The effect of land use on infiltration in Taita Hills, Kenya

Population growth and the conversion of forests to agricultural lands is a typical phenomenon in the highlands of East Africa. Land use changes set pressure on ecosystem services and natural resources, such as fresh water, which is essential for human well-being. Soil forms the largest free fresh water storage on planet. To maximize the potential of groundwater reservoirs in a continually changing environment it is extremely important to understand the factors controlling the infiltration process. The effect of land use on infiltration has been studied broadly but due to the increasing water scarcity and difficult geographical accessibility, the amount of infiltration studies conducted in the highlands of East Africa is low. The aim of this study was to examine whether land use affects infiltration in Taita Hills (3°25' S, 38°20' E), a tropical highland environment in southeastern Kenya, and whether the changes can be explained by the changes in soil properties. Another aim of this study was to examine whether the collected field infiltration data can be modelled with fixed infiltration models, which could potentially decrease the need for time and water consuming field infiltration measurements in future. This study focused on three land use classes: forests, cultivations and grazing lands. The collected field data consisted of field infiltration measurements (n=50) and corresponding soil organic carbon, soil organic nitrogen, soil bulk density, soil moisture and soil texture samples. The effect of land use on infiltration was examined with the one-way ANOVA and pairwise t-test. The relationship of soil properties and steady state infiltration rates was investigated with simple linear regression models. According to the results of this study infiltration rates vary by land use. The mean infiltration rates were 3900, 1700 and 450 mm/h in forests, cultivations and grazing lands, respectively, which correspond a decrease of 59 % and 88 % in infiltration rates when forests are converted to cultivations and grazing lands, respectively. According to the produced simple regression models soil bulk density explains 61.5 % and soil organic carbon and soil organic nitrogen 24.0 and 34.1 % of the variation of infiltration rates, respectively. Initial soil moisture explained 15.6 % of the variation but is believed to reflect the climatic and structural conditions of the soil instead of representing a direct impact on the infiltration process. The results suggest that soil texture does not explain the variation of infiltration, which is most likely due to the homogenous soil across the study area. Horton's infiltration model was found most suitable to model the infiltration process within the study area, although the overall performance of the models of Philip and Green and Ampt were also sufficient. The Modifield Kostiakov model was not found suitable to model the infiltration process of the study area.