Williams, CA; Reichstein, M; Buchmann, N; Baldocchi, DD; Beer, C; Schwalm, C; Wohlfahrt, G; Hasler, N; Bernhofer, C; Foken, T; Papale, D; Schymanski, S; Schaefer, K: Climate and vegetation controls on the surface water balance: synthesis of evapotranspiration measured across a global network of flux towers, Water Resources Research, 48, W06523 (2012), doi:10.1029/2011WR011586

The Budyko framework elegantly reduces the complex spatial patterns of actual evapotranspiration and runoff to a general function of two variables: mean annual precipitation 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 or temporal distribution of rainfall. This paper explores the magnitude of such departures as detected from flux tower measurements of ecosystem-scale evapotranspiration, and investigates to what degree they can be attributed to site characteristics (biome, seasonal rainfall distribution, and frozen precipitation). The global synthesis (based on 167 sites) shows smooth transition from water-limited to energy-limited control, broadly consistent with catchment-scale relations and matching the functional form proposed by Budyko, and explaining 62% of the across site variation in evaporative index (the fraction of mean annual precipitation consumed by evapotranspiration). Climate type and vegetation type act as additional controls, and combine to explain an additional 13% of the variation in evaporative index. For example, warm temperate winter wet sites (Mediterranean) exhibit a reduced evaporative index, 9% lower than the average value expected based on dryness index, and implying elevated runoff. We found that evaporative index is largely insensitive to the fraction of precipitation that is frozen. Seasonal hydrologic surplus explains a small but significant fraction of additional variance in departures of evaporative index from that expected for a given dryness index, particularly for sites with a Mediterranean-type climate. The most surprising result is that grasslands, on average, have a higher evaporative index than forested landscapes, with 9% more annual precipitation being consumed by annual evapotranspiration in grasslands 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.

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last modified 2012-06-24