Effects of land use change on the soil physical properties and stream-flow - in bathi catchment (BC) in Lari Division Kiambu District

Abstract

This study examines effects of land use changes on the soil physical properties and - stream-flow response in the Bathi Catchment (BC) in the Kenyan highlands. Land use changes in the tropics affect soil-water and energy budgets, and possibly leading to climate change. Little information on the impacts of land use changes on soil physical properties and hydro-climatic variability exist in Kenya. Four soil physical properties most prone to land use changes were examined. Land use changes were delineated from aerial photos of 1958 and 1982, planimetrically. Both field and lab measurements were carried out on five categories of land use. These included natural forests (NF), pine plantations (PP), pastures (PA), old cultivated (OC) and newly cultivated (NC) where infiltration rates (L) and hydraulic conductivity (Ks) tests were done. Bulk density (Pb) and soil-moisture content ~F were carried out at the National Agricultural Research Laboratories (NARL). After t = 5 minutes, I, and pF were significantly high in NF and PP with mean values of 16.5 ± 0.01 ern min", 19.96 ± 0.01 cm minot and 48.4% and 47.6% respectively. NF and PP had the lowest bulk density (Pb) of 0.58 ± O.Olg cm-3and 0.62 ± O.Olg cm' respectively. On the other hand, after t = 5 min. comparable rates of 10.52 ± 0.0I cm min , and 12.96 ± 0.01 ern minot coupled with high bulk densities (Ph) of 0.71 ± O.Olg ern" and- 0.68 ± 0.01 g cm-3 respectively were recorded in OC and NC. PA portrayed the ! highest Pb of 0.77 ± O.Olg ern" with mean pF: 42.8%. After t =10 min. I, changed drastically for PA with rates of2.0 ± 0.01 em min-I occurring. Like NF and PP, NC and OC showed almost similar rates :::::10.0± 0.01 em min-I and 8.0-9.0 ± 0.01 respectively. K, measuring soil-water transmissivity was slowest in NF, PP and PA with values of 0.0402 em rnin", 0.0376 cm minot and 0.0107 em min-I respectively. OC and NC recorded 0.0942 and 0.1205 cm minot for each. K, in OC and NC were about three times greater than in NF, PP and PA Mean monthly rainfall and stream-flow data were examined. Ordinary least square regression (OLSR) and moving averages were used to characterize hydro-climatic variability trends. January-March and April-August seasonality was established. Decreasing annual trends were observed with Kinale and Kamae weather stations having significant correlations of35.61% and 41.37% respectively. Similarly, decreasing annual trends with low correlation of 9.0% and 8.25% at Kereita and Uplands stations, respectively were observed. These values were significant at 95% confidence level (CI) for two-tailed t-test. Mean annual stream flow discharge decreased by -0.054 cu.m/s at RGS 3BC 18 since early 1960s with a significant correlation of 61.6%. This study established that dramatic land use changes have occurred in Bathi Catchment 1 since the early] 960s whereby areas under natural forests, shrubs, pines, bamboo as well as riverines decreased. On the other hand, areas under cypresses, settlements, infrastructure and arable land increased. These land use changes have equally altered soil physical properties as well as stream flow characteristics resulting to decline in mean annual precipitation amounts. In order to sustain equitably densely populated rural catchments like Be, it important for farmers to observe proper soil -water balance management practices including agro-forestry, mulching, bee- fanning, creation of hufTer zones and multi-layered gardens.