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

Runoff, river discharges and climate change
Average annual runoff in Europe varies widely, from less than 25 mm in southeast
Spain to more than 3000 mm on the west coast of Norway. Predictions of the effect
of climate change on river flow are uncertain, and the results of different models are
highly variable. Arnell and Reynard (1996), for example, modeled river flows in the
United Kingdom under various climate change scenarios and found that, under all
scenarios, the concentration of flow increased in winter. All models predict that
monthly flow would change by a greater percentage than annual flow. However,
different catchment areas would respond differently to the same scenario;
progressive change would be small compared with variability over a short time scale,
but that it would be noticeable on a decade-to-decade basis.
As a consequence of increased precipitation intensities, peak runoff is subject to
increase, thus increasing the occurrence of flood events. As a general trend,
because runoff is more directly dependent on precipitation rather than on
temperature increase, runoff in higher-latitude areas (e.g. Rhine and Danube) would
decrease in the spring (as a result of less precipitation falling as snow in the winter,
leading to less snow melting in the spring) and increase in the winter. In mild
temperature climates, annual runoff would remain constant, but display a more
extreme seasonal cycle. Catchments with considerable groundwater would display
changes in summer flows that are largely a function of the change not in summer
rainfall but in rainfall during the winter recharge season. In arid and semi-arid
regions runoff would be most immediately linked to precipitation, and a given
percentage change in rainfall would produce a considerably larger percentage
change in runoff. Projected changes in annual water runoff by the year 2050 are
presented in figure III.6.
Infiltration, Deep Percolation and Groundwater Flow
Infiltration is the process by which water enters the soil, moving from the surface to
lower soil horizons. Some of the water entering continues to flow close to the surface
(subsurface runoff), to resurface at some distance or to flow into a stream, while
some moves to greater depths (deep percolation) eventually reaching a groundwater
body (aquifer). Infiltration rates are governed by soil water characteristics (soil water
content, water retention characteristics, and hydraulic conductivity). Water flow in
saturated soils is governed by Darcy’s law (Darcy, 1856). Unsaturated flow is
described by Richards’ equation (Richards, 1931). For a complete review on
infiltration and soil water movement see Rawls et al. (1993).
Aquifers are subterraneous porous medium formations that store and transport large
masses of water. Two soil layers can be distinguished below ground level: in the one
nearest the surface gaps between soil particles are filled with both air and water
(unsaturated zone). Below this layer is the saturated zone, where the gaps are filled
with water. The boundary between these two layers is called water table. As the
amount of water in the aquifer increases or decreases, the water table rises or falls
accordingly. When the entire area below the ground is saturated, flooding occurs
because all subsequent precipitation is forced to remain on the surface. Aquifers are
distinguished in unconfined (when only the bottom is confined by an layer of
impervious material, so that the aquifer is in contact with the atmosphere through the
unsaturated zone) and confined (when the aquifer has impervious layers both at the
bottom and at the surface). Both types of aquifer have a recharge area (where water
infiltrates the ground surface to reach the aquifer) and a discharge area (sea, river,
lake). The movement of water in the aquifer is driven by porous matrix
characteristics such as porosity, hydraulic conductivity, permeability, transmissivity
and storativity (see Smith and Wheatcraft, 1993, for a description of aquifer hydraulic
characteristics and corresponding methods for measurement or estimate). Such
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