Assessing the impact of land use on water and nutrient fluxes in the South-West Mau, Kenya

Abstract

Although tropical montane forests are considered to provide important water-related ecosystem services, such as the supply of clean fresh water, these forests are under significant threat of conversion to agriculture, because of their suitable climate and fertile soils. However, there is little scientific evidence on how land use change affects water and nutrient fluxes in this important ecosystem, especially in Africa. To assess the effect of land use on water provenance, flow paths and nutrient dynamics in the Mau Forest Complex, Kenya, three sub-catchments (27-36 km²) with different land use (i.e. natural forest, smallholder agriculture and commercial tea plantations) were selected within a 1,021 km² catchment. Spatial sampling campaigns showed that land use had a significant effect on nitrate and total dissolved nitrogen concentrations during baseflow, while dissolved organic carbon and nitrogen were more influenced by the hydrological regime and catchment characteristics, such as catchment area. A 2-year high-resolution (10-minute interval) dataset recorded by automatic measurement stations located at the outlet of the sub-catchments showed highest nitrate concentrations in the tea plantation sub-catchment, followed by the smallholder agriculture sub-catchment, caused by leaching of fertilizer to the groundwater. In the natural forest, nitrate entered the stream through shallow sub-surface flow during rainfall events, while surface runoff resulted in dilution of nitrate concentrations in the agricultural sub-catchments. A tracer-based study estimated a mean transit time of ~4 years and showed an increased contribution of groundwater sources to streamflow during high flow in all sub-catchments, emphasising the importance of groundwater in this area. The majority of stream water in the tea plantation sub-catchment originated from springs, while precipitation dominated in the forest and smallholder agriculture sub-catchments. The results clearly show that land use influences water and nutrient dynamics, which could have a detrimental effect on water supply and quality in tropical montane areas and downstream regions.