Hydrometeorology
Climate and Water Regime
The basin’s hydrology is governed by two primary seasons: winter (December–February) and summer monsoon (June–September). Climatically, the region divides into the Himalayan Zone, Sub-Himalayan Zone, and Plains. The Himalayas receive heavy precipitation, sometimes exceeding 50 inches annually primarily as snow, while rainfall in the plains declines south-westward to less than 20 inches, and in Sind often below 5 inches.
Crucially, the Indus system depends heavily on snow, accumulates during the winter and then high summer temperature generates enough energy to gear up flow of water in river channels that continues incessantly for about three months (July-September) and often brims out of banks and brings heavy floods in the upper and lower Indus plains.
Seasonal Flow Patterns
Although perennial, the rivers display strong seasonal variation. Flow is lowest between mid-December and mid-February, rises sharply during spring snowmelt, and peaks during July–August due to combined effect of snowmelt and monsoon rainfall. Of the total annual supply of 170 MAF, the western rivers contribute 137 MAF including approximately 23 MAF from the Jhelum, 26 MAF from the Chenab, and the remainder 88 MAF largely comes from the Indus main stem. The eastern rivers contribute 33 MAF water including 6 MAF (Ravi), 13 MAF (Beas), and 14 MAF (Sutlej).
As these rivers descend into the plains, their hydrological characteristics are further shaped by physical and human geographical factors. Flowing across long alluvial tracts with gentle gradients, the rivers experience significant evaporation, seepage, and floodplain storage, often referred to as valley storage. Downstream discharge may differ considerably from upstream measurements due to these natural losses. The rivers have high capacity of sediment suspensions during flood periods, contributing to shifting channels and the formation of fertile plains. Annual flows vary substantially from year to year despite consistent seasonal patterns, making water storage and regulation essential for irrigation planning.
Water Balance
Significant water losses occur as rivers traverse long plains evaporation, seepage, and percolation are substantial over stretches such as the 1,000-mile Indus and the 500-mile Sutlej. Floodwaters are temporarily stored in riverbanks and adjacent lands, a process referred to as “valley storage.”