407 http://dx.doi.org/10.4314/wsa.v38i3.6 Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 38 No. 3 International Conference on Groundwater Special Edition 2012 ISSN 1816-7950 (On-line) = Water SA Vol. 38 No. 3 International Conference on Groundwater Special Edition 2012 This paper was originally presented at the International Conference on Groundwater: Our Source of Security in an Uncertain Future, Pretoria, 19-21 September 2011. * To whom all correspondence should be addressed.  +27 21 852-0847; fax: +27 86 616-5146; e-mail: ricky@groundwaterafrica.co.za A groundwater-planning toolkit for the main Karoo basin: Identifying and quantifying groundwater-development options incorporating the concept of wellfeld yields and aquifer frm yields R Murray 1 *, K Baker 1 , P Ravenscroft 2 , C Musekiwa 3 and R Dennis 4 1 Groundwater Africa, 54 Irene Avenue, Somerset West, 7130, South Africa 2 Maluti GSM, PO Box 6336, Uniedal, 7612, South Africa 3 Council for Geoscience, Western Cape Unit, POBox 572, Bellville, 7535, Cape Town, South Africa 4 North-West University, Potchefstroom Campus, PO Box 19140, Noordbrug, 2522, South Africa Abstract This paper provides an overview of groundwater-planning tools that were developed during a Water Research Commission project that was initiated due to the need to place the signifcant knowledge on groundwater of the Karoo Basin within the realms of water resource planning. In essence, the project aimed to identify favourable areas of groundwater potential for bulk municipal water supplies, to provide a method to quantify them, and to package the information so that it is assessable for planning purposes. In identifying favourable groundwater areas, the focus turned to developing a detailed transmis- sivity map of the Main Karoo Basin. In order to present yields in an accessible manner to water-supply planners, the same concept used in surface-water resource assessments and dam or reservoir design were adapted and applied to groundwater. Two methods were developed, namely the Aquifer Assured Yield Model and the Aquifer Firm Yield Model (the latter of which was developed into a software package together with the other products). The Aquifer Firm Yield Model provides the historical frm yield and uses historical monthly rainfall data together with recharge, evapotranspiration and basefow to determine aquifer storage in any given month. The frm yield can be considered to defne the upper limit of the groundwater resource. In order to establish possible wellfeld yields, the C-J Wellfeld Model (based on the Cooper-Jacob approxima- tion of the Theis groundwater-fow equation) was developed whereby borehole spacing can be optimised after inputting estimated transmissivity values from the transmissivity map. To aid the planning process, groundwater-quality maps were produced together with the Wellfeld Cost Model which provides an easy way to obtain frst-order cost estimates of the wellfeld options. This paper briefy describes these ‘tools’ that were produced and provides slightly more detail on how the transmissivity maps were developed Keywords: aquifer yield, wellfeld yield, transmissivity, water-balance models, groundwater quality, well-feld costs Introduction This paper describes the resources that were developed during the Water Research Commission (WRC) project entitled ‘The delineation of high-yielding wellfeld areas in Karoo Aquifers as future water supply options to local authorities’ (Murray et al., 2012). This project aimed to identify favourable areas of ground- water potential for bulk municipal water supplies, to provide a method to quantify them, and to package the information so that it is assessable for planning purposes. A software package containing these ‘tools’ was developed which is available from the WRC in a model suite (with the same title as this paper). The package is aimed primarily at hydrogeologists, but components of the suite such as the water-quality maps and the Wellfeld Cost Model would also be useful to others in the water resource plan- ning sector. This paper summarises these ‘tools’. Study area The study area extends over 560 000 km 2 , and is defned by the outcrop of the Main Karoo Basin (Fig. 1), but excludes the folded strata along the southern margin and areas in the north and north-eastern margin that are partly covered by Kalahari sediments. Resources or ‘tools’ for identifying and quantifying suitable groundwater areas A process for identifying and quantifying suitable groundwater targets is outlined in Fig. 2. The items in italics on the right-hand side of the diagram are the resources or tools that were devel- oped during this project. Each step in the process makes use of a number of resources, both existing and new. For example, the frst step in identifying areas of high groundwater potential usually involves identifying existing high-yielding boreholes and aquifers, and identifying drilling targets using geological maps and airborne or satellite imagery. The transmissivity maps are based on the geological maps and can therefore be used in the early stages of identifying drilling target areas. Groundwater quality maps In order to provide guidance on expected groundwater quality and treatment requirements, data from the National Groundwater Archive (NGA) and the Water Management System (WMS) were assessed, merged and mapped; and based on this informa- tion, treatment requirements and associated costs were also mapped. Suffcient data were available for the mapping of salinity (electrical conductivity), sodium, calcium, magnesium,