Analysis of event-based soil hydrologic response for different land use types in upper Njoro river catchment

Abstract

Land use/cover change alters the hydrologic response of river catchments. The response depends on the soil characteristics such as; infiltration capacity, porosity, organic matter content and bulk density. Understanding the effect of the land use change on soil hydrologic response in Upper Njoro River catchment is crucial for formulation and adoption of proper soil and water conservation techniques for increased agricultural production. Additionally, the characteristics of extreme events in terms of magnitude, frequency and duration are key to proper planning and management of water resources. The objective of this research was to evaluate event-based hydrologic soil hydrologic response for different land use types in Upper Njoro River Catchment. Randomized Block experimental design was used where four land use types were purposely selected and blocked against soil hydrologic response parameters and soil properties. The land use types included; natural forest, deforested land, fallow agricultural land and grassland having been indicated to be the major land use types within the catchment. The evaluated soil hydrological response parameters included runoff coefficients, infiltration rate, and water repellency and ponding time and soil properties. Gross rainfall partitioning across planted trees and characterization of annual peak discharges for Njoro River using selected probability distribution functions was also carried out. The findings showed fallow agricultural land, grassland and natural forest to have textural classes of sandy clay loam, clay loam and loam respectively based on USDA textural triangle. The highest and lowest mean bulk densities of 1.36 and 0.96 g/cm3 were found in deforested and natural forest land respectively. The highest mean saturated hydraulic conductivity of 170.21 cm/day, porosity of 0.62 and organic matter content of 4.63% were observed in the natural forest while the lowest values of 24.78 cm/day, 0.47 and 0.75% respectively were found in the deforested land. High water repellency of 18 seconds penetration time and runoff coefficient of 0.0077 were observed in the deforested areas. The quantified rainfall partitions was 43.04% for stem and through flow and 56.96% for interception. The best fitting probability distribution model was found to be Generalized Pareto. Peak flow discharges estimated for 50, 100 and 200 year return periods using the best fitting model included 8.1 m3/s, 10.1 m3/s and 12.3 m3/s respectively. The results showed that deforestation has affected soil hydrologic response and soil properties negatively. The study provided understanding of on-site effect of different land use types on hydrological functioning of soils which in turn affect response of stream flow. Future work should aim at restoring the hydrological functioning of soils within the catchment.