SUMMARY
This paper presents flood analysis of Tarbela sub-catchment using remote sensing (RS) data and geographical information system (GIS) techniques. Tarbela catchment is largely supplemented by glacier and snow melt as well as rain, which are considered as the most critical flood-producing factors in the basin. Among various flood control measures, dams are considered as flood controlling structures. The world's largest earth filled dam, Tarbela Dam, is located in Pakistan. This study focused on rainfall-runoff modeling and estimated surface runoff in the Tarbela catchment with the help of hydrologic simulation model (HEC-HMS with HEC-GeoHMS). Drainage area, stream network and slope of the catchment were generated using Arc Hydro extension of ArcGIS and ASTER DEM of 30m resolution. Soil type and land cover/use characteristics play an important role in the amount of surface runoff reaching the nearby streams. Land use/cover details were derived from LANDSAT satellite imagery. Soil maps and land use/cover were used to develop curve numbers of the Tarbela sub-catchments. In-situ data comprising precipitation, temperature, humidity and wind speed were acquired from manual and automatic weather stations (AWS) of Pakistan Meteorological Department. ERA-Interim data set of precipitation, temperature, evaporation and snowmelt was used for un-gauged area of the catchment. The temperature and precipitation data set of CRU was used for historical climatic background of study area from 1900-2014. The Natural Resources Conservation Services (NRCS) runoff curve number method was selected to estimate precipitation excess and applied for estimation of flood hydrograph. The output of HEC-HMS simulation model was a runoff hydrograph that was validated with the available discharge data at the catchment outlet. The study area is a complex one because the inflow at Tarbela consists of contribution from rain, surface runoff, snow and glacier melt as well as base flow. The contribution of the lower part of a basin comes from rainfall-runoff and snow melt whereas the high altitude region, the runoff derives from snow and glacial melt. The synthetically developed sequence of hydrological data can be used for flood forecasting.