Kp Seiler1 Wz Gu2 W Stichler1

lGSF National Research Centre, D-85758 Neuherberg, Germany (e-mail: [email protected]) 2Institute of Hydrology, Hohai University, Nanjing, P. R. China

Abstract: Groundwater flow is steered by both the groundwater recharge rate and by discharge altitudes above or below sea level; it is further controlled by the hydraulic properties of the aquifer system and often contains a transient flow component affected by natural hydrologic processes. All present groundwater discharges have both recent (< 100 years) and past groundwater recharge components (> 100 years). The ratio of the present to the past groundwater recharge depends on the climate zone: it is large in humid and small in arid areas, hence at low recharge rates transient, and at high groundwater recharge rates steady-state conditions prevail. Developing groundwater management strategies while neglecting any transient response of groundwater resources, and conducted in sensitive recharge/discharge areas like dry lands, results in either over-estimates or under-estimates of safe yields of groundwater resources, and thus may lead to non-sustainable resource development. The consequence of this would be groundwater depletion and often also a deterioration of the hydraulic properties of the aquifer system by subsidence, which both take place only after a long period of time.

Few issues have produced as much scientific and political attention and controversial debate as the increase of greenhouse gases (CO2, N2O and CH4) in the atmosphere from combustion processes, their effect on global warming, and its consequences for the life and health of human beings and ecosystems including groundwater regime.

The global warming scenario, from the beginning of the industrial age until the end of the twentieth century, caused an increase in the average climate temperature of about 0.6°C and an average sea level rise of about 0.1 to 0.2 m. In the same period of time rainfall increased in some areas in the north of the northern hemisphere and decreased in some areas of the tropics and subtro-pics of Africa and southeast Asia (IPCC 2001).

Climate modelling predicts for the end of the twenty-first century a temperature increase of a minimum of 1.4°C to a maximum of 5.8°C as compared to the beginning of the industrial period (IPCC 2001). The consequences of this include: an increase of evaporation, transpiration and precipitation, but also aridity; a higher variability of the rate and intensity of precipitation, and further decrease of precipitation in South Africa, Australia, the Near East, Mediterranean and in Central America; a shift of the magnitude and time of peak stream flow in mountain areas from smooth to strong, from mid- or late spring at present, to the end of the winter season in the future; increased melting and calving of polar ice; the Arctic Sea will be partly or completely free of ice during the summer period, leading to a decrease in the overall albedo; a further mean sea level rise from

1990 to 2100 of 0.1 m to 0.9 m, exceeding the rate observed in the twentieth century, hence damaging coastal lowland areas and moving the salt water-fresh water interface landwards, a reduction of the permafrost in the northern hemisphere by about 0.5 m and the disappearance of lakes in grasslands of the same areas.

Thus, in the future global warming is expected to have a major impact on the magnitude of both surface and subsurface runoff. Experience has shown that such changes show up instantaneously in compartments with short turn-over times such as rivers, but in freshwater reservoirs with long turn-over times, such as groundwater, they appear with delay times of decades or even millennia. Table 1 compiles mean turn-over times (MTT) for important surface and subsurface freshwater reservoirs. It can be seen that surface water has MTTs of days, shallow groundwater of years, deep groundwater and glaciers of centuries to millennia; MTTs of lakes are in between those of surface and subsurface reservoirs.

Globally, the predicted trend of precipitation changes related to the present water availability on continents would not result in serious problems for the Americas and Australia (> 15000 m3/ capita/year; Table 2) as compared to a total water demand (direct and indirect water) of 1000 to 1500 m3/capita/year. In contrast, some areas in southern Europe, north Africa, and the Near and Far East already suffer, or will suffer in the near future, from water scarcity.

We are living in an exceptional geological period, the Cenozoic glacial period, with natural,

Table 1. Mean turn over times (MTT) of waters in different reservoirs

Reservoir type

MTT

Cold glaciers

>100 000 years

Temperate glaciers

<500 years

Connate water

> 1 000 000 years

Deep groundwater

>100 years

Shallow groundwater

<100 years

Lakes

c. 15 years

Rivers

c. 15 days

Atmospheric water vapour

c. 10 days

dramatic climate changes, to which man-made short-term changes become an additional, accelerating challenge. Against this background, short-term solutions to management and protection of freshwater resources have been discussed in detail, but developments of water resources, lasting for more than a few generations, have not yet been well considered. To compile such information, past long-term changes of groundwater discharge and heads can be analysed in water-sensitive areas such as dry, cold or flood areas, using geohydraulic and environmental isotope techniques. Then, these results can be extrapolated to the future and allow a better estimate of the consequences of expected climate changes for subsurface waters. Generally the focus is on short-term strategies for development of groundwater resources to address the challenges caused by climate changes rather than long-term strategies for long-lasting solutions. This paper deals with one of the many aspects of the response of subsurface water resources to climate changes (e.g. global warming), especially the transient behaviour/response of subsurface water in dry lands.

Table 2. Available water (1921-1985) in different world regions as related to a yearly bulk discharge from continents of 44 800 km3

Continent Percentage of bulk Available water in discharge (m3 / capita / year)

Table 2. Available water (1921-1985) in different world regions as related to a yearly bulk discharge from continents of 44 800 km3

Continent Percentage of bulk Available water in discharge (m3 / capita / year)

Asia

31.5

3920

Europe

6.7

4200

Africa

9.8

5720

N America

18.4

17 400

S America

28.0

38 200

Australia &

5.6

83 700

Oceania

Data from UNESCO (1999).

Data from UNESCO (1999).

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