ESTIMATION OF SPATIALLY DISTRIBUTED GROUNDWATER RECHARGE AND ITS QUALITY EVALUATION FOR IRRIGATION, MODJO RIVER CATCHMENT, CENTRAL ETHIOPIA

Abstract

Knowledge of groundwater resource potential is important for groundwater management and sustainable use. Recharge is an important factor in evaluating groundwater resources but difficult to quantify. Hence estimation of groundwater recharge requires modeling of the interaction between all the important processes in the hydrological cycle. In this study the long term seasonal and annual groundwater recharge of Modjo river catchment (2,202 km2) was estimated and recharge map were developed through a grid based physically distributed model, WetSpass. Long term average hydro-meteorological data and spatial pattern of watershed physical grid maps were used as main inputs for the model. All input maps for the model were prepared using ArcGIS 10.2 spatial analysis tool. Soil, land use and runoff coefficient parameters in dbase files, season independent gridded base map of topography, slope, and soil were used in the model; whereas precipitation, potential evapotranspiration, temperature, wind speed, groundwater depth and land use map were prepared and employed by the model, in ASCII grid format for both winter and summer seasons. For irrigation water quality analysis, the groundwater samples were collected from well and borehole of different site and analyzed in lab for necessary parameters. From the result, it is found that the long-term temporal and spatial average annual rainfall of 933 mm was distributed as: surface runoff of 164 mm (17.6%), evapotranspiration of 686 mm (73.5%), and recharge of 83 mm (8.9%). Thus an average of 183Mm3 of groundwater will be recharged per year or 5802 l/s from the catchment area. It has also seen that the water balance components are dependent on the soil type and land use classes, thus high values of groundwater recharge are observed in the grassland (31%) and agricultural land (29%) with silty loam and silty clay soils. This is due to relatively good permeability of these soils and gentle topography which favors more water infiltration contributing to recharge. Plotting of the chemical analysis of water samples collected in the area using piper graph shows that Na-Ca-HCO3 water type in the recharge area, intermediate water type Ca-Mg-Na-HCO3 in northern and central part of the subbasin and Ca-Mg-HCO3 type towards the east and most of the water is clustered in the left part of the diagram, suggesting that the water in the study area is a Calcium-Sodium-Magnesium-Bicarbonate type, which is characterized by a high concentration of HCO3 and Ca. Flood control dams (artificial recharge) practice was recommended in this study area to harvest the excess water (simulated annual surface runoff 361Mm3) which is helpful in one way to reduce soil erosion and in the other way to enhance more recharge to groundwater. From water quality analysis; the concentrations in the water sample shows below the maximum allowable limits for irrigation (i.e.EC<2000 μS/cm and SAR<9) and therefore the groundwater could be used safely for irrigation.