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

eduction in the rate of evaporation from a catchment despite a temperature driven
increase in evaporative demands. In areas with sufficient available moisture (regions
in northern Europe), increases in temperature would lead to increases in evaporation
and evapotranspiration: water in the atmosphere can affect the radiation balance of
the planet through water vapor greenhouse and cloud greenhouse effects and
through reflection of sunlight by clouds and ice. Arnell (1996) estimated for a sample
of UK catchments that the rate of actual evaporation would increase by a smaller
percentage than the atmospheric demand for evaporation, with the greatest
difference in the “driest” catchments, where water limitations are greatest.
Figure III.5. Annual potential evapotranspiration map for Europe (EEA, 1996).
Runoff and river discharge
Runoff is the movement water over land, and consists of that portion of precipitation
that is not evaporated, transpired, or infiltrated in the soil. The magnitude of runoff
after a precipitation event is determined chiefly by the soil characteristics and soil
conditions prior to the precipitation event: precipitation on saturated soil is likely to
produce a greater runoff than the same event on the same soil under dry conditions.
Soil cover also plays a role, as intercept of water by vegetation can delay runoff or
decrease it. Water flowing over land can be trapped in small irregularities on the
ground (depression storage), or trickle to form networks, thus routing from small
streams to larger channels to sea. High-intensity precipitation events or
long-duration/low-intensity precipitation (but also snowmelt or dam and levees
operation) can cause excess runoff, which in turn can lead to flooding. The study of
runoff and channel routing processes is of fundamental importance for protection of
human settlements and infrastructures.
Peak discharge originating from excess runoff can be calculated with a variety of
methods. For small to medium size watersheds the most widely used methods are:
the rational method (see Pilgrim and Cordery, 1993, and references therein, for a
description of the method), the U.S. Soil Conservation Service method (see Singh,
1982, for a review of main procedures and subsequent developments), and the
regional flood frequency methods (see Stedinger et al., 1993 for a review). Fread
(1993) provides a review of channel flow routing.
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