) data of upper Swat catchment above Kalam gauging station was acquired from WAPDA. Normal monthly values over the period and average monthly values of each year were determined for stream flow, precipitation and temperature together with average monthly values of weighted and maximum temperature. Snowmelt regime was ascertained from plot of normal values of flow, precipitation and temperature. Using temperature index approach, average monthly flow over the snowmelt months (April, May and June) in terms of mm depth over the catchment was regressed on all the temperature indices using exponential, power and third degree polynomial functions. T was found the best max 2 index for snowmelt with R as 0.902 for the third degree polynomial function. Runoff coefficient (ROC) for the total precipitation was conceptualized and through iteration was found as T /100. The optimized value of max ROC was used to segregate rain induced and snowmelt induced runoff. The segregated snowmelt induced runoff was again regressed on T using the same max 2 function which slightly improved R to 0.916. The model was tested for four years of data and forecasted flow was found reasonable in the context of simplicity of the approach.