Timescale Dependence of Environmental Controls on Actual Evapotranspiration Along Climate and Ecosystem Gradients

Abstract

The interactions of actual evapotranspiration (ETa) with environmental variables vary across temporal scales; however, how these multi‐timescale interactions change along climate and ecosystem gradients remains unclear. To address this, the FLUXNET2015 data set was analyzed using wavelet coherence analysis (WTC). At four selected sites with contrasting climates, the WTC power spectra revealed that daily ETa showed stronger coherences with meteorological variables (e.g., net radiation‐Rn), and its coherences with leaf area index and soil moisture content (SWC) increased with site dryness. The 90‐day coherences of ETa with meteorological variables weakened, while became more pronounced with SWC particularly in dry regions. By comparison, annual coherences of ETa with meteorological variables restrengthened, reflecting their shared seasonality driven by climatic and phenological cycles. Across all sites, the global coherence coefficient (GCCw) showed that ETa was more correlated with meteorological drivers at high‐frequencies (half‐hourly to daily). Particularly, a sigmoid relationship emerged between GCCwETa‐Rn and aridity index (Φ), likely owing to the weakened ETa‐Rn coupling in non‐growing seasons at humid sites. At medium frequencies (daily to three monthly), GCCw between ETa and meteorological variables decreased considerably as subdaily meteorological influences on ETa were averaged out, while GCCwETa‐SWC increased with Φ. Similar to the results of annual coherences, seasonality resulted in restrengthened GCCw between ETa and environmental variables at low frequencies (three monthly to yearly). Among ecosystems, high‐frequency ETa responses to meteorological variables were weaker under dense vegetation, while the differences in medium‐frequency ETa responses to influencing variables decreased noticeably, largely owing to comparable phenologic controls on ETa.