When Drought Is Not Drought: How Metrics Bias Estimates of Ecosystem Water Limitation

Anthropogenic warming and shifts in global precipitation regimes are increasing rainfall variability and the frequency of intense precipitation events, raising uncertainty about how conventional meteorological drought indices capture ecosystem water limitation. This uncertainty arises from two key issues: temporal aggregation, as most meteorological drought indices are based on monthly integrated summaries, and drought definition, because atmospheric water deficits do not necessarily translate into vegetation water stress. Here, we quantified ecosystem gross primary productivity (GPP) responses to drought using three approaches: (1) monthly meteorological drought identified from monthly standardized precipitation evaporation index (SPEI), (2) daily meteorological drought defined by daily SPEI, and (3) ecological drought defined using an indicator of actual vegetation water limitation. Comparing monthly and daily SPEI-based drought isolated the effects of temporal aggregation, while comparing daily meteorological and ecological droughts quantified the influence of drought definition. Across 130 eddy-covariance sites spanning multiple biomes and climate zones, we analyzed 6691 drought months identified using monthly SPEI. Daily SPEI data revealed that within months classified as meteorological drought, nearly 50% of days did not coincide with atmospheric water deficits owing to daily rainfall variability. As a result, monthly SPEI overestimated GPP responses (20.60% ± 5.28% increase) relative to daily SPEI (4.61% ± 1.21% increase). In contrast, daily meteorological drought metrics underestimated drought-induced suppression of GPP by ~46% compared with ecological drought, demonstrating that atmospheric drought metrics do not fully reflect water stress experienced by vegetation. Our results show that both temporal resolution and drought definition can bias estimates of ecosystem drought responses, underscoring the need for finer-scale temporal and ecologically relevant measures of vegetation water limitation to improve understanding of biosphere-climate interactions in an increasingly drought-prone world.