Peak discharge dynamics in response to vegetation cover change and variability of precipitation events in steep tropical mountain catchments: case of the western Rift Valley escarpment of Northern Ethiopia

Abstract:

Tesfaalem G. Asfaha^{1, 2}, Amaury Frankl¹, Mitiku Haile³, Amanuel Abraha ³, Jan Nyssen¹

¹Department of Geography, Ghent University, Krijgslaan 281 (S8), B-9000 Ghent, Belgium

² Department of Geography and Environmental Studies, Mekelle University, Mekelle, Ethiopia

³Department of Land Resources Management and Environmental Protection, Mekelle University, Mekelle, Ethiopia

* Corresponding author: tesfitga@gmail.com

Abstract

The hydrologic characteristic of mountain catchments is responsive to changes in vegetation cover and rainfall variability. In the 1980s, deforestation of the catchments in the western Rift Valley escarpment of Ethiopia resulted in huge amount of peak discharge. Conversely, recent reforestation interventions have improved the vegetation cover. In this study, 332 peak discharge events from 11 steep catchments were measured in three rainy seasons (2012-2014) with the objective of analyzing runoff response of mountain catchments with contrasting vegetation cover. Seven rain gauges were installed in and nearby the catchments. Peak discharge events were calculated using the Manning’s equation from measurements of maximum discharge height at 11 crest stage gauges. Percentages of land cover were detected from Google Earth Imagery. Morphometric variables were computed from digital elevation model and topographic maps. Correlation analysis between precipitation (P_d) and peak discharge (Q_p) events showed positive relation (R² = 0.32 to 0.94, P < 0.05) in all the catchments. Average catchment-specific peak discharge coefficient (C_p) showed a strong decreasing relation with vegetation cover (R² = 0.85, P < 0.01), catchment length (R² = 0.37, P < 0.05), time of concentration (R² = 0.43, P < 0.05) and average soil depth (R² = 0.42, P < 0.01) while positively with catchment slope gradient (R² = 0.37, P < 0.05). A stepwise multiple regression analysis revealed that 99% (P < 0.01) of the variability of catchment-specific peak discharge coefficient can be predicted by vegetation cover and infiltration number. Overall, this study demonstrates that in steep mountain catchments, runoff response is determined by precipitation events and strongly by percentage of vegetation cover.

Key words: runoff, crest stage gauge, mountain streams, land cover.