Dont judge a soil by its cover - Organization of the soil profile controls the risks of runoff and erosion in the 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. Understanding surface runoff drivers is 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 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. 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. 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 this region. Additionally, field data indicate that saprolites by weathering of volcanic bedrock are becoming exposed due to soil erosion, and that shallow saprolitic layers greatly enhance the variability in soil hydraulic properties along hillslopes. Erosion rates in the region are amongst the highest in the world. Saprolite may become more extensively exposed in the soil profile by this ongoing erosion. The implications of this exposure for hillslope hydrology, the design and longevity of soil and water conservation practices, and food security in the Ethiopian highlands remain unknown, but may represent a large departure, or regime shift from current conditions.