Diverging hydrological drought traits over Europe with global warming

9 Climate change is anticipated to alter the demand and supply of water at the earth’s surface. Since 10 many societal impacts from a lack of water happen under drought conditions, it is important to 11 understand how droughts may develop with climate change. This study shows how hydrological 12 droughts will change across Europe with increasing global warming levels (GWL of 1.5, 2 and 3 K 13 above preindustrial temperature). We employ a low-flow index derived from river discharge 14 simulations of a spatially-distributed physically-based hydrological and water use model, which was 15 forced with a large ensemble of regional climate model projections under a high emissions 16 (RCP8.5) and moderate mitigation (RCP4.5) pathway. Different traits of drought, including 17 severity, duration and frequency, were investigated. The projected changes in these treats identify 18 four main sub-regions in Europe that are characterized by somehow homogeneous and distinct 19 behaviours with a clear southwest/northeast contrast. The Mediterranean and Boreal sub-regions of 20 Europe show strong, but opposite, changes at all three GWLs, with the former area mostly 21 interested by stronger droughts (with larger differences at 3 K) while the latter sees a reduction in 22 droughts. In the Atlantic and Continental sub-regions the changes are less marked and characterized 23 by a larger uncertainty, especially at the 1.5 and 2 K GWLs. Combining the projections in drought 24 hazard with population and agricultural information shows that with 3 K global warming an 25 additional 11 million people and 4.5 million ha of agricultural land will be exposed to droughts 26 every year, on average. These are mostly located in the Mediterranean and Atlantic regions of 27 Europe. 1.5, 2 3 K represent different We use streamflow as indicator of as it represents the in over The indicator is derived from daily simulations for the river network, are obtained with a continental and with of 11 for and We performed value

With the raising awareness of climate change, a number of local and regional studies have  To further deepen the understanding on this issue, we evaluate changes in hydrological 86 droughts across Europe between present climate and climate corresponding to different GWLs. We 87 look specifically at 1.5, 2 and 3 K global warming, which represent different climate change 88 mitigation targets. We use streamflow deficit as an indicator of drought as it represents the 89 integrated deficiency in water budget over the upstream catchment. The indicator is derived from 90 daily streamflow simulations for the pan-European river network, which are obtained with a 91 continental spatially-distributed hydrological and water use model forced with an ensemble of 11 92 bias-corrected regional climate projections for RCP4.5 and RCP8.5. We performed extreme value 134 The hydrological drought index used in this study is analogous to the low-flow indicator used  According to the theory of runs (Yevjevich, 1967), a continuous period with river flow values 140 below the defined low-flow threshold is considered as a drought event, of which the severity is 141 quantified by the total deficit (D, represented by the area enclosed by the threshold and the 142 streamflow time series). Other key traits of drought are the duration, quantified by the number of 143 drought days (N), and the temporal frequency of the events, which can be expressed as return period 144 (T).

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In order to avoid the inclusion in the analysis of minor events, two post-processing corrections 146 were applied after selection of the events below the threshold: 1) consecutive events with an inter-147 event time smaller than 10 days were pooled together (Zelenhasić and Salvai, 1987), and 2) small 148 isolated events (of duration less than 5 days) were removed from the analysis (Jakubowski and    The spatial data of population and agricultural land were summed over NUTS 2 statistical   it will be adopted in all the subsequent analyses.

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The strongest increase in drought severity is projected for Portugal, Spain and Greece, where 210 the fraction of rivers with an increase in deficit of more than 50% at 3 K is 99, 80 and 75%, 211 respectively. If climate stabilizes at 2 K, streamflow drought severity is lower than at 3 K, but still 212 at least 50% higher than in the baseline for halve of the rivers of Portugal and Spain, and 35% of 213 Greece. Capping global warming at 1.5 K would further limit the increase in severity, with only 21, 214 20 and 14% of the rivers of Portugal, Spain and Greece experiencing an increase in drought severity 215 of more than 50%.

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Over the Atlantic region (apart from Iceland), streamflow droughts will in general also become 217 more severe with global warming. The south of France shows a pattern towards more severe flow 218 deficits with warming that is similar to that projected for most of the Mediterranean. For the other 219 parts of the Atlantic sub-region the changes are less pronounced. Keeping warming to 2 K or below 220 would limit the increase in severity for most of the region to below 25% compared to the baseline.

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At 3 K warming, the increase in severity could reach up to 50%. In some parts of the Atlantic sub-  increase by more than a month/year.

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In the Continental sub-region, the area that shows a decrease in drought duration is around 80% 255 at 1.5 K, which slightly reduces in extent with increasing warming. Yet, over this area droughts will

Population and agricultural land exposed to drought
298 Figure 4 shows the changes with respect to the baseline in population exposed to streamflow 299 drought at country scale (percentage relative changes are also reported as numbers next to the bars).

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Total changes for the four macro-regions and the entire domain (TOT) are summarised in Table 1.

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Aggregated over the whole domain, about 1.5 million fewer people will be annually exposed to 302 drought at 1.5 K GWL compared to the baseline period, which reverses to an increase of about 2.5 303 and 11 million people/year compared to baseline human exposure at 2 and 3 K GWLs, respectively.

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This is because at 1.5 K the increase in population exposed annually in the Mediterranean (2.4 305 million) and Atlantic (less than 0.1 million) sub-regions is outweighed by the reduction in exposure GWLs (i.e., -3.9, -5.4 and -4.7 million/year at 1.5, 2 and 3 K, respectively).

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Spain is projected to have the largest absolute increase in population exposed to drought with 312 global warming, with an almost doubling (+3.8 million/year) of the number of people exposed to 313 drought each year at 3 K GWL. In relative terms, the relative increase in population exposure at 3K 314 is also high in Portugal (+81%), United Kingdom (+58%) and France (+52%). The largest absolute 315 decrease in population exposed is observed for Germany at 1.5 and 2 K GWL (-1.

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This study analysed how the main characteristics of hydrological droughts will change over 396 Europe due to global warming. Projections in drought severity, duration and frequency based on 397 river water deficits highlight some common features and spatial patterns in future drought 398 conditions across Europe. The Mediterranean sub-region, which already suffers most from water 399 scarcity, will experience the strongest negative effects of climate change on drought conditions.

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With increasing global warming, streamflow deficits in this region will happen more frequently, 401 become more severe and last longer. Symmetrically, the Boreal sub-area will face a consistent 402 decrease in drought severity, duration and frequency.