If it's a drainage basin then it means all water within a particular catchment (rivers, stores, groundwater)
So how does deforestation affects these.
Plants intercept and store water. So same water is recycled back to the basin when evapotranspiration causes rainfall. So lack of trees means less evapotranspiration & therefore reduced water supply.
Plant roots also facilitates percolation. This percolating water can accumulate underground as groundwater. The groundwater can be resupplied back to rivers as baseflow
Lack of trees increase runoff which can channel water out of a basin if there's no storage mechanism that can intercept & retain the water within the basin