Climate Change and Water Supply - Consequences of Climate Change and Potential Adaptation Strategies - page 03



Rising air and water temperatures

Higher air temperatures increase the vertical temperature gradient in lakes and reservoirs. Thermal stratification tends to become more stable; full circulation - required for the renewal and oxygen supply of the hypolimnion, which is generally crucial for raw water abstraction, occurs more rarely, decreases in length or may never re-occur in isolated cases. Moreover, prolonged periods of heat will result in deeper epilimnion strata, thus reducing the depth of the hypolimnion. Increasingly insufficient deep mixing with ensuing deep water replenishment has already been observed in Lake Constance during the winter season.

Higher temperatures generally accelerate biological and chemical processes in water bodies. This tends to adversely affect raw water quality; the extent of the impact however also depends on other parameters such as the availability of nutrients and oxygen. These may foster the growth of algae which may result in algal blooms and a concurrent formation of odours and flavours, as well as, the release of bacterial exo- and endotoxins. Another consequence may be delayed phyto- and zoo-plankton growth, resulting in the degeneration of today’s food chains within aquatic communities.

Higher air and soil temperatures may also lead to higher drinking water temperatures in distribution networks. Whether higher temperatures increase the risk of microbial growth and contamination depends very much on the general condition and operation of the supply system. In networks with a given tendency towards microbial recontamination, this tendency will be fortified by higher temperatures.

Higher air and water temperatures tend to favour the proliferation of a variety of waterborne pathogens. Any impairment of drinking water is possible only in exceptional cases. In general, the monitoring and treatment of raw waters potentially at risk (surface water, near-surface groundwater and spring water) is already focusing on the presence of pathogens.


Indirect consequences

Higher soil temperatures promote conversion and mineralisation processes in soils and, consequently, the pollution of seepage water. These processes depend on sufficient soil moisture. The trend towards drier summers increase seasonal topsoil desiccation which inhibits mineralisation processes. The conversion and displacement of the accumulating substances is delayed until infiltration of the vadose zone occurs in autumn, which may result in a considerable mobilisation of substances (e.g. of nitrate) and subsequent pollution of seepage water.

Intensification of agriculture with growing demand for irrigation, fertiliser and pesticide use triggered by the cultivation of energy crops for biofuel production, the extension of the vegetation period, and decreasing precipitation during that period.

Extreme weather events like rain storms, hailstorms or droughts may lead to crop failures or even ruin the entire standing growth, causing plants not to absorb fertilisers and/or the fertilisers to remain in the plant residues, which may result in massive nitrate pollution of groundwater.

Conflicts about the use of locally or regionally available water resources: locally and temporarily, water resources may not be sufficient to satisfy the demand of all users (i.e. water suppliers, households, farmers, commerce and industries) in a region.

Peak demand increases: the dry summer of 2003, for instance, demonstrated that water consumption increases during dry and hot periods. As a result, the gap between average and peak water demands grows. The situation becomes even worse in regions with decreasing average water demand caused by e.g. a population decrease, a change in industrial consumption etc. Under such circumstances water suppliers face new challenges for the design, construction, and operation of water supply systems.

As extreme weather events and flood events are likely to increase in frequency and intensity, dam and reservoir management will have to focus more on flood protection, which in turn may result in reduced storage capacities for raw water abstraction.