Climate Change Projections

The Rhine originates in Switzerland as a mountain river, fed by glacier water, snow-melt and rainfall. As a result, the hydrograph upstream of Rheinfelden (the Swiss part of the Rhine catchment) is largely determined by high discharges in spring due to snow-melt. Between Rheinfelden and Lobith (where the Rhine enters The Netherlands), the Rhine changes character from a mountain river, though various stages typical for medium high mountains, to a lowland river. Elevation in this region is between 100 and 1500 m, and the current climate ranges from alpine towards the moderate humid climate of the North Sea region. Inflow into the river in this stretch is determined by a large precipitation surplus in winter and a precipitation deficit during summer. On crossing the Dutch-German border at Lobith, the river has become a typical lowland river. Here its hydrograph is determined equally by the snowfall and snowmelt regime of the Alps and by the seasonal effects of precipitation surplus.

The SRES HA-2 scenario (see Chapter 2) simulates an increase in average temperature for the entire catchment upstream from Lobith (for the location see Fig. 7.2) from 8°C in the current situation to 9°C for the period 2010-2039 and 12°C for the period 2070-2099. For the catchment upstream from Rheinfelden (Fig. 7.2), the yearly temperature increase ranges from 6°C (current situation) to 7°C (2010-2039) to 10°C (2070-2099) (Fig. 7.3).

Fig. 7.2. Locations of Lobith and Rheinfelden stations.

The calculated precipitation changes are given in Fig. 7.4. For the entire basin above Lobith, average yearly precipitation will change from 2.6 mm/day (current situation) to 2.7 mm/day (2010-2039) and 2.6 mm/day (2070-2099). For the area above Rheinfelden, the projections are 4.1 mm/day (current situation), 4.2 mm/day (2010-2039) and 4.1 mm/day (2070-2099). Hence, the changes in annual water availability in the Rhine catchment will be relatively small. The temporal distribution through the year, however, is expected to change significantly. It is projected that the Rhine will change from a combined snowmelt-rain-fed river to an almost entirely rain-fed river. Precipitation projections show a decrease in runoff in summer and an increase in winter and spring. Although the actual precipitation in summer is expected to increase, the accompanying increase in evapotranspiration will result in a net decrease in effective precipitation in summer and therefore in a reduction in river discharges. Such a change will mainly affect areas that already are sensitive to drought. Additional consumption of water for irrigation may create additional depletion of water reservoirs and groundwater. The frequency of peak-flow events may increase the number of flood events in the downstream part of the river (Kwadijk, 1993; Grabs, 1997; Middelkoop et al, 2000, 2001).

Figure 7.5 (graph for Lobith) shows the variability in precipitation for the entire

Lobith

O 15

O 15

feb mar apr may jun jul aug sep oct nov dec

-2070-2099

-2010-2039

• current situation

Rheinfelden

jan feb mar apr may jun jul aug sep oct nov dec

Fig. 7.3. Temperature in the SRES HA-2 scenario for a downstream and upstream station (Lobith and Rheinfelden).

jan feb mar apr may jun jul aug sep oct nov dec

Fig. 7.3. Temperature in the SRES HA-2 scenario for a downstream and upstream station (Lobith and Rheinfelden).

catchment and the catchment upstream from Rheinfelden. Variability in precipitation for each month is here defined as the 95th percentile of the monthly precipitation for the analysed period minus the 5th percentile of the monthly precipitation. The figure shows that the variability in precipitation increases largely under the climate change projections.

Lobith

-Ar-2070-2099 --m— 2010-2039-current situation

Rheinfelden

2070-2099 • ■ 2010-2039-current situation

Fig. 7.4. Precipitation for the SRES HA-2 scenario (Lobith and Rheinfelden).

Impacts from Climate Change

Hydrology

The SRES scenarios used in this study result in a decrease of summer discharge and an increase in winter discharge. The results of the simulations of the SRES HA-2 scenarios with the Rhineflow model are given in Fig. 7.6 with the 10th and 90th percentile runoff intervals. A clear change in mean runoff is visible. The increased runoff in winter, when soils are saturated, is mainly a result of changing temperatures and increased precipitation, which greatly affects the mechanism of snowfall—snowmelt. The decreased summer runoff is caused by increased temperature and related

—2070-2099 -9— 2010-2039-current situation

-*r- 2070-2099 -m— 2010-2039-current situation

Fig. 7.5. Variability in daily precipitation (SRES HA-2 scenario Lobith).

increased évapotranspiration and by reduced contributions from snowmelt. In addition, Fig. 7.6 shows a larger variability in runoff, illustrated by the broadening of the variability bands around the mean runoff in the projection periods.

An increase in precipitation may lead to increased peak flows and hence increased flood risks in winter. The increased temperatures in summer could lead to higher local precipitation extremes and associated flood risks in small catchment areas (Kwadijk, 1993).

The projected changes in climate and hydrology will affect various user functions of the River Rhine. Higher average and extreme temperatures will enhance the demand for freshwater, in particular for agriculture and direct human consumption. Changes in precipitation patterns, particularly over regions that already are sensitive,

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