Estimation of the preliminary groundwater reserve using numerical models

English: The Water Act requires the determination of the Groundwater Reserve to secure the basic human needs and to protect the environment (aquatic ecosystems in terms of the Act). In other words, it is necessary to determine the allocatable groundwater resources and its use may not cause unacceptable negative impacts. As numerical models have been greatly developed in assessing groundwater resources in recent years, it is used to estimate the Groundwater Reserve. An investigation into existing groundwater numerical models has been done in order to select the suitable numerical model for determining the Groundwater Reserve. The MODFLOW program is selected for the present work because of the widespread of applications and the existence of user-friendly graphical interfaces, e.g. Chiang and Kinzelbach (in press). The procedure of estimating the allocatable groundwater resources can generally be divided into two steps: The first step is to estimate the total accessible groundwater resources by means of the regional groundwater balance. Groundwater fluxes, recharge, abstraction, and leakage to/from surface water bodies are balanced over the whole catchment. The methodology is used for the Pienaars River catchment, where the groundwater recharge is the only inflow term to the aquifer and its determination will directly affect the result. The interaction (water exchange) between groundwater and surface water (rivers and wetland) cannot be quantified without additional data, especially river flow rate and evaporation measurements in proper positions. To date, most available computer models are unable to compute the dynamic interaction between river stages and the groundwater levels. As a result, in most MODFLOW (or other groundwater models) applications, the river stages are often assumed to be known. This is, however, not applicable in many cases, where the change of the surface water profiles (river stages) due to groundwater or surface water abstractions must be predicted. A computer program which is able to couple the groundwater and surface water flow models, will be useful for computing and predicting both the groundwater and surface water levels and their interactions. The second step is to estimate possible negative environmental impacts due to groundwater abstractions. The estimated impacts are scales for decision-makers to give allowances to future abstractions. There are many possibilities to perform the second step, for example post-auditing or by using numerical models. The former is often not appreciated, because most groundwater systems are difficult or impossible to restore once they are damaged or contaminated. The latter is superior due to its predictive capability and its flexibility at the price of the data requirement. Because of the complexity of the saltwater intrusion processes, the program SEAWATER is developed to solve the steady-state salt-/freshwater sharp interface for using with MODFLOW. SEAWATER calculates the interface by means of an iterative process, which uses the Ghyben-Herzberg approximation to determine the location of the interface. The results of the program are compared with several analytical solutions.