Hydrological And Erosion Processes In The Ethiopian Highlands

Abstract

With the objective to ameliorate the impact of recurrent drought and severe erosion, nonindigenous soil and water conservation structures are ubiquitous in the Ethiopian highlands. Nevertheless, erosion and drought remain critical problems in the sub-humid and humid highlands. The less than optimum performance of the non-indigenous practice is caused in part by not taking the varying landscape and climate conditions into account that determine the spatial pattern of runoff and erosion and thereby the performance of soil and water conservation practices. This dissertation research was conducted with the objectives of understanding runoff generation processes and spatial and temporal runoff and erosion patterns at different (plot and watershed) scales. In addition, we investigated the potential of biochar, charcoal, and deep-rooted crops to improve soil hydraulic properties and greenhouse gas emissions. Field and laboratory experiments were conducted in the 113 ha Anjeni watershed during the 2012 and 2013 rainy monsoon phases. Field infiltration tests were conducted and soil samples were analyzed for selected soil parameters. Soil column experiments were conducted to assess the effects of biochar and charcoal amendments on moisture retention and permeability of soils. Runoff and erosion rates were measured from 24 runoff-erosion plots and at watershed outlet. In addition, gas samples were collected using static chambers and analyzed for nitrous oxide and methane emissions. Results showed that while poor soil conditions (acidic, high clay and low organic matter content) are common, saturation excess runoff was the dominant runoff mechanism. Rainfall intensity would exceed median infiltration rate only 21% of the time. Soil degradation level and tillage significantly affected runoff. Sediment concentration and yield increased with greater plot length from 3 m to 15 m, but decreased at a plot length of 30 m. Sediment rating coefficients were also affected by plot size and with the progression of the rainy monsoon phase. Wood charcoal improved permeability of soils near saturation (10 and 30 kPa). However, effect of charcoal on runoff and erosion at plot level was not significant. Significant differences were observed between seasonal nitrous oxide emissions, with greater fluxes observed during the end of the rainy monsoon phase. Overall findings of this dissertation research imply that hillslope runoff and erosion rates are greatly variable both spatially and temporally. Taking the spatial pattern and mechanisms of runoff generation into account is of paramount importance for improving the performance of newly installed soil and water conservation practices.