GROUNDWATER RECHARGE USING BASEFLOW RECESSION ANALYSIS IN SOUTHWESTERN NIGERIA

Postgraduate

ABSTRACT

Baseflow recession analyses of Yewa and Ona rivers were carried out with a view to determining and comparing groundwater recharge in parts of the sedimentary and Basement Complex terrains of Southwestern Nigeria. Daily discharges of the rivers for ten years (1988 – 1997) were analysed. Groundwater recharge in Yewa Basin, located in a sedimentary area was 27.3 mm while that in Ona Basin, which is mostly urban and located within the Basement Complex, was 63.6 mm, representing 2.3% and 5.3% of the annual precipitation of the study area respectively. Baseflow were 21.5 mm and 65.3 mm, representing 34% and 8% of the average annual total runoff of Yewa and Ona rivers respectively. Ona Basin had a comparatively lower groundwater recharge than Opeki Basin (with 137.4 mm), which is in a similar terrain but in a rural environment of Southwestern Nigeria, indicating the negative effects of vegetation clearing and developmental activities, typical in urban environments, on infiltration and consequent increase in direct runoff and reduction in groundwater recharge. Identification and preservation of groundwater recharge areas, especially in urban environments, is therefore imperative in the sustainable management of groundwater in the study area. 


INTRODUCTION

Groundwater recharge can be defined as an addition of water to a groundwater reservoir (Beekman and Xu, 2003). It normally takes place though infiltration of precipitation, but it can also occur as seepages from surface water bodies (like lakes, rivers and canals) and lateral or vertical interaquifer flow (MacDonald et al., 2005). Typically, it travels vertically downwards through the unsaturated zone to the water table, from where it flows according to the hydraulic gradient, until it reaches an area of discharge where it occurs as springs or seepages, providing part of the dry season flow and constituting part of the streamflow hydrograph. Streamflow hydrographs are usually considered as consisting of two components; baseflow and direct runoff (Hammond and Han, 2006). The baseflow is regarded as the underlying dry season runoff and derives from delayed interflow and groundwater flow, while the direct runoff is the part that derives from surface or near-surface flow (Mazvimavi, 2004). A number of techniques for separating baseflow from direct runoff can be found in the literature (UNESCO, 1972; Tallaksen, 1995; Chapman, 1999; Eckhardt, 2005; Hammond and Han, 2006). Separation of the stream hydrograph into the two components is necessary in investigations concerning water balance of catchments and the relationship between surface water and groundwater. Such investigations enable the determination, not only of the groundwater contribution to the total stream flow, but are also useful for adequate water resources management and modelling (Carter and Driscoll, 2006), estimation of sustainable yield from aquifers and prediction of the impacts of underground construction (Rodhe and Bockgård, 2006).

Determination of the available groundwater resources and estimation of sustainable yields from aquifers have become essential in Nigeria considering the prevalent exploitation of groundwater in the country, which has raised concerns about its sustainability and the need for reliable estimates of groundwater recharge (Goni et al., 2005; Goni, 2006). Groundwater development in Nigeria has increased tremendously over the last decade and many governmental and non-governmental

organizations seek to improve the critically limited access to potable water, especially in the rural areas, through the construction of water wells.  The  importance of groundwater, therefore,  in the overall  development of Nigeria's economy cannot be over-emphasised (Goni, 2008)  It has been estimated that over 75% of the population rely on groundwater for domestic consumption only (Goni, 2006) and that over 50 percent of the accessible and renewable fresh water resources in the country has been committed already (Musa, 1997). This means that even in Nigeria, where the available groundwater and surface water resources have been estimated to be 50 million trillion ℓ/year (Akujieze et al., 2002) and 224 trillion ℓ/year (Hanidu, 1990) respectively, competition for water may soon become as serious as it is in some other parts of the world that are less endowed.

The evaluation of groundwater recharge is commonly done by the consideration of the difference between rainfall and evapotranspiration estimates, taking into account any surface – runoff (Ogunkoya, 2000; Sjodin et al., 2001). In this study, however, the evaluation of groundwater recharge is done by analysing the stream hydrographs and recession equations of the two major rivers in Yewa and Ona drainage basins. The method has been used for the evaluation of Opeki drainage Basin, also in southwestern Nigeria, by Idowu and Martins (2007), and the results obtained were compared with the results of this study in order to provide a broad view of southwestern Nigeria. While the Yewa is located within the sedimentary terrain, the area where this study covers in Ona Basin, Figure 1 (i.e. upstream of Fidiwo, where the gauging station was located), is located entirely within the Basement Complex, where crystalline rocks occur, thereby making a comparison of groundwater recharge between the sedimentary rock and Basement Complex terrains of Southwestern Nigeria possible. The characteristics of the two drainage basins are presented in Table 1, while the average monthly discharges of the Yewa and Ona rivers are presented in Figure 2.

The Ona Basin falls within the Pre-Cambrian rocks of southwestern Nigeria, which is part of the Nigerian Basement Complex. The major rock types are schist-quartzites, granite-gneiss, banded gneiss, augen-gneiss, and migmatites (Jones and Hockey 1964; Olayinka et al., 1999), with minor intrusions of pegmatite, aplites, quartz veins and dolerite dykes. Gneisses are migmatized in places, and characterized by predominantly mediumsized grains while schist-quartzites occur as elongated ridges striking NW-SE (Olayinka et al., 1999). The sedimentary rocks of Yewa Basin are part of the layered sequence of rocks comprising five sedimentary formations – Abeokuta (Cretaceous), Ewekoro (Paleocene), Ilaro (Eocene), Coastal Plains Sands (Pleistoceneoligocene) and the Alluvial deposits (Recent). The formations are essentially a succession of sands, clays, shales and gravels with limestone occurring in the paleocene Ewekoro Formation (Jones and Hockey 1964).