GROUNDWATER RECHARGE AND FATE OF GROUNDWATER STORAGE OF THE WEYBO RIVER CATCHMENT WELAYITA-HADIYA ZONES SOUTHERN ETHIOPIA

BACKGROUND   Most activities come to rely on groundwater resources than on surface water resources mainly due to their sustainability and quality   The Potentialities of Groundwater resource is mainly factored by the Rate of Recharge  Changes in the Geo-Environment of the catchment and the entire Omo-Gibe Basin is manifested by changes in Recharge Rates

OBJECTIVES Estimating the Optimum Groundwater Recharge   Showing the Temporal-Trend of Recharge w.r.t. the Temporal Changes of the Hydro- Meteorological Parameters Assessing the Fate of Groundwater Storage To Show the Major Controlling Parameter of Groundwater Recharge in the Catchment

60 55’ 42 - 7010’28N 370 31’ 39 - 37046’40E AREAL EXTENT = 574Km2 PERIMETER = 124Km 60 55’ 42 - 7010’28N 370 31’ 39 - 37046’40E   DISTANCE FROM ADDIS ABABA AND ALTERNATIVE ROADS Along Addis Ababa – Butajira – Hosaina Road = 300Km Along Addis Ababa – Shashemene – Welaita Road = 410Km Along Addis Ababa – Durame – Areka = 440Km PERIMETER = 124Km   AREAL EXTENT = 574Km2 DISTANCE FROM ADDIS ABABA AND ALTERNATIVE ROADS   Along Addis Ababa – Butajira – Hosaina Road = 300Km Along Addis Ababa – Shashemene – Welaita Road = 410 Along Addis Ababa – Durame – Areka = 440Km

Elevated areas are found in the boundaries of the area PHYSIOGRAPHY    50% of the area has a slope 2-6%,  25% has 0-2%, 15% has 6-12%,  6% has 12-24% Elevated areas are found in the boundaries of the area   Topographic relief ranges from 1800-1900m from the highest peak Damota to the lowest Weybo valley

GEOLOGY The main lithologic units that are outcropped in the study area and near to its adjacent watersheds are the teritiary volcanics   The Nazareth Groups (of Miocene to Pliocene age) and the flood basalts (Eocene to early Miocene age) are the dominant units that widely cover the study area About 90% of the area is covered by the Nazareth Group, which comprises of a series of rhyolite-trachyt flows, ignimbrites, pumice and ash falls The Nazareth Group unconformably overlies the early flood basalts

HYDROGEOLOGIC UNITS AND AQUIFER SYSTEMS AQUIFER FORMATION HYDROGEOLOGY HYDROGEOLOGIC UNITS AND AQUIFER SYSTEMS AQUIFER FORMATION -Weathered and fractured ignimbrite/welded tuff -Sediments associated with weathered pumice -Weathered and fractured rhyolites and Trachytes TYPES OF AQUIFERS IN THE CATCHMENT   -Dominantly leaky aquifers

HYDROGEOLOGICAL MAP OF THE STUDY AREA

Geological structures and hydraulic conductivity values     HYDRAULIC CHARACTERISTICS   Correlation has been made between Geological structures and hydraulic conductivity values  A greater extent of the study area possesses a low permeability zone.

GROUND WATER FLOW SYSTEMS AND POTENTIOMETRIC CONTOURS   Regional, intermediate and local flow systems

RECHARGE AND DISCHARGE ZONES    Zonation based on topography  

Flow lines tend to diverge from recharge areas and  Zonation based on peizometric patterns Flow lines tend to diverge from recharge areas and converge toward discharge zones  

 Zonation Based on Hydro chemical Trends

POTENTIAL EVAPOTRANSPIRATION = 1074.4mm HYDROMETEOROLOGY PRECIPITATION = 1340 mm TEMPRATURE = 19.210C RELATIVE HUMIDITY = 63.5% MEAN WIND SPEED = 2.45m/s POTENTIAL EVAPOTRANSPIRATION = 1074.4mm ACTUAL EVAPOTRANSPIRATION (AET) = 960.3mm  

STREAM DISCHARGE OF WEYBO RIVER (i) Scaling up the stream flow values Qmouth = (A2/A1) Qgauged Q = Qan (Pan/P) (ii) The analogue method

GROUNDWATER RECHARGE (GWR)  CONVENTIONAL WATER BALANCE APPROACH GWR = 88.19mm/annum  STREAM HYDROGRAPHS ANALYSES Mean long-term minimum flow = 62.5mm/year Seasonal recession method = 74.5mm/year  CHLORIDE MASS BALANCE = 123.5mm/year

THE OPTIMUM GROUND WATER RECHARGE ESTIMATION BASED ON GROUNDWATER BUDGET ESTIMATED GROUNDWATER OUTFLOW = 78.56 GW INFLOW ESTIMATED FROM WATER BALANCE AND SEASONAL RECESSION METHOD = 81.5mm THE OPTIMUM GROUND WATER RECHARGE = 81.5  

TREND OF GROUNDWATER RECHARGE

FATE OF GROUNDWATER STORAGE Equating the linear equation: y = -587x + 93.07 Groundwater storage will be heavily affected and appreciable change in storage will be seen after 30 years now on

RECHARGE TREND W.R.T. CONTROLLING HYDRO-METEOROLOGICAL PARAMETERS Mean precipitation is slightly decreasing, however, its rate of declination is not comparable

mean temperature shows an increasing trend mean wind speed shows an increasing trend

potential evapotranspiration is slightly increasing

CONCLUSION -Groundwater recharge is computed using 4 different Methods   -Groundwater recharge is computed using 4 different Methods and four different values are obtained. The optimum=81.5mm -The decline in groundwater recharge is highly attributed to changes in the environment -Measurable changes in groundwater storage will be seen in a 30 years period of time if environmental changes are keeping on the same rate CONCLUSION CONCLUSION -The groundwater recharge estimated from measurements of chloride yields an over estimated result.

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