Comparison between Global Hydrological Models and observations: trend and drought analysis in England and Wales

Global hydrological models (GHMs) have been widely used in scientific studies to describe the hydrological cycle at large scale. Many improvements have been made to these models, e.g., to include human influences, such as reservoir operations, water uses and land cover changes. Given these enhancements, the question is now how well these models can reflect trends and drought characteristics found in streamflow observations in relatively small catchments, where flow might be altered by different (local) human influences. To answer this question, this study compares discharge simulated data from six different GHMs with observations in 189 gauging stations around England and Wales. A common characteristic of the considered catchments is their relatively small size (10-10000 km2) and part of the challenge consists in studying the ability of the models to describe hydrological conditions and human influences at such small scale. The selected catchments consist of a mix of catchments with near-natural flow as well as catchments with different human influences, such as groundwater abstraction and reservoir management. For all catchments, discharge trends and drought event characteristics have been determined for both observed and modelled time series and the outcomes have been compared. Results show that GHMs (especially the course-resolution ones) simulate catchments areas larger than the observed ones. This overestimation influences simulated streamflow and impacts on the results of the trend and drought analyses. The identified trends and drought characteristics, in fact, vary among GHMs and from GHMs to observations. It is evident the impact of groundwater abstraction emerging in the most affected regions, whereas the influence of the other human activities on drought characteristics is not so clear.