Groundwater balance modelling with Darcy's Law

Abstract

The sustainability of groundwater resources is important for the environment, the economy and communities where surface water is scarce. It is a hidden resource, but additional information can be extracted by combining groundwater measurements and lithological information with groundwater flow equations in groundwater models. The models convert data and knowledge about the groundwater systems into information, such as relative inflow and outflow rates and water-level predictions that can be readily understood by groundwater managers. The development of models to effectively inform groundwater management policies is, however, a complex task that presents a fundamental scientific challenge. This thesis presents methods and results for water balances calculated using groundwater flow models. Groundwater flow modelling methods and approaches are discussed, and their capabilities and limitations are reviewed. Two groundwater systems are studied for the Great Artesian Basin (GAB) and for the irrigation area near Bowen, Queensland. Three approaches to water balance modelling are applied in comprehensive model-development frameworks that take into account model objectives, data and knowledge availability and sensitivity analysis techniques. The three models show numerical methods of increasing complexity. The Bowen study area is wellsuited to the least-complex method because data collection has been a priority there. As a contrast, the GAB is a large, poorly-monitored basin for which more knowledge of the groundwater system can be gained from its simulation by the steady state and transient groundwater flow models. The Bowen impact assessment model calculates dynamic historical water balances. The GAB aquifer models are high-complexity representations of the groundwater system that include predicted responses of the system to changes in hydrological conditions. These are comprehensive and well-documented attempts to model these systems. They provide a platform for scenario investigation and future improvements. Darcy’s Law was used in a GIS (Geographic Information System) to calculate dynamic water balances for an aquifer near the Queensland town of Bowen. This is the first time this approach has been applied to generate a complete groundwater balance. Over the period 19891997 the model estimates average total inflows to be 87 % groundwater recharge by rainfall and irrigation return flow, 12 % river recharge and 1 % inflow across the study area boundary. Outflows are estimated to be 66 % evapotranspiration, 28 % water bore discharge, 4 % discharge to the ocean and 2 % groundwater loss to rivers. Analyses show that evapotranspiration is the most uncertain parameter value. The GIS method was found to be useful for calculating water balances more accurately than analytical methods, because of their simplifying assumptions, and less time consuming than the more complex numerical models developed for the GAB aquifer.