Surface Water – Groundwater Interactions and Effects of Irrigation on Water and Soil Resources in the Awash Valley
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Date
2009-07
Authors
Tsegaye, Gedion
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Publisher
Addis Ababa Universty
Abstract
With an area of some 115,523 km2, the Awash basin mainly comprises the north-western and south-eastern
highlands and the rift valley. The basin is endowed with abundant surface water and groundwater resources that
require proper management if they are to be wisely utilized in a sustainable manner. With the Awash river as the
major resource, the Awash valley is the most developed part in Ethiopia in terms of irrigated agriculture.
The Awash river water balance has been computed with recent level of irrigation development for a period of five
years. The results show that the Awash river is generally influent with transmission loss of some 2.1 BCM and
6.8 BCM for the reaches Koka-Metahara and Metahara-Tendaho respectively during the given period. These
values include potential indirect recharge as well as loss due to over-bank flooding and subsequent evaporation.
One such place where considerable loss (about 500 MCM/yr from evapotranspiration) occurs is Gedebassa
swamp, situated in the middle part of the Metahara - Tendaho reach.
Influent conditions may be valid as groundwater tables are relatively deeper than the Awash river in the valley
and highly permeable river bed sub-reaches exist (alluvial sediments and fractured/faulted volcanic rocks).
Nevertheless, there are intermediate reaches where the Awash may also be effluent. These include the vicinities
of major irrigation schemes and the sub-reach immediately downstream of Koka reservoir where groundwater
table is shallow.
The Awash river receives considerable surface runoff inflow (about 2.7 BCM/yr) in the middle and lower valleys,
mainly from the western sub-catchments. About 60% of this amount is acquired upstream of Gedebassa swamp
and the rest 40% downstream. In the upper valley, the Awash river receives inflow of some 420 MCM/yr, which is
less than one-fifth of what is received in the reach between Metahara and Tendaho. And yet, from the overall
irrigation abstractions (currently less than 1 BCM/yr), more than 60% take place in the upper valley.
With computed overall irrigation efficiency of less than 45%, the Amibara Irrigation Schemes (AIS) in the middle
valley have been studied at semi-detailed level. Groundwater hydrograph analysis based on peizometers’ data
has shown that surface irrigation in Amibara is responsible for rising groundwater tables with average rates of 0.3
to over 1 m/yr in the 70s up to 90s. Further investigations on possible Awash river-groundwater as well as canal
water-groundwater interactions in Amibara have been made possible by conducting groundwater flow analysis.
The prepared depth to groundwater map portrays that average groundwater levels are within 3m (shallow depth)
in one-third of AIS.
As evidenced from analysis results of bulk water and surface soil samples collected as part of this study, the
Awash river is regarded as suitable for irrigation with moderate salinity and low sodicity; the groundwater in
Amibara is generally saline (85% of AIS above 5 mS/cm); and about 20% of AIS soils are saline and 3% salinesodic.
The groundwater levels have currently reached close to surface, due to recharge from irrigation, that evaporation
has now become one of the major outputs other than groundwater outflow. The relatively higher evaporation rate
in Amibara favours upward movement of salts from the saline, shallow groundwater during the fallow period. In
addition, the shallow groundwater has further hindered effective leaching of soluble salts below the root zone,
necessitating measures of groundwater level control by sub-surface drainage installation or other means.
Description
Keywords
Water and Soil Resources