Hydrological droughts in the 21st century, hotspots and uncertainties from a global multimodel ensemble experiment

Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by biascorrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO2 and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty

(a) Comparison of per drought event simulated deficit volumes (m³) under pristine conditions (climate variability only) and under transient human water consumption with those calculated from observed streamflow in a logarithmic scale over 23 major river basins that are affected by human water consumption

<p><strong>Figure 1.</strong> (a) Comparison of per drought event simulated deficit volumes (m³) under pristine conditions (climate variability only) and under transient human water consumption with those calculated from observed streamflow in a logarithmic scale over 23 major river basins that are affected by human water consumption. The observed streamflow was taken from the selected GRDC stations closest to outlets. (b) Frequency distribution of correlation coefficient and slope per river basin from (a). Five worst drought events were selected from each of 23 major river basins. Note that for fair comparison, deficit volume was calculated with the threshold level <em>Q</em>₈₀ that was derived respectively from each streamflow time series: from the GRDC observations; from the simulated streamflow under pristine conditions; from the simulated streamflow under transient human water consumption. River basins (GRDC stations; station number; available period used) selected: Orinoco (Puente Angostura; 3206720; 1960–1990), Parana (Corrientes; 3265300; 1960–1992), Nile (El Ekhsase; 1362100; 1973–1985), Blue Nile (Khartoum; 1663100; 1960–1983), White Nile (Malakal; 1673600; 1960–1996), Orange (Vioolsdrif; 1159100; 1964–1987), Zambezi (Katima Mulilo; 1291100; 1964–2002), Murray (below Wakool Junction; 5304140; 1960–2002), Mekong (Mukdahan; 2969100; 1960–1994), Brahmaputra (Bahadurabad; 2651100; 1969–1993), Ganges (Hardinge Bridge; 2646200; 1965–1993), Indus (Kotri; 2335950; 1967–1980), Yangtze (Datong; 2181900; 1960–1989), Huang He (Sanmenxia; 2180700; 1960–1989), Mississippi (Vicksburg; 4127800; 1960–2000), Columbia (The Dalles; 4115200; 1960–2000), Mackenzie (Norman Wells; 4208150; 1961–2002), Colorado (Yuma; 4152050; 1965–1990), Volga (Volgograd Power Plant; 6977100; 1960–2002), Dnieper (Dnieper Power Plant; 6980800; 1960–1985), Danube (Ceatal Izmail; 6742900; 1960–2002), Rhine (Rees; 6335020; 1960–2002), Elbe (Wittenberge; 6340150; 1960–2002).</p> <p><strong>Abstract</strong></p> <p>Over the past 50 years, human water use has more than doubled and affected streamflow over various regions of the world. However, it remains unclear to what degree human water consumption intensifies hydrological drought (the occurrence of anomalously low streamflow). Here, we quantify over the period 1960–2010 the impact of human water consumption on the intensity and frequency of hydrological drought worldwide. The results show that human water consumption substantially reduced local and downstream streamflow over Europe, North America and Asia, and subsequently intensified the magnitude of hydrological droughts by 10–500%, occurring during nation- and continent-wide drought events. Also, human water consumption alone increased global drought frequency by 27 (±6)%. The intensification of drought frequency is most severe over Asia (35 ± 7%), but also substantial over North America (25 ± 6%) and Europe (20 ± 5%). Importantly, the severe drought conditions are driven primarily by human water consumption over many parts of these regions. Irrigation is responsible for the intensification of hydrological droughts over the western and central US, southern Europe and Asia, whereas the impact of industrial and households' consumption on the intensification is considerably larger over the eastern US and western and central Europe. Our findings reveal that human water consumption is one of the more important mechanisms intensifying hydrological drought, and is likely to remain as a major factor affecting drought intensity and frequency in the coming decades.</p

Sensitivity of estimated hydrological drought frequency (figure 3) to the different percentile thresholds (<em>Q</em>₇₀,<em>Q</em>₈₀, and <em>Q</em>₉₀) for pristine conditions (climate variability only) and for transient consumptive water use (transient consumption) over the period 1960–2010 over (a) the Globe, and for each continent; (b) Asia, (c) North America, (d) Europe, (e) Africa, (f) South America, and (g) Oceania

<p><strong>Figure 4.</strong> Sensitivity of estimated hydrological drought frequency (figure <a href="http://iopscience.iop.org/1748-9326/8/3/034036/article#erl471582fig3" target="_blank">3</a>) to the different percentile thresholds (<em>Q</em>₇₀,<em>Q</em>₈₀, and <em>Q</em>₉₀) for pristine conditions (climate variability only) and for transient consumptive water use (transient consumption) over the period 1960–2010 over (a) the Globe, and for each continent; (b) Asia, (c) North America, (d) Europe, (e) Africa, (f) South America, and (g) Oceania. The frequency was derived from the sum of the number of drought events below threshold levels (<em>Q</em>₇₀,<em>Q</em>₈₀, and <em>Q</em>₉₀) for each year over the globe and for each continent. The frequency was indexed per year by dividing the sum by the average drought frequency of the pristine condition calculated with each percentile threshold over the period 1960–2010.</p> <p><strong>Abstract</strong></p> <p>Over the past 50 years, human water use has more than doubled and affected streamflow over various regions of the world. However, it remains unclear to what degree human water consumption intensifies hydrological drought (the occurrence of anomalously low streamflow). Here, we quantify over the period 1960–2010 the impact of human water consumption on the intensity and frequency of hydrological drought worldwide. The results show that human water consumption substantially reduced local and downstream streamflow over Europe, North America and Asia, and subsequently intensified the magnitude of hydrological droughts by 10–500%, occurring during nation- and continent-wide drought events. Also, human water consumption alone increased global drought frequency by 27 (±6)%. The intensification of drought frequency is most severe over Asia (35 ± 7%), but also substantial over North America (25 ± 6%) and Europe (20 ± 5%). Importantly, the severe drought conditions are driven primarily by human water consumption over many parts of these regions. Irrigation is responsible for the intensification of hydrological droughts over the western and central US, southern Europe and Asia, whereas the impact of industrial and households' consumption on the intensification is considerably larger over the eastern US and western and central Europe. Our findings reveal that human water consumption is one of the more important mechanisms intensifying hydrological drought, and is likely to remain as a major factor affecting drought intensity and frequency in the coming decades.</p

Previous data and model based assessments of hydrological drought

<p><b>Table 1.</b>  Previous data and model based assessments of hydrological drought. </p> <p><strong>Abstract</strong></p> <p>Over the past 50 years, human water use has more than doubled and affected streamflow over various regions of the world. However, it remains unclear to what degree human water consumption intensifies hydrological drought (the occurrence of anomalously low streamflow). Here, we quantify over the period 1960–2010 the impact of human water consumption on the intensity and frequency of hydrological drought worldwide. The results show that human water consumption substantially reduced local and downstream streamflow over Europe, North America and Asia, and subsequently intensified the magnitude of hydrological droughts by 10–500%, occurring during nation- and continent-wide drought events. Also, human water consumption alone increased global drought frequency by 27 (±6)%. The intensification of drought frequency is most severe over Asia (35 ± 7%), but also substantial over North America (25 ± 6%) and Europe (20 ± 5%). Importantly, the severe drought conditions are driven primarily by human water consumption over many parts of these regions. Irrigation is responsible for the intensification of hydrological droughts over the western and central US, southern Europe and Asia, whereas the impact of industrial and households' consumption on the intensification is considerably larger over the eastern US and western and central Europe. Our findings reveal that human water consumption is one of the more important mechanisms intensifying hydrological drought, and is likely to remain as a major factor affecting drought intensity and frequency in the coming decades.</p