Temporal variation of suspended sediment transport in the Koga catchment, North Western Ethiopia and environmental implications

Abstract

Event sediment transport and yield were studied for 45 events in the upstream part of the 260 km 2 agricultural Koga catchment that drains to an irrigation reservoir. Discharge and turbidity data were collected over a period of more than a year, accompanied by grab sampling. Turbidity was very well correlated with the sediment concentrations from the samples (r = 0.99), which allowed us to estimate the temporal patterns of sediment concentrations within events. The hysteresis patterns between discharge and sediment concentrations were analysed to provide insight into the different sediment sources. Anticlockwise patterns are the dominant hysteresis patterns in the area, suggesting smaller contributions of suspended sediment from the river channels than from the hillslopes and agricultural areas. Complicated types of hysteresis patterns were mostly observed for long events with multiple peaks. For a given discharge, sediment yields in August and September, when the catchment was almost completely covered with vegetation, were much smaller than during the rest of the rainy season. The hysteresis patterns and timing suggest that the sediment availability from the agricultural areas and hillslopes affects sediment yields more strongly than does peak discharge. Two distinct types of sediment rating curves were observed for the season when the agricultural land was covered with vegetation and when it was not, indicating the dominating contribution of land use/cover to sediment yields in the catchment. The rate of suspended sediment transport in the area was estimated as 25.6 t year À1 ha À1 .

Sustaining reservoir use through sediment trapping in NW Ethiopia

“…This result agrees with the findings of previous studies in northwest Ethiopian highlands. For example, ~25 t ha -1 y -1 (Setegn et al, 2010;Yeshaneh et al, 2014) and 8.6-55 t ha -1 y -1 (Mekonnen et al, 2015b). The result was also within the range of 3-49 t ha -1 y -1 found by Tamene et al (2006a) ) is the contribution of Minizr catchment.…”

“…Different approaches exist to estimate surface runoff, soil erosion and sediment yield from a catchment including models Keesstra et al, 2014a), sediment rating curves and river samplings (Yeshaneh et al, 2014), bathymetric surveys (Tamene et al, 2006a) and trapped sediment analysis (Baade et al, 2012;Stefanidis & Stefanidis, 2012). Each approach has its advantages and disadvantages in relation to data requirement and availability, easiness for application and cost effectiveness.…”

“…: (i) erosion models (Van Rompaey et al, 2001;Keesstra et al, 2014b), (ii) suspended sediment discharge measurements at the outlets of sub-catchments (Syvitski & Milliman, 2007;Yeshaneh et al, 2014) and (iii) field survey methods during and after rainstorms (Bewket & Sterk, 2003;.…”

“…Figure 2.1 shows the brown, high sediment laden water of the Koga and Minizr rivers, draining into the Koga reservoir. Yeshaneh et al (2014) found that the upper part of Koga catchment (~98 km 2 ) alone contributed ~252 000 t y -1 (25.6 t ha -1 y -1…”

“…Sediment discharge from a catchment is highly variable both temporally and spatially (Hagmann, 1996;Yeshaneh et al, 2014;Buendia et al, 2016), which is because the processes driving catchment sediment dynamics have proved to be highly variable in space and time (Liu et al, 2012). Over recent decades, there has been an increased interest in quantifying sediment loads of intra-annual (within a year) variability in addition to inter-annual (between years) in order to improve understanding of suspended sediment loads at a higher temporal resolution (Smith et al, 2003;Seeger et al, 2004).…”