Terrestrial water fluxes dominated by transpiration

Abstract

Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km(3) per year) (refs 1, 2, 3, 4, 5). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth's continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km(3) of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…Most importantly, these analyses highlight how model improvements need to focus on biological controls on E in addition to physical mechanisms, especially given the predominance of transpiration fluxes in total evapotranspiration (Jasechko et al, 2013). This will require continued and expanded efforts to monitor root and canopy demographic processes in relation to variability in available water, nutrients and light.…”

“…This is because the method does not equally sample the net radiation distribution, due to a bias towards cloud-free periods arising from the need to exclude periods when the canopy was wet. For this reason, we did not attempt to estimate the transpiration fraction of evapotranspiration, though it is likely a large fraction for the forest sites (Jasechko et al, 2013). The slope of leaf-level stomatal conductance versus photosynthesis is the parameter m in the Ball-Woodrow-Berry-Collatz (BWBC) semi-empirical model of stomatal conductance (Collatz et al, 1991) (see Table A3), the inverse of which we refer to as intrinsic water use efficiency of photosynthesis (iWUE), following the definition of Beer et al (2009).…”

The SPARSE model for the prediction of water stress and evapotranspiration components from thermal infra-red data and its evaluation over irrigated and rainfed wheat

“…Evapotranspiration is an important, yet difficult to estimate (Jasechko et al, 2013), component of the water cycle, especially in semi-arid lands. Its quantification is crucial for a sustainable management of scarce water resources.…”

Hydrological modeling of soil-water availability in the Caatinga biome

“…Comparing the future predictions to the simulations carried out by with measured weather data for the Caatinga biome, on average actual transpiration increased by 36%, actual soil evaporation and interception losses decreased by 16% and 34%, respectively. As pointed out by Jasechko et al (2013), biological fluxes (transpiration) play a bigger role in water fluxes than physical fluxes (evaporation). This is expected because plant roots are able to take up stored soil-water and moving deeper sources of water to the atmosphere, whereas evaporation is only effective for water at or near the soil surface.…”

“…On the other hand, vegetation plays a role in climate as well, mainly on the regional scale, and replacement of a vegetation type by another will affect evapotranspiration and other climate factors simultaneously. This process may result directly in land-atmosphere feedbacks, suggesting that a land-cover change may modify precipitation cycle dynamics, and thus, play an important role in the water balance of a land surface (JASECHKO et al, 2013; STERLING; DUCHARNE; POLCHER, 2012).…”