Spatio-temporal Dynamics of Land Use Change on Rivers in Tropical Watersheds: a Case Study of Ruiru and Ndarugu Basins, Kiambu County, Kenya

Abstract

Land use dynamics are known to cause considerable modifications to the environment which can cause severe impacts to aquatic natural resources. This study addressed the impacts of land use change in the Ruiru and Ndarugu river basins in Kiambu County of Kenya. The objectives of the study were to determine the land use and land cover trends, implications of land use practices on physico-chemical and macro-invertebrate richness and to create an impact model for the assessment of land use effects on hydrological parameters in the Ruiru and Ndarugu river basins of Kenya. The hypotheses used to guide the study were that no change occurred in land use practices between 2005 and 2015 in the study basin; land use and land cover systems in the basins do not have significant impact on water quality and macro invertebrate composition and; simulated water quality parameters do not significantly differ from measured water quality parameters in the basins. The data used in the study included spatial datasets made up of satellite imagery for the study area sourced from Landsat 7 and 8 (TM and ETM), administrative boundaries sourced from Survey of Kenya, Soil data from Soil Survey of Kenya, digital elevation model sourced from United States Geological Survey (USGS), climate data from world weather database, all of which were secondary data. Other secondary datasets used in the study included rainfall and temperature data from Kenya Meteorological Department and long term water volume flow data from Water Resources management Authority (WRMA). The study also used primary data collected in situ within the basins on the physical-chemical water quality parameters (Water Temperature, Dissolved Oxygen, pH, Electrical Conductivity and Turbidity). The resulting datasets were used to assess the land use change dynamics impacts on water quality using the Soil and Water Assessment Tool (SWAT) model which is a spatial distributed GIS-based hydrological model. The land use change assessment indicated that a significant change in land use and land cover had occurred between 2005 to 2015, with both watersheds changing to a more urbanized landscape; land use and land cover systems had significant influence in water quality parameters and macro invertebrate composition in the two basins and; the SWAT outputs closely matched with the in situ water quality measurements. The study concluded that the changes in land cover and land use were mainly due to increasing population pressure, and land cover is projected to continue to change towards an urbanized landscape, land use and land cover changes contribute to changes in water quality parameters and; the SWAT model with local modifications is a suitable tool for monitoring and management of land use, water quality and macro invertebrate assemblages in the Ruiru and Ndarugu basins. It is recommended that the use of Intergrated Watershed Management (IWM) based on scientific approaches to be adopted by both the County and National governments; the use of ecosystem management techniques in basin management to be incorporated in land use policies (such as the adoption of blue-green infrastructure) and; the use of hydrological models such as SWAT in data-scarce regions must allow the modification that suits local conditions to avoid over or under-estimation of hydrological parameters.