Climate Change and the European Water Dimension - Agri ...

ack to the oceans via transport in streams, rivers, lakes, reservoirs, wetlands and
groundwater bodies, to begin its cycle anew.
Approximately 42,000 km 3 of precipitation flows each year to the oceans through the
world’s rivers. Some of the precipitation will seep down into the Earth’s surface and
become groundwater. Some of it will be taken up by plants and subsequently
released in gaseous form back into the atmosphere via transpiration. A substantial
quantity of water is returned to the atmosphere in this manner, thereby shortcircuiting
the full hydrologic cycle. It is estimated, for example, that a hectare of corn
can transpire about 30-40 m 3 of water into the atmosphere each day. Nevertheless,
because of their enormous surface area, the most important source of water in the
atmosphere is evaporation from the oceans, which comprises nearly 90% of the total
global evaporation. Indeed, more water evaporates from the oceans than is directly
precipitated back, thus creating the driving force for the hydrologic cycle.
III.2. HYDROLOGIC CYCLE COMPONENTS AND CLIMATE CHANGE
Geographical Distribution of Resources
The magnitude of each component of the hydrologic cycle in a region or country is
determined by a number of factors, including the amount of water received from
precipitation, inflow and outflow in rivers and aquifers (this factor is particularly
important in transnational water bodies) and the amount lost by evaporation and
evapotranspiration (influenced to a large extent by local land use/land cover).
Human activities also greatly affect individual components of the hydrological cycle,
through actions such as water abstraction from ground and surface water bodies,
through irrigation, and morphological changes. Methods for calculating the availability
of freshwater resources vary considerably from country to country, making
comparison difficult. Rees and Cole (1997) have developed a method of estimating
the renewable freshwater resources across the EU. The method uses data from
hydrometric (river gauging) networks, supplemented by an empirical freshwater
balance model that relates runoff to precipitation and potential evaporation.
Freshwater resources vary considerably across the European Region: annual runoff
ranges from over 3000 mm in western Norway, to 100 mm over large areas of
Eastern Europe, and to less than 25 mm in inland Spain (figure III.3). The average
annual runoff for the member countries of the European Environment Agency (EEA,
1998) is estimated to be about 3100 km 3 per year. This is equivalent to 4500 m 3 per
capita per year for a population of 680 million (Stanners and Bourdeau, 1995). The
population of the WHO European Region is some 870 million, but figures for total
runoff are not available. Specific processes govern the generation of each individual
component in the hydrologic cycle. The following paragraphs offer a description of
each component, and how each could be possibly affected by climate change.
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