Implications of the KNMI'14 climate scenarios for the discharge of the Rhine and Meuse; comparison with earlier scenario studies

In this assessment we investigate potential changes in discharge for the rivers Meuse and Rhine due to climate change using:
1) The KNMI’14 scenarios for the Rhine and Meuse basins
2) A selection of 183 simulations from the recently developed Coupled Model Inter-comparison Project (CMIP5) datasets, that are based on the IPCC representative concentration pathways of the 5th IPCC assessment report.

To simulate discharge for the gauging stations (amongst others) Borgharen and Lobith and to simulate the flow into the main river, the hydrological rainfall - runoff models (HBV) for the Rhine and Meuse were used. Hereto the KNMI’14 and CMIP5 climate scenario sets were down-scaled to the sub-catchments of the hydrological model. For the calculation of the distribution of (extreme) high discharges for Rhine (Lobith) and Meuse (Borgharen) rivers, the Generator of Rainfall and Discharge Extremes (GRADE) was used. For these calculations, the historical time-series for precipitation and temperature were resampled to synthetic time-series of 50.000 years using the KNMI weather generators for the Rhine and Meuse basins.

For the Rhine the hydraulic SOBEK model was run to simulate the propagation of the flood wave and to include the effect of flooding on the simulated flow at Lobith. Additionally the most extreme high flows are post-processed to include flooding occurring at very extreme discharges in the dike rings upstream of the Netherlands, between Wesel and Lobith. Finally, changes in both high and low flow statistics have been calculated.

The resulting discharge projections were compared with existing discharge projections i.e. those based on KNMI’06 and the results from the international AMICE (Meuse) and RheinBlick2050 (Rhine) projects. The comparison focussed on the annual cycle of the mean discharge, the mean annual minimum 7-day flow, the mean annual maximum flow and extreme flows with long return periods. It should be noted that the comparison with earlier results is also influenced by changes in the data handling as well as in the model set up since 2006. These changes include improvements of down-scaling methods, extension of the historical time-series, improvements of the method to account for the climate induced change in potential evaporation, improved representation of the Swiss lakes (for the Rhine basin) and, finally, the recalibration of the hydrological models used.

The results show that the implications of the KNMI’14 scenarios on both rivers are a general tendency towards increasing discharges in winter and spring and decreasing discharges in (late) summer. For the Rhine and Meuse the mean winter and mean annual maximum discharge are projected to increase whereas the mean summer and mean annual minimum 7-day discharge are projected to decrease. According to most scenarios, mean annual discharge shows a clear increase as well.

The range of the change in (extremely) high discharges for all KNMI’14 scenarios is relatively small for 2050 (for the 1250-year event between 4250 and 4450 m3/s for the Meuse and between 15,210 and 15,950 m3/s for the Rhine when flooding is taken into account) and increases in 2085 (for the 1250-year event between 4110 to 4760 m3/s for the Meuse and between 14,950 and 17,100 m3/s for the Rhine when flooding is taken into account). These ranges of the change in discharge are consistent with the ranges of the change in extreme multi-day precipitation in the KNMI’14 scenarios. Yet, the width of the ranges in the CMIP5 projections for the winter months seems slightly larger than for the KNMI’14 scenarios. This indicates that the range of change in extreme discharges projected for 2050 may be somewhat underestimated in KNMI’14.

The effect of upstream flooding is taken into account for the Rhine. This includes the effect of the potential flood areas between Wesel and Lobith, which are taken into account by correcting the discharges calculated by Sobek above 16,000 m3/s (start of flooding around Emmerich) for the potential flooding volumes and considering the maximum flow over the dikes between Wesel and Lobith. The correction of the Sobek results was needed, because the Sobek model does not incorporate correctly the flooding between Wesel and Lobith. The result is that for very long return periods (above ~1000 years) the differences between the scenarios become small, largely due to the limited discharge capacity of the Rhine between
Wesel and Lobith. The maximum discharge at Lobith will be between 17,500 and 18,000 m3/s.

Comparison of the new, KNMI’14 based, discharge projections with the existing discharge projections results in the following conclusions:
* Generally the trends in discharge envisaged by the KNMI’14 scenarios for the Rhine and Meuse are comparable with those envisaged in most of the existing scenarios (AMICE WET, KNMI’06 and RheinBlick2050). There are (a) larger differences between the dry and wet seasons and (b) more water in the wet (winter and spring) period and less in the dry (late) summer, autumn period (so both increase and decrease of precipitation).
* Specifically, the KNMI’14 scenarios for the Rhine result in higher extreme discharges compared to the KNMI’06 scenarios. For both 2050 and 2085 the KNMI’14 scenarios give for the 1250-year event at most 600 m3/s larger discharges (respectively for GL and for WH) than the earlier KNMI’06 W+ scenario. For the Rhine the W+ scenario roughly lies between the KNMI’14 GH and WL scenarios in 2050.
* For the Meuse the KNMI’14 WH scenario gives comparable results to the KNMI’06 W+ scenario. For 2050 KNMI’14 scenarios give for the 1250-year event at most 200 m3/s larger discharges (GL) than the earlier KNMI’06 W+ scenario. For 2085 the difference
becomes smaller, the 1250-year discharge for WH is 100 m3/s higher than the KNMI’06 W+ scenario.