The future for global water assessment

The global water cycle is a fundamental component of our climate and Earth system. Many, if not the
majority, of the impacts of climate change are water related. We have an imperfect description and
understanding of components of the water cycle. This arises from an incomplete observation of some
of the stores and fluxes in the water cycle (in particular: precipitation, evaporation, soil moisture and
groundwater), problems with the simulation of precipitation by global climate models and the wide
diversity of global hydrological models currently in use. This paper discusses these sources of errors
and, in particular, explores the errors and advantages of bias correcting climate model outputs for hydrological
models using a single large catchment as an example (the Rhine). One conclusion from this analysis
is that bias correction is necessary and has an impact on the mean flows and their seasonal cycle.
However choice of hydrological model has an equal, if not larger effect on the quality of the simulation.
The paper highlights the importance of improving hydrological models, which run at a continental and
global scale, and the importance of quantifying uncertainties in impact studies.