Abstract
Multi-year droughts constitute a natural experiment for testing the hydrological legacy across different climate regimes. Evapotranspiration (ET) responses to drought have not been fully elucidated because the underlying mechanisms involve complex interactions of climate, soil, and vegetation. This study collected monthly hydrometeorological data during 1982–2011 from 1069 catchments worldwide to investigate multi-year droughts and their impacts on ET and to analyse the underlying mechanisms of ET. As expected, multi-year droughts generally induced ET reductions across different climate regimes, while ET exhibited asymmetric hydrological responses with a greater portion of precipitation (P) being partitioned into ET in water-limited regions, and a smaller portion of P being partitioned in ET in energy-limited regions. Multi-year droughts induced soil moisture droughts and leaf area index decreases, which often led to increases in the vapour pressure deficit, while inconsistent responses occurred for air temperature. Interestingly, when long-term catchment wetness [i.e., (precipitation − streamflow) / precipitation] exceeded 0.50 we found that catchment wetness played a vital role in controlling the response of catchment properties to climate changes. During the multi-year droughts, attribution analysis revealed the enhanced role of available water (represented by P) in controlling ET, followed by available energy and vegetation. Although vegetation played vital role in controlling ET in energy-limited regions, it was P that controlled ET during drought periods. Taken together, these findings indicate that ET responses and their underlying mechanisms induced by multi-year droughts vary under different climate regimes. Specifically, a two-stage mechanism drove the ET responses in wet catchments, suggesting that catchments with higher catchment wetness (>0.50) could more effectively mitigate the impact of multi-year droughts.
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