| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Ground Water > Groundwater level |
Brief Description: 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 paleowaters (fossil groundwater) stored from earlier periods of wetter climate: the reduction of these reserves is comparable to 'mining'. In alluvial plains, reduction in streamflow reduces the rate of natural recharge to aquifers: in Bangladesh it is estimated that local groundwater levels have dropped nearly 3 m because of upstream dams and diversions of the Ganges. 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. However, springs may be perennial, intermittent, or periodic, and their discharge may depend on changes in climate, tides, and local underground conditions such as changes in rock stresses. Significance: 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. The availability of clean water is of fundamental importance to the sustainability of life. It is essential to know how long the resource will last and to determine the present recharge: groundwater mining is a terminal condition. Human or Natural Cause: There are natural changes in groundwater levels because of climate change (drought, pluvial episodes), but the main changes are due to human abstraction. In many places artificial recharge of aquifers is accomplished deliberately by pumping or as an indirect result of irrigation. Environment Where Applicable: Wherever groundwater is extracted for human use (drinking, irrigation, industrial use), or where it influences the ecosystem (e.g. in wetlands). Paleowaters are of particular importance in aquifers in arid and semi- arid regions. Scale: patch to landscape / regional Types of Monitoring Sites: Boreholes, wells or springs representative of the particular aquifer. Method of Measurement: Monitoring of the depth to the water table is carried out using manual measurements, automatic water-level recorders, or pressure transducers. Standard hydrogeological methods are used to calculate a water balance. Current recharge rates must be calculated taking account of climatic variability over recent decades. Frequency of Measurement: Minimum monthly intervals to reflect seasonal as well as annual changes. The state of fossil aquifers should be assessed at about 5 year intervals. Limitations of Data and Monitoring: Water levels need to be measured both seasonally and annually over decades to determine overall trends. Overall accuracy of manual methods is around 1 cm, but this can be reduced to millimetres with automation. Applications to Past and Future: Paleowaters may provide an archive of past climatic variations. Possible Thresholds: 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 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. |
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