Estimation of Groundwater Recharge in Makutupora Basin Located in a Semi-arid Region in Central Tanzania using Soil Moisture Balance (SMB) and Chloride Mass Balance (CMB) Techniques

Abstract

ABSTRACT The objective of this study is to assess the groundwater recharge in the Makutupora basin, Central Tanzania using the soil moisture balance (SMB) and chloride mass balance (CMB) methods. The SMB method considered the estimation of both evapotranspiration in CROPWAT 8.0 software and surface runoff by the curve number (CN) method. The CN of the basin was determined based on hydrological soil group (HSG), antecedent moisture condition (AMC) and land use type using Arc GIS 10.2. Distributed land-use maps, slope and soil texture maps, daily climatological data (e.g., precipitation, temperature, wind speed, etc.), chloride concentration in rainfall and groundwater are the basic data used in this study. Input data were prepared in the form of digital maps using GIS and remote sensing tools and finally excel models were created to compute recharge using a set of given conditions. The results indicate that the average chloride concentration measured in the groundwater samples of the study area is 282.88 mg/L; much higher than the average concentration in rainwater samples which is 3.47mg/L while the average potential evapotranspiration is 2280 mm/year with an average rainfall of 680mm/year. Runoff computed using the CN method is ranging from 7.6-246mm/year or 0.68-22%. The final recharge from SMB is ranging from 0.025-117.86mm/year or 0.006-10% of annual rainfall equivalent to an average annual volume of 41.58Mm3 while recharge from CMB ranges from 2.57-44.6mm/year or 0.38-6.56% of annual rainfall equivalent to an average annual volume of 23.85Mm3. The results show that CMB underestimated the recharge compared to the SMB method. The reason for this is thought to be caused by the high chloride concentration analysed in groundwater compared to rainfall samples which may be caused by increased chloride concentration as water passes through the rock formations (unsaturated zone) to the saturated zone. Further lack of atmospheric chloride deposition data may be another contributing factor.