Topography Impacts Hydrology in the Sub-Humid Ethiopian Highlands

Demesew A Mhiret,Kristine Walraevens,Wubneh B Abebe,Benjamin F Zaitchik,Fasikaw A Zimale,Dessalegn C Dagnew,Minychl G Dersseh,Tammo S Steenhuis,Seifu A Tilahun,Christian D Guzman

doi:10.3390/w14020196

Abstract

Understanding the relationship between topography, hydrological processes, and runoff source areas is essential in engineering design, such as predicting floods and implementing effective watershed management practices. This relationship is not well defined in the highlands with a monsoon climate and needs further study. The objective of this study is to relate topographic position and hydrological response in tropical highlands. The research was conducted in the Debre Mawi watershed in the northwest sub-humid Ethiopian highlands. In the monsoon rain phase of 2017 and 2018, groundwater depth, infiltration rate, and surface runoff were monitored at the upslope, midslope, and downslope positions. Surface runoff rates were measured in farmer fields through distributed V-notch weirs as estimates of positional runoff. Average water table depths were 30 cm deep in the downslope regions and 95 cm in the upslope position. The water table depth affected the steady-state infiltration rate in the rain phase. It was high upslope (350 mm h−1), low midslope (49 mm h−1), and zero downslope. In 2017, the average runoff coefficients were 0.29 for the upslope and midslope and 0.73 downslope. Thus, topographic position affects all aspects of the watershed hydrology in the humid highlands and is critical in determining runoff response.

Highlights

Thousands of studies have been conducted to understand the amount of runoff generated in response to a hydrological event
The study was conducted in the 95 ha Debre Mawi watershed in the sub-humid highlands of Ethiopia
The surface runoff, groundwater table depth, and soil type are highly linked with the topographic position of a landscape unit

Summary

Introduction

Thousands of studies have been conducted to understand the amount of runoff generated in response to a hydrological event. In the humid Ethiopian highlands, where the volcanic-derived soils have a high infiltration capacity, soil moisture governs runoff processes [1,2]. It means that any rain will infiltrate unless the soil is saturated, at which point all rain that is not lost as interflow will become runoff [5,6]. Studies based on the topographic wetness index map [12,13] confirmed that topography is a significant factor in catchment response. Downslope regions close to the watershed outlet are in the lowest topographic position, saturating during the last half of the rain phase [2,14]

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Conclusion