Global trends in future hydrological drought

Climate change very likely impacts future hydrological drought characteristics worldwide (i.e. duration and intensity of drought in runoff). However, the magnitude and sign of the change is largely unknown. In this study we quantify impact of climate change on future low flows and hydrological droughts characteristics on a global scale. The global hydrological and water resources model PCR-GLOBWB was used to simulate future river runoff at 0.5 degree globally on a daily basis. The newly available CMIP5 climate projections were obtained through the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP). The model was forced with the daily transient climate fields taken from five GCMs and four underlying emission scenarios (here accounted for by using four Representative Concentration Pathways or RCPs). The monthly Q80 (20 percentile lowest flow) was used to evaluate the changes in low flows and as the threshold level for the hydrological drought characterisation. The monthly threshold was smoothed to allow for drought calculations on a daily basis. The thresholds in the future remained transient and were calculated over the climatology of the last 30 year period to reflect the adaptation of society to new conditions. Trends in the thresholds and drought characteristics were analysed over the period 2000-2099. Results for most GCMs and all RCPs showed that Q80 discharge (low flow metric) indicates a significant negative trend in large parts of South America, Central Africa, the Mediterranean and South East Asia. Under the higher greenhouse gas emission scenarios of RCP6.0 and RCP8.5, the USA and Central and South Europe were also projected to have drier conditions. For all future projections Russia and Canada were expected to get wetter during the 21st century. Under RCP6.0 and RCP8.5 scenarios, the results generally showed the strongest negative changes in future low flow. The results simulated with most GCMs agree well over many parts of the world, however, for Central Asia and Northern Europe the variability among GCMs was large. The drought analysis showed that the duration and deficit volume were expected to increase even with the use of transient thresholds (30-year window), which already consider for the drier low flow conditions (i.e. lower future Q80). Drought duration was projected to increase in more than 80% of the world and deficit volumes are likely to increase in 70% of the world. Drought events will get more prolonged and severe in North America, Southern Europe and South East Asia, where projected drought duration increased by 20 to 100% and the increase in deficit volumes ranged from 20 to 300%, depending on location and RCP. The projected change in future hydrological drought in large parts of the world shows that adaptation likely has to anticipate on drier hydrological conditions. The increase in hydrological drought duration and intensity in vast areas of the world will lead to severer drought events, which puts sustainable regional water supply at risk and urges water resources managers, policy-makers and stakeholders to timely design pro-active measures.