Estimation of the preliminary groundwater reserve using numerical models

[摘要] English: The Water Act requires the determination of the Groundwater Reserve to secure thebasic human needs and to protect the environment (aquatic ecosystems in terms of theAct). In other words, it is necessary to determine the allocatable groundwaterresources and its use may not cause unacceptable negative impacts. As numericalmodels have been greatly developed in assessing groundwater resources in recentyears, it is used to estimate the Groundwater Reserve.An investigation into existing groundwater numerical models has been done in orderto select the suitable numerical model for determining the Groundwater Reserve. TheMODFLOW program is selected for the present work because of the widespread ofapplications and the existence of user-friendly graphical interfaces, e.g. Chiang andKinzelbach (in press).The procedure of estimating the allocatable groundwater resources can generally bedivided into two steps:The first step is to estimate the total accessible groundwater resources by means of theregional groundwater balance. Groundwater fluxes, recharge, abstraction, and leakageto/from surface water bodies are balanced over the whole catchment. Themethodology is used for the Pienaars River catchment, where the groundwaterrecharge is the only inflow term to the aquifer and its determination will directlyaffect the result. The interaction (water exchange) between groundwater and surfacewater (rivers and wetland) cannot be quantified without additional data, especiallyriver flow rate and evaporation measurements in proper positions. To date, mostavailable computer models are unable to compute the dynamic interaction betweenriver stages and the groundwater levels. As a result, in most MODFLOW (or othergroundwater 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 waterprofiles (river stages) due to groundwater or surface water abstractions must bepredicted. A computer program which is able to couple the groundwater and surfacewater flow models, will be useful for computing and predicting both the groundwaterand surface water levels and their interactions.The second step is to estimate possible negative environmental impacts due togroundwater abstractions. The estimated impacts are scales for decision-makers togive allowances to future abstractions. There are many possibilities to perform thesecond step, for example post-auditing or by using numerical models. The former isoften not appreciated, because most groundwater systems are difficult or impossibleto restore once they are damaged or contaminated. The latter is superior due to itspredictive capability and its flexibility at the price of the data requirement.Because of the complexity of the saltwater intrusion processes, the programSEAWATER is developed to solve the steady-state salt-/freshwater sharp interface forusing with MODFLOW. SEAWATER calculates the interface by means of aniterative process, which uses the Ghyben-Herzberg approximation to determine thelocation of the interface. The results of the program are compared with severalanalytical solutions.

[发布机构] University of the Free State