Effect of small catchment dams on downstream vegetation of a seasonal river in semi-arid African savanna.
Abstract
Seasonal rivers in semiarid African savannas support riparian woodland and hydromorphic grassland, key habitats for wildlife or livestock. We investigated the effect of numerous, small, agricultural dams built on low-order streams within the Kolope-Setonki subcatchment of the Limpopo river, South Africa. We examined how reducing or changing the pattern of water flow in the main rivers will affect the riparian or hydromorphic vegetation. Existing hydrological models demonstrate a mean annual water flow of 7.6×106 m3, ranging from zero to 215×106 m3. The catchment area affected by agricultural dams increased from 2% to 50% between 1955 and 1987. Dams are believed to have curtailed the flow during years of low flow. The water requirement of four key species was assessed from their elevation above and distance from the river such that Faidherbia albida>Schotia brachypetala - Xanthocercis zambesiaca>Combretum imberbe. This order did not accord with the relative degree of mortality and canopy dieback. Twenty-nine percent of F. albida trees were dead and the remainder had lost an average of 31% of canopy volume; S. brachypetala and X. zambesiaca exhibited negligible dieback and no mortality; whereas 10% of C. imberbe were dead and the remainder had lost an average of 8% of canopy volume. Tree size and water availability influenced mortality and canopy dieback of F. albida and C. imberbe. Mortality of F. albida affected trees of all except the smallest size, although dead trees were on average larger than living trees. Dead trees of C. imberbe were at a greater elevation above or distance from the river. The likelihood of F. albida experiencing dieback was greater at higher elevations above the river and for larger trees, whereas that of C. imberbe was greater at greater distances from and higher elevations above the river if a tree was large. Woody cover on hydromorphic grasslands, mainly of Acacia tortilis, increased from 8 to 37% between 1955 and 1987. Most of the increase occurred between 1965 and 1977, coincident with the extended 1960s drought and the above-average rainfall of the 1970s in a system that had been exposed to sustained, severe grazing by livestock. The cumulative impact of many small farm dams on downstream rivers is apparently to reduce flow during critical dry years to levels causing dieback of F. albida and C. imberbe and desiccation of hydromorphic grassland. The water requirements of this vegetation need to be determined for policy makers to address the conflicting water needs of agriculture and indigenous vegetation.