Effects of soil type, vegetation and land use on infiltration rate and related soil physical properties in two semi-arid catchments in Laikipia District Triticum aes tivum l.)

Abstract

Effects of soils, vegetation and land use on infiltration rates and related soil physical properties of the semi-arid Sirima and Hukogodo catchments in Laikipia district were investigated. Representative six soils in Sirima and four soils in Hukogodo, surveyed at detailed level, were assessed. Within soil types, four site condition treatments with respect to vegetation cover and land use designated as Tr_ee/Bush _(TB), Open Grass (OG), Bare Ground (BG) and Cultivated Surface (CS) were studied. Infiltration rates were measured during a dry and a wet season by a double cylinder infiltrometer. The top three soil horizons bulk density, saturated hydraulic conductivity, organic carbon, texture, antecedent water content and water retention were also investigated . In each catchment, within horizons, all the soil physical parameters except texture were consistently significantly different (p<.05) among site conditions; but rarely among soils. Also, within soils among site conditions, the surface horizon physical parameters exhibited significant differences (p<.05) more often than those of subsurface horizons. Infiltration rates were significantly different (p<.05) among soils and site conditions in both catchments. The

difference among soils in Sirima was due to the occurence of vertic and non-vertic ~oils; while in Hukogodo it was due to the occu'ren ce of so iIs formed in situ and those deve loped from alluvial material. Among site conditions, in both areas and seasons, intake rates were highest under TB, intermediate on OG and lowest on BG. Cultivation temporarily increased infiltration rates above those of TB sites, however, due to clod collapse and subsequent high susceptibility to surface sealing , most of the increase was eliminated within a short period. In both areas, within each season, bulk density, carbon results saturated hydraulic significantly correlated conductivity and with infiltration organic rates and indicate stronger surface horizon correlations relative to subsurface horizons. All investigated parameters except clay and silt in Sirima and clay in Mukogodo were identified in multiple regression as significant in influencing variation in infiltration rates.Soil cracking and swelling in Sirima during the dry and wet season respectively, partially obscured the influence of the physical parameters on infiltration rates. Consistently, Sirima soils retained more water than Mukogodo soils at all suctions, but the later had more available water. In both areas, BG surface soils retained significantly (p<.05) less water and also held the least amount of available water. Cultivation reduced water retention of surface soils, but had no significant influence on water availability. Depletion of vegetation due to excessive livestock grazing and cultivation resulted in deterioration of surface soil physical properties important in soil water intake, storage and availability.