Shift from transport limited to supply limited sediment concentrations with the progression of monsoon rains in the Upper Blue Nile Basin

Abstract

Long‐term erosion monitoring data in the Ethiopian highlands are only available from the Soil Conservation Research Program (SCRP) watersheds including the Anjeni watershed. The 113 ha Anjeni watershed was established in 1984 and fanya juu terraces were installed in 1986. Runoff and erosion data are available from three different plot sizes and at the watershed outlet. The objective of this study was to investigate how erosion processes and sediment rating parameters vary with plot size and the progression of the rainy monsoon phase. We analyzed runoff and sediment loss data from 40 plots and the watershed outlet. The dataset included erosion data from 24 newly constructed plots (3 m length) during the rainy monsoon phase of 2012 and 2013, and 16 long‐term plots (with 12, 16, 22, and 24% slopes and 3, 15 and 30 m lengths) and the watershed outlet during the period between 1986 to 1990. Sediment concentration (C) was fitted to runoff (Q) using a power regression equation (C = aQb). Sediment concentration and yield increased with increasing plot length from 3 m to 15 m, but sediment yield decreased as plot length increased to 30 m.The coefficients (a and b) were affected by plot size and the progression of the rainy monsoon phase. As plot size increases, the a value increased, while the b value decreased. Greater a values were observed during the beginning of the monsoon phase, while values of b were greater towards the end of the monsoon phase. Overall findings suggest that erosion from cultivated fields is primarily controlled by transport limitations at the beginning of the monsoon phase, while towards the end of the monsoon phase, as surface covers emerge, sediment availability will be reduced, and thus sediment source would be a limitation. Copyright © 2016 John Wiley & Sons, Ltd.

Impact of Soil Depth and Topography on the Effectiveness of Conservation Practices on Discharge and Soil Loss in the Ethiopian Highlands

“…In most watersheds, sediment concentrations are elevated in the beginning of the rain phase after the soils are plowed and rills form [22,23,28,[44][45][46][47]. Once the rills have formed, sediment concentrations decrease.…”

“…To discuss the differences in discharge and runoff coefficients between the two watersheds, we note that prior experimental studies have found that the main runoff generation mechanism in the (sub) humid Ethiopian highlands is saturation excess runoff [15,20,22,24,[45][46][47][48]. Saturated excess overland flow occurs at locations either where the soils become saturated above an impeding layer or where the regional groundwater reaches the surface.…”

Evaluating erosion control practices in an actively gullying watershed in the highlands of Ethiopia

“…After gullies are rehabilitated erosion rates decrease rapidly (Akale et al ., ). In addition, high erosion rates of up to 125 Mg ha ‐1 yr ‐1 have been reported for plot studies (Mitiku et al ., ; Bewket and Teferi, ); but these large losses are moderated by the time the water reaches the outlet (Bayabil et al ., ).…”

“…Previous hydrological studies in the sub-humid and humid Ethiopian highlands have found that the median infiltration rate in the volcanic surface soils is frequently exceeded. Saturation excess overland land flow is therefore common (Liu et al, 2008;Steenhuis et al, 2009;Tilahun et al, 2013Tilahun et al, , 2015Bayabil et al, 2010Bayabil et al, , 2016Bayabil et al, , 2017. This phenomenon is sometimes labeled infiltration excess when the limited infiltrability of the subsurface is the main defining characteristic (Bewket and Sterk, 2002).…”

Hydrogeology of Volcanic Highlands Affects Prioritization of Land Management Practices

“…Thus, despite the implementation of government-imposed water conservation practices, the land has become more degraded in humid and subhumid highlands [26,61], and sediment concentrations in streams have been increasing [62,63]. Annual soil losses equivalent to 1-8 mm per year are not uncommon in small watersheds, with the upper end of the sediment losses being from watersheds that have active gully formations [64,65]. Restoring natural resource bases before soils are fully degraded is important because the harm caused in one year cannot be repaired in a human lifetime [66].…”