Application and comparison of groundwater recharge estimation methods for the semiarid Yola area, northeast, Nigeria

Abstract

Then Bima Sandstone aquifer is the main water resource in the semi-arid Yola area of Northeastern, Nigeria. The rapid increase in water demand in this area has prompted the need to estimate available groundwater recharge for sustainable utilization. Groundwater recharge estimation methods and results are compared: chloride mass balance method (5.81 to 9.65 mm/year); water budget method (52.45 to 86.20 mm/year); Darcian method (16.38 to 104.04 mm/year) and hydrograph separation method (133.6 to 645.81 mm/year). The Chloride mass balance and the Darcian methods are well suited to identify the existence of recharge and enable one to determine good estimates of aerial and point values. A major limitation of the chloride mass balance is that there may be other sources of chloride in the soil (eg halites) other than the chloride contained in the rainwater. On the other hand in the Darcian method the hydraulic conductivity of the soil is poorly known due to heterogeneities and its
variation with saturation. The water budget method is bedeviled by a number of limitations which include lack of lysimeter for measuring
evapotranspiration but is simple and can be estimated as a residual in a continuity equation. A major drawback of the hydrograph separation method is that it estimates baseflow at lower elevations in a watershed which is assumed to be equal to recharge that occurred at higher elevation. It is however one of the few integrative measurement of recharge.
The merits and demerits of each recharge method in terms of accuracy and applicability were also highlighted in this study. This study has shown that recharge occurs to some extent in even the most arid regions, though increasing aridity are characterized by a decreasing net downward flux and greater time variability. Thus as aridity increases, direct recharge is likely to become less important and indirect recharge more important in terms of total recharge to an aquifer. It is therefore concluded that estimates of direct recharge are likely to be more reliable than those of indirect recharge.