Abflußentwicklung in Teileinzugsgebieten des Rheins - Simulationen
für den Ist-Zustand und für Klimaszenarien
D. Schwandt (April 2004)
This report "Development of runoff in subcatchments of the River Rhine –
simulations of the current state and for climate change scenarios" investigates
the impacts of possible future climate changes on runoff and runoff regime in
selected subcatchments of the River Rhine. The regional climate in the selected
subcatchments Mosel (up to gauge Cochem), Sieg (gauge Menden 1) and Main
(gauge Kemmern) is affected by the middle mountain ranges. In a first step,
important model processes are parameterized according to catchment characteristics.
A representation of the regional hydrology is then produced by using the hydrological
model HBV-D. Based on time series of daily measurements (temperature, precipitation)
at stations within the catchment, this representation can be used to realistically
simulate time series of runoff and discharge. In all examined areas, the quality
of simulations of the calibration and validation periods for the current state
(standard period of measurements 01/01/1961-12/31/1999) can be regarded as good
to excellent. To aid the catchment-specific, extensive and time-consuming data
processing, a working environment for the hydrological model HBV-D has been developed.
It is based on program extensions of the geographical information system ArcView and
further programs. The working environment HBV-Params contains a graphical interface
that gives both experienced hydrologists and students full control and enables
them to flexibly derive parameter values and edit parameter and control files.
In contrast to previous versions with only rudimentary working environments,
HBV-D can therefore be utilized for research as well as for educational purposes.
In a second step, the current states of areal precipitation, areal temperature
and simulated mean discharge (MQ) are compared to the corresponding states for
two scenarios of future climate changes (100 years later, 2061-2099).
These scenarios are based on simulated global circulations of one model
run for each of two global circulation models (GCM). These global circulations
are regionalized (downscaled) using a statistical approach into scenario time
series of daily values (temperature, precipitation – input for the hydrological
model) at control stations within the individual catchments. For the second half of
the 21st century, both regionalized climate change scenarios indicate
increases in the mean annual areal temperature and mean annual sum of precipitation,
along with a high variability of the latter. The seasonal (monthly) changes in
temperature, precipitation and mean discharge between scenario state (2061-2099)
and current state indicate increases in temperature (higher in summer than in winter)
as well as a general increase in precipitation sums (strong fluctuations between
individual months.) In the hydrological simulations for all investigated catchments,
this results in considerably higher mean discharges from November to March and small
increases in mean discharge for the other months. The magnitude of the increases in
discharge depends on the individual climate change scenario, one showing higher
increases than the other during the summer half-year and vice versa for the winter
half-year. The main reason for the simulated strong increase in mean discharge during
winter half-year is, in spite of higher temperatures, the still relatively low
evapotranspiration which allows higher precipitation to be directly transformed
into higher discharges. The comparison of the investigated catchments shows decreasing
amounts of changes in the sum of precipitation from West to East in individual months.
This may indicates the importance of continentality under changed climatic conditions
in Southwest Germany. For the modification of measured time series (temperature,
precipitation), which can be easily converted as synthetic scenarios into simulated
hydrological results, amounts of change are derived from regionalized (downscaled)
climate change scenarios. The derivation of synthetic scenarios directly from GCM
output at individual GCM gridpoints yielded unsatisfactory results. Only the future
itself can show whether the timing and amount of changes in temperature and precipitation
used in (synthetic) climate change scenarios come close to reality. An assessment of
possible developments in runoff regime and specifically mean discharge under possible
changed climatic conditions in the investigated catchments is already feasible today.
Simulations based on scenarios are one way to establish unknown future boundary
conditions for the estimation of regional impacts of possible changes of the climate
system. Nevertheless, all types of modelling and simulation of natural systems are
linked with uncertainties. Rather large uncertainties persist regarding the future
development of the socio-economic system and the complexity of the climate system
and earth system. Furthermore, besides data uncertainties associated with the
measurement of hydro-meteorological values, uncertainties associated with individual
components of the model chain emission scenarios/gas cycle model – GCM/regionalization
– hydrological model, which form a cascade of uncertainty, have a great influence on
the trustworthiness of the simulation results (which are understood as one shown value
within a range of results). In the young field of climate impact research the use of
• robust hydrological models that adequately describe temperature-dependent processes,
• long time series (at least 30 years long) of measurements,
• concurrent comparisons of climate change scenarios, based on different GCMs (and, if possible, different emission scenarios)
should be considered for reasons of scientific thoroughness and to improve comparability of regional impact studies.
