Human and climate impacts on the 21st century hydrological drought
Introduction
Climate change is expected to increase drought intensity and frequency worldwide as a result of change in precipitation patterns and rising temperature (Burke et al., 2006, Lehner et al., 2006, Feyen and Dankers, 2009, Dai, 2011, Dai, 2013, Prudhomme et al., 2014, Trenberth et al., 2014). Drought is generally related to meteorological extremes and is induced by below-normal precipitation (Wilhite and Glantz, 1985, Wilhite, 2000, Mishra and Singh, 2010). Lack of precipitation causes meteorological drought and agricultural drought over the region, but further propagates into hydrological drought via the drainage network (Tallaksen et al., 1997, Sheffield and Wood, 2007, Tallaksen et al., 2009, Sheffield et al., 2012, Van Loon et al., 2014). Various studies analysed the severity, frequency and trends of hydrological droughts using large-scale hydrological models that enable the analysis of drought over continental to global scales (Hisdal et al., 2001, Fleig et al., 2006, Feyen and Dankers, 2009, Tallaksen et al., 2009, Corzo-Perez et al., 2011, Van Huijgevoort et al., Jul. 2013, Van Huijgevoort et al., 2014, Alderlieste et al., 2014). However, the anthropogenic impact on drought is generally less well known and such impact has rarely been explored. Few exceptions are recent studies by Dai, 2011, Dai, 2013, Sheffield et al., 2012 who indicated that anthropogenic global warming is likely responsible for intensifying meteorological droughts, primarily due to enhanced evaporative demand and altered monsoon circulation over regions such as Africa and Asia. Another exception by Wada et al. (2013) showed that human water consumption substantially intensifies the magnitude of hydrological droughts regionally by 10–500%, and it alone increases global drought frequency by 30%. However, no study has yet provided a comprehensive overview of human and climate impacts on future hydrological drought at the global scale. Prudhomme et al. (2014) provided future projections of hydrological drought based on a large ensemble of five Global Climate Models (GCMs) from the latest CMIP5 (Coupled Model Intercomparison Project Phase 5), four emission scenarios or Representative Concentration Pathways (RCPs) and seven Global Hydrological Models (GHMs). Yet, they considered only the effect of climate on hydrological drought using the streamflow simulated under natural or pristine conditions such that anthropogenic influence (e.g., irrigation and reservoir regulation) on resulting drought is not explicitly incorporated.
The severe impacts of large-scale droughts have historically showed the need to improve understanding of drought mechanisms so that our society can be better prepared (Trenberth et al., 1988, Gleick, 2000, Andreadis et al., 2005, Seager, 2007, Gleick, 2010, Pederson et al., 2012). Thus, providing a comprehensive overview of future drought projections considering both human and climate impacts is a vital step, ensuring future water and food security. Here, we present for the first time a full global analysis of the impact of human activities (irrigation and reservoir regulation, Wada et al., 2013) and climate change on hydrological drought. We simulated streamflow both under natural or pristine conditions and under conditions including human influences using the global hydrological and water resources model PCR-GLOBWB (Van Beek et al., 2011, Wada et al., 2011, Wada et al., 2011, Wada et al., 2014) with five GCMs from the latest CMIP5 and four emission scenarios (here represented by RCPs 2.6, 4.5, 6.0 and 8.5). We incorporate human-induced change by including human water use for irrigation and reservoir regulation parameterized by the latest extensive global reservoir data set (GRanD, Lehner et al., May 2011). Another innovative aspect of this study is that we apply a transient spatially-distributed threshold or (30-year window) identifying drought characteristics that reflects changes in the hydrological regime over time (Wanders et al., 2014), while most studies used the threshold calculated over the control or historical period (e.g., 1971–2000). A transient threshold assumes adaptation to long-term changes in the hydrological regime as the drought is defined by a deviation from normal conditions (i.e. normal implies decadally updated 30-year averages according to the WMO guidelines) (World Meteorological Organization, 2007, Arguez and Vose, 2010). Our study stands out from earlier work by presenting for the first time the human impact on future hydrological droughts using the latest multi-model climate projections and multi-emission scenarios.
Section 2 of this paper presents a brief description of the global hydrological and water resources model PCR-GLOBWB, climate forcing data, the drought identification method and the simulation protocol. In Section 3 the simulation results are presented and the human and climate impacts on future hydrological drought are evaluated globally and per river basin. Section 4 discusses the advantages and the limitations of our approach and the associated uncertainties, and provides conclusions from this study.
Section snippets
Model simulation of streamflow
The state-of-the-art global hydrological and water resources model PCR-GLOBWB was used to simulate spatial and temporal continuous fields of discharge and storage in rivers, lakes, and wetlands at a 0.5° spatial resolution (Wada et al., 2010, Van Beek et al., 2011, Wada et al., 2014). In brief, the model simulates for each grid cell and for each time step (daily) the water storage in two vertically stacked soil layers and an underlying groundwater layer. At the top a canopy with interception
Climate impact on a global scale
On a global scale the impact of climate change on the low flow regime (, Eq. 6) has been evaluated and compared for the control and the future period (Fig. 2). It is shown that climate change has a negative impact on the low flow regime (decrease of 10% or more) in South-America, Australia, Southern-Africa, Southeast Asia and the Mediterranean. Positive impacts on the low flow regime are found in Northwest Africa and large parts of Northern Europe, Russia and Canada. Differences
Discussion and conclusions
In this study the impact of climate change, and human water use and reservoirs on projected hydrological drought characteristics for the 21st century has been studied. Obtained future simulation results were compared to the control period or the pristine scenario (climate change only) and the relative contribution of humans was compared to the impact of climate change. The impact of climate change on the low flow regime and hydrological drought characteristics is projected to be severe. Large
Acknowledgments
NW was funded by a grant from the user support program Space Research of NWO (contract number NWO GO-AO/30). This work has been supported by the framework of ISI-MIP funded by the German Federal Ministry of Education and Research (BMBF) (Project funding reference number: 01LS1201A). We thank anonymous reviewers and guest Editor (Ashok Mishra) for their constructive suggestions, which helped to improve the manuscript.
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