Evaporation enhancement drives the European water-budget deficit during multi-year droughts

[摘要] In a warming climate, periods with lower than averageprecipitation will increase in frequency and intensity. During such periods,known as meteorological droughts, the decline in annual runoff may beproportionally larger than the corresponding decline in precipitation.Reasons behind this exacerbation of runoff deficit during dry periods remainlargely unknown, and this challenges the predictability of when thisexacerbation will occur in the future and how intense it will be. In thiswork, we tested the hypothesis that runoff deficit exacerbation duringdroughts is a common feature across climates, driven by evaporationenhancement. We relied on multidecadal records of streamflow andprecipitation for more than 200 catchment areas across various European climates,which distinctively show the emergence of similar periods of exacerbatedrunoff deficit identified in previous studies, i.e. runoff deficit on theorder of −20  % to −40  % less than what expected from precipitationdeficits. The magnitude of this exacerbation is two to three times larger forbasins located in dry regions than for basins in wet regions, and isqualitatively correlated with an increase in annual evaporation duringdroughts, in the order of +11  % and +33  % over basins characterizedby energy-limited and water-limited evaporation regimes, respectively. Thus,enhanced atmospheric and vegetation demand for moisture during dry periodsinduces a nonlinear precipitation-runoff relationship for low-flow regimes,which results in an unexpectedly large decrease in runoff during periods ofalready low water availability. Forecasting onset, magnitude, and durationof these drops in runoff have paramount societal and ecologicalimplications, especially in a warming climate, given their supporting rolefor safeguarding water, food, and energy. The outcome that water basins areprone to this exacerbation of runoff deficit for various climates andevaporation regimes makes further understanding of its patterns ofpredictability an urgent priority for water-resource planning and managementin a warming and drier climate.

[学科分类] 妇产科学