Journal Article

Terrestrial water fluxes dominated by transpiration

Jasechko, Scott, Gibson, John J., Birks, S. Jean, Fawcett, Peter J., Sharp, Zachary D., Yi, Yi

2013

Overview

An analysis of the relative effects of transpiration and evaporation, which can be distinguished by how they affect isotope ratios in water, shows that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration and using half of all solar energy absorbed by land surfaces. Plants dominant in water-flux calculations Water fluxes from the land surface to the atmosphere are divided between evaporation, and transpiration from leaf stomata. Although a seemingly basic division between the physical and biological, there is still no consensus on the global partitioning between the two fluxes, resulting in uncertainties as to responses to future climate variations. Now, Scott Jasechko and colleagues use the isotopic signatures of transpiration and evaporation from a global data set of large lakes and reveal that enormous quantities of water — as much as 90% of total terrestrial evapotranspiration — are cycled through vegetation via transpiration. One conclusion to be drawn from this study is that the accuracy of biological — rather than physical — fluxes should be prioritized in work to improve climate models. 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 CO 2 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 6 . 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/449/1734 / 704/106/242 / 704/106/286 / 704/158/2466 / Atmosphere / Atmosphere - chemistry / Carbon Dioxide - analysis / Carbon Dioxide - metabolism / Catchment scale / Climate / Climate change / Climate models / Climatology. Bioclimatology. Climate change / Continents / Deserts / Earth, ocean, space / Ecosystem / Evaporation / Evapotranspiration / Exact sciences and technology / External geophysics / Fractionation / Fresh Water - analysis / Fresh Water - chemistry / Groundwater discharge / Humanities and Social Sciences / Isotopes / Lakes / letter / Meteorology / Monte Carlo simulation / multidisciplinary / Oceans and Seas / Photosynthesis / Plant Transpiration - physiology / Plants - metabolism / Rain / River ecology / Rivers / Science / Solar energy / Stomatal conductance / Surface water / Terrestrial environments / Transpiration / Uncertainty / Vegetation / Volatilization / Water in the atmosphere (humidity, clouds, evaporation, precipitation) / Water Movements / Water resources / Water use