Climate and vegetation controls on the surface water balance: Synthesis of evapotranspiration measured across a global network of flux towers

TLDR

The Budyko framework reduces spatial patterns of evapotranspiration and runoff to a function of mean annual precipitation and net radiation, but observed departures suggest additional controls such as vegetation type. The study investigates how departures from the Budyko curve, measured at flux towers, are attributed to biome, rainfall seasonality, and frozen precipitation. It uses 167 flux‑tower sites with 764 tower‑years to quantify evapotranspiration and relate deviations to biome, rainfall seasonality, and frozen precipitation. The synthesis shows a smooth transition from water‑ to energy‑limited control, explaining 62 % of evaporative‑index variation, with climate and vegetation adding another 13 %, and reveals that Mediterranean sites have lower indices, grasslands higher than forests, and overall the Budyko framework is supported by global FLUXNET data.

Abstract

The Budyko framework elegantly reduces the complex spatial patterns of actual evapotranspiration and runoff to a general function of two variables: mean annual precipitation (MAP) and net radiation. While the methodology has first‐order skill, departures from a globally averaged curve can be significant and may be usefully attributed to additional controls such as vegetation type. This paper explores the magnitude of such departures as detected from flux tower measurements of ecosystem‐scale evapotranspiration, and investigates their attribution to site characteristics (biome, seasonal rainfall distribution, and frozen precipitation). The global synthesis (based on 167 sites with 764 tower‐years) shows smooth transition from water‐limited to energy‐limited control, broadly consistent with catchment‐scale relations and explaining 62% of the across site variation in evaporative index (the fraction of MAP consumed by evapotranspiration). Climate and vegetation types act as additional controls, combining to explain an additional 13% of the variation in evaporative index. Warm temperate winter wet sites (Mediterranean) exhibit a reduced evaporative index, 9% lower than the average value expected based on dryness index, implying elevated runoff. Seasonal hydrologic surplus explains a small but significant fraction of variance in departures of evaporative index from that expected for a given dryness index. Surprisingly, grasslands on average have a higher evaporative index than forested landscapes, with 9% more annual precipitation consumed by annual evapotranspiration compared to forests. In sum, the simple framework of supply‐ or demand‐limited evapotranspiration is supported by global FLUXNET observations but climate type and vegetation type are seen to exert sizeable additional controls.