Adaptive simulation of the impact of changes in land use on water resources in the lower Aswa basin

In the lower Aswa basin, Uganda, the changes in land use due to complex demographic and social economic factors are among the numerous challenges facing management of the limited water resources. The current study analysed the degree to which water yield in the Aswa basin could be changed by altering the vegetation cover (here considered to be agricultural use and forest) at the basin and sub-basin level, and whether manipulation of vegetation cover can complement water resource management objectives in the study area. The distributed hydrological process Soil Water Assessment Tool (SWAT) model was used to simulate the impact of the changes in vegetation cover on water balance. The impact was compared with the water balance simulated using the year 2001 as baseline. The results showed that: 37.5% afforestation at the basin scale can reduce water yield by 15.85%; using 53.7% of the land for agriculture can increase water yield by 27.6%, while a combination of 23.2% forest and 52% agriculture can increase water yield by 24.85%. The location of forest and agricultural land cover with respect to rainfall regime also indicated a notable impact on sub-basin water balance. In particular, afforestation in sub-basins receiving less than 900 mm annual rainfall considered as dry showed minimum change in surface runoff and net water yield, while in sub-basins receiving more than 900 mm annual rainfall afforestation showed notable change in water yield. In this way, afforestation in dry sub-basins can be used to offset the afforestation pressure in the wet sub-basin without altering the basin water balance.