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.