Section A - Groundwater Management: Best Practice Guideline
1. The Management Objective
The amount of groundwater stored in an aquifer at any instant of time depends on the dynamic relationship between recharge inputs, through the overlying land surface and from rivers, and outflow to surface waters and pumped abstraction. Aquifer storage provides a buffer between highly variable, climatically-driven recharge processes and the less variable outflow that supports surface water ecology. Abstraction of groundwater for human use, and some kinds of land use changes, alters the dynamic balance between "natural" recharge and the state of surface waters. The resource management objective is to determine the regime of abstraction that results in acceptable environmental effects.
2. Concepts of Sustainable Yield
2.1. A MISCONCEPTION
Occasionally, there appears in the media a statement to the effect that "...we must conserve groundwater for our grandchildren...". This may arise from a concept of groundwater as a static, finite body of water, which is mined by any abstraction. What should be passed on to future generations are the beneficial results of a well-managed, dynamic water resource system.
2.2. GROUNDWATER BUDGETS
One resource management approach is to attempt to estimate all the input and output components of the groundwater budget, and then make a decision about what can be safely allocated for abstraction on a "sustainable" basis. In practice, it is very difficult to measure independently the quantity of recharge from rivers or the natural outflow from an aquifer to surface waters, for example. Selection of the recharge proportion to be used as sustainable yield can become an arbitrary choice that may bear no relation to the effects of abstraction.
2.3. DYNAMIC STORAGE
The amount of groundwater present in an aquifer at any instant of time depends on the recent (months or years) history of climatically-driven recharge processes, natural outflow to surface waters, and abstractions for human use. The recharge processes are highly variable through the land surface, and less so for river recharge. Natural outflows are less variable than the recharge history, because of the smoothing effect of groundwater storage, and abstraction is potentially measurable and manageable.
This is the picture of a dynamic water storage responding to inflows that have varying degrees of randomness. At most locations, this water storage is intimately connected to the surface water environment and the ecology has adapted to the local regime of natural variability of water supply. Abstraction of groundwater for human use alters the regime of variability, and therefore affects the natural environment. The conceptual basis for management should be understanding of system behaviour rather than a budgetary approach.
3. Scale of Abstraction Effects
The theory of groundwater wells shows that the piezometric effect of groundwater abstraction propagates away from the well in a radial direction, and that this effect decreases in magnitude with distance. Most applications of this theory are directed to estimating the effects of drawdown from a well, in terms of interference with other wells or influence on stream-aquifer interaction. The distance scale of these investigations is usually up to a few kilometres, and the time scale is up to a few months. These methods are well established and are not considered any further in this report. However, it is important to realise that these applications of well theory limit the estimation of effects to magnitudes that are significant or are practically measurable.
Any abstraction from an aquifer has an effect that eventually propagates throughout the whole aquifer. This effect may be a lowering of piezometric levels or induced additional recharge from a river. The effect from any one well may be infinitesimal in terms of practical measurement, but the cumulative long-term effects of many wells can be very significant. The result is that every user of groundwater from an aquifer is a contributor to environmental effects such as reduction of low flows in streams or salt water intrusion, which are determined by natural outflow to surface waters at the whole-aquifer scale.
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