Organization of the soil profile controls the risks of runoff in the humid Ethiopian Highlands

Erosion of agricultural land endangers the livelihood of millions of people who depend on natural resources. Soil and water conservation practices (SWCP) are intended to reduce runoff production and erosion. Under-standing surface runoff drivers are crucial for the effective design and implementation of SWCPs. We present field observations of complex vertical soil profiles and variations in lithology along hillslopes ridge-channel transects from the Debre Mawi watershed in the upper Abay (Blue Nile) basin of the Ethiopian Highlands. Our study generates the first depth-and hillslope-distributed soil water retention curves, textural profiles and measures of hydraulic conductivity for the region. We provide guidance into preferred pedotransfer functions (PTFs) and their sensitivity to clay fraction. Hydrological processes associated with the observed vertical profiles and variations on them were simulated with the Hydrus-2D software package, revealing that dominant runoff generation mechanisms were sensitive to the vertical organization of soil texture, particularly at depths of 30 cm or more below the soil surface for soil profiles that have well-drained surface soil textures. This suggests that current practices of evaluating soil properties based on surface texture only is insufficient for understanding hillslope hydrology and mitigating soil erosion in these regions. In regions where comprehensive soil hydraulic profile data is not available, our study quantifies the uncertainties in both PTFs and hydrological fluxes due to variation of soil profile texture present; these can aid decision makers in data collection and accounting of errors for the intended application of interest.