This video shows how I computed sub-basin areas for the Kaveri river in the Indian states of Karnataka (KA) and Tamil Nadu (TN). Data for drainage areas came from AQUASTAT and was processed in QGIS open source software. I then created this video using Flashback Express 5 screen recording software.
Intersect layers for administrative boundaries and sub-basin areas to obtain sub-basin areas by state
Extract sub-basin areas by state layers into two separate layers (KA and TN)
Dissolve the two layers to get total area by state
The values I computed for drainage areas: 35462 sq km (KA) and 44422 sq km (TN)
India Water Resource Information System (India-WRIS) provides these values for drainage areas: 36240 sq km (KA) and 48730 km2 (TN)