Global Drought Assessment using a Multi-Model Dataset

Large-scale models are often applied to study past drought (forced with global reanalysis datasets) and to assess future drought (using downscaled, bias-corrected forcing from climate models). The EU project WATer and global CHange (WATCH) provides a 0.5o degree global dataset of meteorological forcing (i.e. WATCH Forcing Data, WFD), which was used as input for a suite of global hydrological models (GHMs) and land surface models (LSMs). Ten GHMs and LSMs have been run for the second half of the 20th C and seven for the whole century. Spatio-temporal drought characteristics were derived from gridded time series of daily and monthly aggregated runoff using the threshold level, and non-contiguous and contiguous approaches. GHMs and LSMs were intercompared and to some extent also tested against observations to explore to what level these models capture past drought events. This paper will present an overview of results. Global maps showing drought summary statistics (e.g. average duration) and distribution of drought clusters across the globe for major documented drought events will be presented. In addition, area in drought and the occurrence of the maximum drought cluster will be discussed. The main results from a number of studies are: (i) drought characteristics across the globe vary dependent on the selected window of years, (ii) GHMs and LSMs broadly identified major drought events in a number of large river basins around the world, (iii) drought events obtained with individual GHMs an LSMs may substantially deviate from those derived with a catchment scale hydrological model (selected EU WATCH river basins), but the multi-model ensemble mean agrees rather well, (iv) use of different calculation methods for reference evapotranspiration has little to substantial influence on drought characteristics dependent on the climate region (Köppen-Geiger), (v) groundwater systems are as important as climate for the development of drought in runoff. Understanding of past drought events will be discussed in the context of the assessment of uncertainty of projected drought and potential improvements of GHMs and LSMs