Flow regime change in an endorheic basin in southern Ethiopia

Abstract

Abstract. Endorheic basins, often found in semi-arid and arid climates, are particularly sensitive to variation in fluxes such as precipitation, evaporation and runoff, resulting in variability of river flows as well as of water levels in endpoint lakes that are often present. In this paper we apply the indicators of hydrological alteration (IHA) to characterise change to the natural flow regime of the Omo-Ghibe Basin in southern Ethiopia. Little water resource infrastructure has been developed in the basin to date, and it is considered pristine. The basin is endorheic and is the main source of flow to Lake Turkana in the East African Rift Valley. The water level in Lake Turkana shows significant fluctuation, but increase of its level can be observed over the past 20 years. The reasons are currently not well understood.Of the five groups of hydrological characteristics in the IHA (magnitude, timing, duration, frequency and variability), only those related to magnitude were found to show significant trends, with the main trend being the increase of flow during the dry season. This trend was not reflected in climatological drivers such as rainfall, evaporation and temperature (which shows a positive trend), but rather is attributed to the substantial changes in land use and land cover in the basin. The change in the basin hydrology is apparent mainly in the more humid part of the basin. The significant shift from forest and woodland to grassland and cropland results in a decrease of actual evaporation and subsequent increase in (dry season) runoff. The long-term trend of the increasing levels in Lake Turkana are related to these trends in dry season flows, while shorter-term fluctuations of the lake levels are attributed primarily to anomalies in consecutive wet and dry season rainfall.

Detection of trends in hydrological extremes for Ethiopian watersheds, 1975–2010

“…Land cover conversions such as from natural vegetation to cropland and settlement are generally assumed to enhance total and peak flow volumes (Legesse et al 2010). However, this assumption was not supported by empirical data particularly for large-sized watersheds in Ethiopia (Gebrehiwot et al 2014;Worku et al 2014). Thus, we found it difficult to explain the increasing signals that we found for Qann, Qwet, and high-flow indicators (Qmax1day, Qmax7day, and Qmax30day) with land cover conversations.…”

“…Findings in our study are generally different from previous research works (Bewket and Sterk 2005;Melesse et al 2009;Tesemma et al 2010;Rientjes et al 2011;Gebrehiwot et al 2014) that reported no significant changes for annual, wet season and high-flow magnitudes and downward trends for low-flow magnitudes, particularly in the upper Abbay River Basin. However, there are some recent studies (Worku et al 2014;Degefu and Bewket 2017) that found evidences of upward changes in dry weather flow and extreme lowflow magnitudes. Another study (Berahanu et al 2014) had also reported decreasing changes in the mean zero flow days and coefficient of variation in streamflow magnitudes for Ethiopian rivers.…”

“…Mixed and opposite trend signals also observed between adjacent gauging stations in the same and neighboring river basins. The observed complex trend signals between adjust streamflow stations may be attributed to the presence of very complex climate, topography, land cover, and land use systems that showed greater variation with short horizontal distances over the highland part of the country (Bewket and Conway 2007;Bewket 2014, 2017;Gebrehiwot et al 2014;Worku et al 2014).…”

“…Previous studies were focused on Abbay (Blue Nile) River Basin and its sub-catchments (Bewket and Sterk 2005;Melesse et al 2009;Tesemma et al 2010;Rientjes et al 2011;Gebrehiwot et al 2014) and a few on Omo-Ghibe River Basin (Worku et al 2014;Degefu and Bewket 2017). These studies reported the absence of systematic trends in mean annual, mean seasonal, and extreme hydrological flow magnitudes, except Worku et al (2014) and Degefu and Bewket (2017) who reported that there was a tendency towards upward change in low-flow indices. The aim of this study is therefore to provide a more comprehensive analysis of trends in streamflow amount and hydrological extremes for a large number of watersheds across the Ethiopian highlands.…”

Analyzing Changes in the Flow Regime of the Yangtze River Using the Eco-Flow Metrics and IHA Metrics

“…The most widely used hydrologic metrics are called indicators of hydrologic alteration (IHA), which include 33 hydrologic parameters describing a wide array of flow characteristics, including the magnitude of monthly runoff, the magnitude of annual extreme streamflow, the timing of annual extreme flow, the frequency and duration of high and low flow pulse and the rate and frequency of streamflow changes [13]. IHA metrics have been successfully applied to evaluate hydrological alterations in a suite of watersheds located in arid, semi-arid, and humid regions [14][15][16]. However, previous studies have also concluded that there is statistical redundancy in the parameters of IHA and that some parameters are inter-correlated [17,18].…”