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 end-point 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.
Highlights
Understanding the hydrology of a river and its historical flow characteristics is essential for water resources planning, developing ecosystem services, and carrying out environmental flow assessments
The natural flow regime in the Omo–Ghibe Basin was analysed based on the daily flow data for the 20–30 years of available data at each of the 12 selected stations
These show the annual FDC, as well as the FDC for the dry and wet months. These curves are developed for two 13-year periods (1982–1995, and 1996–2008), to analyse whether there are any clear changes to the distribution between the two periods, and whether these corroborate trends found in other indices. These results suggest that the dry season and annual flows are higher in the second period for the majority of the stations, while the wet season flows remain almost the same
Summary
Understanding the hydrology of a river and its historical flow characteristics is essential for water resources planning, developing ecosystem services, and carrying out environmental flow assessments. Additional to characterising the NFR, the parameters used to describe the NFR support the analysis of change resulting from changing climatological and hydrological characteristics (Poff et al, 1997a; Assani et al, 2010) Anthropogenic activities such as damming, impounding, land use land cover (LULC) change, diversion and abstraction of water and geomorphological change can impact the natural flow regime. Longer-term climate change will influence hydrological flows (Risbey and Entekhabi, 1996; Sankarasubramanian et al, 2001) These changes will result in changes to hydrological characteristics such as magnitude, duration, timing, frequency and rate of change of flow, and it is important to study these as they provide indication of the wellbeing of the riverine ecosystem (Lytle and Poff, 2004)
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