Isotope constraints on water, carbon, and heat fluxes from the northern Great Plains region of North America

[1] Water vapor flux facilitates the transfer of large amounts of mass and energy from terrestrial ecosystems to the atmosphere, yet the proportions of this flux ascribed to evaporation and plant transpiration are poorly constrained. Here we used a water-isotope mass balance approach to partition evaporation and transpiration water vapor fluxes in the northern Great Plains region of western Canada. Utilizing the proportion of gross watershed area that contributes to annual river flow, we estimated that of the mean annual precipitation (∼490 mm), ∼7% was transferred to the atmosphere via direct evaporation from water bodies and soils, whereas ∼50% was returned to the atmosphere by plant transpiration. Further, through the explicit coupling of transpiration and photosynthesis, we estimated that plant transpiration in the watersheds corresponded to an annual photosynthetic carbon flux of ∼48.9 × 10¹² g C or ∼325 g C m⁻². Although uncertainty related to this estimate of photosynthetic carbon uptake was substantial owing to the paucity of regional estimates of plant water-use efficiency, it was similar to the flux of carbon released through soil respiration and other independent estimates of primary productivity. The water-isotope mass balance approach in partitioning evaporation and transpiration fluxes at a regional scale was promising, although results revealed that the flux estimates would be greatly improved by longer-term data on the isotope composition of river water, precipitation, and atmospheric moisture, as well as through detailed regional-scale measurements of plant water-use efficiency under various environmental and climatic conditions.