The resource groundwater compared to surface waters is so valuable because it is available throughout the year and (normally) cleaner. Therefore, it seems necessary to investigate the possible influences of global climate changes on this precious natural good. This can be done by observing the other sections of the water cycle, since it responds very sensitively to climate changes. This is due to the fact that higher temperatures in the atmosphere and of the ocean surfaces lead to an increased energy exchange between surface and atmosphere. Warm air can induce a higher water vapour concentration than cold air (the warming up of a mass of air by two Kelvin already increases the uptake capacity of water vapour by 10 to 20 percent).
Global climate models show that when the atmospheric CO2-content doubles (which is expected by about 2070) the average global precipitation per year will rise by 3 to 15 percent, and the global evapotranspiration (the total water evaporated from plants and open water surfaces) by 5 to 10 percent. 10 to 20 percent more rainfall are expected in the mid-latitudes in winter, but there will be less rain in summer. In arid and semi-arid regions precipitation amounts are expected to decrease even further.
The forecast for tropical and subtropical regions is more uncertain, but changes are expected to be less pronounced than in the temperate zones. Generally a shift can be expected towards higher precipitation amounts, higher variability of rainfalls and more extreme occurrences.
Climate change makes an impact
Groundwater resources are generally influenced by the following factors:
- Changes in annual precipitation amounts, intensity and seasonal distribution
- Changes in the evapotranspiration due to temperature changes, but also in the vegetation
- Occasionally increased abstraction of groundwater (in some places the only water resource throughout the year) in drier periods.
In this context, surface-near aquifers which feed the bigger part of their water into rivers, lakes, wetlands and springs are supposed to react in the most sensitive way. Due to their slow flow rate and the long dwelling periods, groundwater systems generally react much slower to climate changes than surface waters. Extreme examples for this phenomenon are arid regions, where fossil groundwater is not seldom pumped, i.e. water that has formed several thousand years ago under totally different climatic conditions.
Regional trends
Groundwater resources under the impact of climate changes give rise to fears of the following scenarios:
the thermic dilatation of the upper layers of the ocean surfaces as a result of higher temperatures leads to a rise of the ocean level. This again causes an increased saltwater intrusion into the aquifers and estuaries in coastal regions (e.g. India, China, Egypt, the Netherlands). The saltwater/groundwater boundaries move inland. In agricultural irrigation the use of saltier groundwater and a reduced drainage will result in an increasing salting of soils.
Model calculations concerning the impact of global warming on the new formation of groundwater in selected rugged lime aquifers in Europe show the following scenario: in maritime zones (e.g. Great Britain) groundwater renewal is expected to increase, in the continental zone (e.g. Germany) it is supposed to decrease in summertime. In Mediterranean zones (e.g. Spain) groundwater renewal is expected to decrease by 15 percent.
In addition to the changes in the water balance mentioned, water management will be facing a further problem: certain methods used for the calculation of groundwater renewal (e.g. the chloride method, which uses the chloride dissolved in storm water as tracer) start out from constant climatic framework conditions. These methods cannot be unrestrictedly used in precipitation conditions changed to such an extent.
In how far the demonstrated direct consequences of global warming outweigh the indirect ones, i.e. those generated by man in response to climate changes (e.g. the impacts of the increased demand for groundwater for agricultural irrigation or increasing dry periods), still remains mere speculation.
(Source: aqua press Int. 2/2001)
Dr. Harald Kunstmann
Information & Contact:
Fraunhofer-Institut für Atmosphärische Umweltforschung (IFU)
Dr. Harald Kunstmann
Kreuzeckbahnstraße 19
D-82 467 Garmisch-Partenkirchen
Tel. +49 88 21 183 208
Fax +49 88 21 183 243
