Abstract
The Department of Water Affairs and Forestry (DWAF) is currently carrying out detailed hydrogeological studies within the northern Sandveld area, Western Cape, South Africa. In addition, preliminary studies funded by DWAF, have also been carried out in the area assessing hydrological, ecological and botanical components. The area is receiving much attention due to its environmental uniqueness (part of the area is a Ramsar site) and its significant groundwater resources, which are being utilised for agricultural purposes and municipal water supply. In certain areas this groundwater abstraction is impacting on surface water flows, groundwater and associated ecosystems. In order to gain an understanding of the systems and their inter-relationships within the area, it is necessary to carry out water balance calculations. In carrying out such calculations, the quantification of the “input” component and associated temporal and spatial variability is important to address. With special emphasis on groundwater recharge, the area may be receiving both direct groundwater recharge, (as a consequence of precipitation), and distant groundwater inflow, via the large faults that transect the area, which may be importing groundwater from the inland recharge areas (i.e. the Cederberg Mountains). Earlier groundwater recharge studies within the area and in geologically similar settings indicate a wide range in recharge values. With special reference to the primary aquifers, earlier conservative recharge values indicate 8% recharge, with values as high as 15% being obtained. The current study indicates direct recharge values in the region of 0.2 to 3.4%. Assessment of water balance calculations indicate these recharge values are more realistic. The groundwater recharge quantification has huge significance for the Resource Directed Measures that are also being carried out in the area, in line with the requirements of the South African National Water Act of 1998. The Resource Directed Measures are aimed at ensuring water resource use and development is balanced by protection measures thereby guaranteeing the sustainable use of the resource. Water SA Vol. 30 (5) 2005: pp.75-81
Highlights
In order to optimally manage a groundwater resource that is being utilised, it is highly beneficial to carry out water balance calculations
The study area is located in the northern Sandveld, Western Cape, South Africa
Currently time series data is mainly available for the municipal supply wellfields within the area. An analysis of this data was carried out using a number of methods, including the chloride method, the Saturated Volume Fluctuation (SVF) Method, the Cumulative Rainfall Departure (CRD) Method and the Extended model for Aquifer Recharge and soil moisture
Summary
In order to optimally manage a groundwater resource that is being utilised, it is highly beneficial to carry out water balance calculations. One of the components of the water balance equation that needs to be determined is the rate of groundwater input This input can be subdivided into three main components, namely: direct (vertical) recharge; recharge from river flow; and lateral inflow. It is important to define groundwater inputs and especially direct recharge as significant economic benefit is being derived from groundwater usage by the agricultural sector, whilst the fragile ecosystems within the area are very groundwater dependent and showing signs of being stressed and significantly impacted. De Vries and Simmers (2002) state that the quantification of groundwater recharge is a prerequisite for efficient and sustainable groundwater resource management in arid regions They state that recharge is defined in the general sense as the downward flow of water reaching the water table, forming an addition to the groundwater reservoir. This paper discusses the main challenges associated with determining vertical groundwater recharge in an arid to semi-arid environment
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