Temporal and spatial variations in future hydrological processes under a changing climate in the humid river basins of west flowing rivers of India

The present study examines and compares the spatial and temporal variations in the future streamflow across four South Indian river basins, humid river basin of Chaliyar (CHAL) and dry sub-humid river basins of Ithikkara (ITHK), Vamanapuram (VMPM), and Thamiraparni (TMPR). To simulate the future streamflow, an ensemble of five general circulation models (GCMs) was integrated into Soil Water Assessment Tool (SWAT). Simulations were conducted for three future periods: the near future (2011–2040), mid future (2041–2070), and far future (2071–2100) under two scenarios, climate change alone and climate–land use/land cover (LU/LC) change. Compared to the humid basin (CHAL), which shows an overall increase in stream flow during the southwest monsoon (up to ~25 m³/s), and a moderate rise during the northeast monsoon (up to ~10 m³/s), the dry sub-humid basins (ITHK and VMPM) exhibit a contrasting trend with a significant decline in streamflow during the southwest (up to ~30 m³/s) and northeast monsoons (up to ~46 m³/s). In between these extremes, TMPR exhibits spatially mixed trends, with some regions experiencing a significant increase and others a decline; on average stream flow rises by up to ~10 m³/s during both monsoon seasons. These insights can help in developing an effective river basin management plan and show the need for river basin-scale impact studies in a region for various scenarios.

In humid river basin, Chaliyar, streamflow is expected to decrease in June followed by an increase in July to September, indicating a possible shift in the onset of the southwest monsoon.