Aquifer Depletion

Groundwater is the major source of water in many regions, supplying a large proportion of water globally. In the USA, more than half the drinking water comes from the subsurface: in arid regions it is generally the only source of water. In these areas, the availability of groundwater is of fundamental importance to both livestock and humans. It is essential to know how long the resource will last and to determine the present rates of recharge. | Pressure State Response Further reading

Pressure

Groundwater is replenished from precipitation and from surface water, but the rate of abstraction (withdrawal by humans) may exceed the rate of natural recharge, leading to reduction of the resource. Some aquifers, especially in arid and semi- arid regions, contain paleo-waters (fossil groundwater) stored from earlier periods of wetter climate and where current rates of replenishment are low (or in some cases virtually non-existent). The reduction of these reserves of water is comparable to mining – once they are depleted to levels where abstraction is no longer feasible or is uneconomic, these resources will no longer be available. In alluvial plains, reduction in streamflow reduces the rate of natural recharge to aquifers.

State

There are natural changes in groundwater levels because of climate change (drought, pluvial episodes), but the main changes are due to human abstraction. At some sites it is possible to artificially recharge aquifers directly by pumping or indirectly as a result of irrigation.

Examples: In Bangladesh it is estimated that local groundwater levels have dropped nearly 3 metres because of upstream dams and diversions of the Ganges. Botswana and Namibia have already reached their internal water supply limits. Almost all the states of the Arabian peninsula, plus Jordan, Israel and Libya, already consume much more water than their annual renewable supply.

Measurement on a regular basis of water levels in wells and boreholes or of spring discharge provides the simplest indicator of changes in groundwater resources. Measurements need to be carried out at boreholes, wells or springs that are representative of the particular aquifer in question and these data spatially integrated (e.g. using GIS systems) to obtain the overall picture over both space and time.

Monitoring of the depth to the water table is carried out using manual measurements, automatic water-level recorders, or pressure transducers. Standard hydro-geological methods are used to calculate a water balance. Current recharge rates must be calculated taking account of climatic variability over recent decades. Measurements must be made at a minimum of monthly intervals in order to reflect seasonal as well as annual changes.

A threshold is crossed when the rate of abstraction exceeds the rate of recharge, and a sustainable renewable resource becomes a non-renewable mined one. When pumping a well or borehole exceeds the rate of lateral inflow, the well dries out and a threshold has been crossed, though the situation may reverse itself when pumping ceases or when recharge increases.

State is therefore characterised by:

- Numbers and density of boreholes
- Change in groundwater levels
- Change in rates of abstraction from existing boreholes
- Rates of construction of new boreholes (as a result of pressure from the livestock sector)

Response

- There is an obvious need for careful and comprehensive monitoring of groundwater resources, and rates of recharge;
- There is also a need for the development and enforcement of regulations concerning sustainable rates of water abstraction;

In addition, the following points are also relevant:

- The issue of water pricing requires careful attention;
- In some cases, the settlement of cross-border water rights issues will be increasingly important, especially in those areas where aquifer replenishment and groundwater use are spatially distinct;
- Improved integration between land and water management, ensuring sustainability of flora, fauna and habitats;
- Research into how issues of land tenure and property rights, including common property concerns, affect the way water is managed and water services delivered;
- Research into the most appropriate role for and the most effective way of incorporating indigenous knowledge into water decision-making and management.

Further Reading:

Perkins, J.S., Thomas, D.S.G., Allison, R.J. 1993. Environmental responses and sensitivity to permanent cattle ranching, semi arid western central Botswana. Landscape sensitivity. 1993, 273 286

Landsberg, J., James, C.J., Morton, S.R., Hobbs, T.J., Stol, J. Drew, A. & Tongway, H. 1997. The Effect of Artificial Sources of Water on Rangeland Biodioversity. Final Report to the Biodiversity Convention and Strategy Section of the Biodiversity Group, Environment Australia. Environment Australia and CSIRO. Commonewalth of Australia.

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