Water fluxes and their control on the terrestrial carbon balance: Results from a stable isotope study on the Clyde Watershed (Scotland)

The gradients between precipitation and runoff quantities as well as their water isotopes were used to establish a water balance in the Clyde River Basin (Scotland). This study serves as an example for a European extreme with poorly vegetated land cover and high annual rainfall and presents novel water stable isotope techniques to separate evaporation, interception and transpiration with annual averages of 0.029 km³ a⁻¹, 0.220 km³ a⁻¹ and 0.489 km³ a⁻¹, respectively. Transpiration was further used to determine CO₂ uptake of the entire basin and yielded an annual net primary production (NPP) of 352 × 10⁹ g C (Giga gram) or 185.2 g C m⁻². Compared to other temperate areas in the world, the Clyde Basin has only half the expected NPP. This lower value likely results from the type of vegetation cover, which consists mostly of grasslands. Subtracting the annual heterotrophic soil respiration flux (R_h) of 392 Gg (206.1 g C m⁻² a⁻¹) from the NPP yielded an annual Net Ecosystem Productivity (NEP) of −40 Gg C, thus showing the Clyde Watershed as a source of CO₂ to the atmosphere. Despite the unusual character of the Clyde Watershed, the study shows that areas with predominant grass and scrub vegetation still have transpirational water losses that by far exceed those of pure evaporation and interception. This infers that vegetation can influence the continental water balances on time scales of years to decades.