An evaluation of priority and fractional methods of water allocation in the sand river catchment, South Africa.
The development and apportionment of water resources is a critical issue, both globally and locally in South Africa. This is particularly true in the development and allocation among states sharing watercourse systems. The competition inherent in access to water resources is increasing. In particular, pressure is being placed on water resources from several activities including irrigation, domestic consumption and industrial requirements. Water allocation mechanisms are therefore critical to sustain the existing allocatable water resources while attempting to combine both efficiency and equity principles. The National Water Act of South Africa (Act 36 of 1998) (NWA (36, 1998)) incorporates both institutional and legal policy which promotes the efficient, equitable and sustainable management of water resources. The aims of the NWA (36, 1998) are achieved by a movement away from a Riparian Rights system (a property adjacent to a water course is allowed reasonable use) to an Administrative System (Hallowes et al., 2008). The inception of an Administrative System for the allocation of water in South Africa is vital given that a number of catchments in South Africa have reached a state of being fully developed and more than 50% of the 19 water management areas in South Africa are water stressed, i.e. the demand exceeds the supply (DWAF, 2004). The NWA (36, 1998) makes allowance for only one right to water; that being the Reserve, which consists of two components, the ecological requirement and basic human needs. The management of the resource is important because the NWA (36, 1998) states that the water resources within South Africa are to be protected, used, developed, conserved, managed and controlled in accordance with the National Water Resources Strategy (DWAF, 2004). The water allocation method currently applied in South Africa is referred to as a Prioritybased River and Reservoir Operating Rule (PRROR) institutional arrangement. Under PRROR, when there is a risk of a reservoir or river failing to meet the supply demanded, restrictions are applied to abstractions. The priority extends not only to those who have the priority of use but which users will relinquish water to the higher priority users and by what quantity. Disadvantages of PRROR include the inability of the Water User to manage their water to meet their needs and are then forced into using it when the water is available. Possible alternate allocation methods include Fractional Water Allocation and Capacity Sharing (FWACS), public water allocation and prior rights systems. The PRROR as currently implemented leads to high priority sectors having dominance over access to water which may lead to those sectors not using water efficiently. The introduction of FWACS creates an atmosphere of water awareness and being responsible for managing water use. In this study, the MIKE BASIN model was used in the simulation of the processes of the PRROR and the FWACS allocation methods. The model routes water based on rules specified for the allocation method under review. The efficiency of each allocation method was evaluated in terms of the reliability of supply to Water Users. In the catchment used as a case study (Sand River Catchment), limited information on Environmental Water Requirement (EWR) was available and the EWRs were set as minimum flows at each reservoir and then set as a minimum flow requirement at a downstream node to prevent Water Users downstream of the dam from immediately abstracting the EWR release. Based on data used in the case study and the rules applied to each scenario, the results from the initial study indicated that PRROR provides a 4% higher reliability of supply in comparison to FWACS in the catchment under investigation. This is true when the supply to a Water User is similar between scenarios. However, if the fractions allocated in FWACS are varied away from this baseline, results indicate that a 50% increase on the original FWACS fractions provides for better reliability of supply. Thus the results show that although PRROR is an alternative method for determining water allocation to water users, FWACS+50 is able to improve on the water reliability of supply within the Sand River Catchment.