Hydrological processes in East African tropical montane headwater catchments influenced by land use

Tropical montane forests play an important role in the supply of fresh water, but are also characterised by fertile soils and a suitable climate for agriculture. Therefore, the conversion of montane forests to agricultural land to meet the increasing demand for food is an important threat for these forests. Such land use changes likely affect the hydrological cycle, but relatively little is known about these effects, especially in African mountain regions. This study assessed how hydrological processes are influenced by land use in the Mau Forest, Kenya. Three subcatchments (27 to 36 km(2)), characterized by either tropical montane forest, smallholder agriculture or commercial tea plantations within a 1,021 km(2) catchment, were instrumented with automatic measurement stations for water level and water quality. The tea plantations showed the highest nitrate concentrations in the stream (1.71 +/- 0.15 mg N L-1). Concentrations were lower in the smallholder agriculture (0.84 +/- 0.06 mg N L-1) and lowest in the montane forest (0.40 +/- 0.04 mg N L-1). Although lower annual specific discharge was measured in the smallholder agriculture subcatchment (530 +/- 188 mm yr(-1)) than in the tea plantations (607 +/- 122 mm yr(-1)) and the tropical montane forest subcatchments (659 +/- 153 mm yr(-1)), interannual and spatial differences in precipitation made it difficult to attribute the observed differences to land use. The investigation of hydrological processes using trace elements and stable isotopes of water showed that the contribution of groundwater to streamflow increased during high flow in the agricultural subcatchments. This was confirmed by an in-depth investigation of nitrate and discharge dynamics. Further differences between the catchments were obtained from the analysis of nitrate hysteresis loops for rainfall events, which indicated the occurrence of surface runoff in the agricultural subcatchments. Nevertheless, the young water fraction (< 0.2 years old) in all subcatchments was low (< 15 %). This indicates that groundwater makes an important contribution to streamflow, irrespective of land use. Therefore, it seems that, despite small differences in hydrological processes, land use seems to have a larger effect on water quality than on water quantity in this tropical montane region.