Water and carbon balances in a hemi-boreal forest

Abstract The carbon and water fluxes and their inter-relations are key aspects of ecosystem dynamics. In this study, regionalization was used in transferring parameters from the GR4J-Cemaneige model calibrated in Reola hydrographic basin to predict daily flows in Kalli basin; both watersheds are located in the southeast of Estonia. Evapotranspiration data was collected from the MODIS sensor of the Terra satellite and from the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR Estonia). Precipitation data was collected from Tartu–Tõravere and SMEAR Estonia stations and river flow from Reola hydrometric station. The year 2011 was used for model warm-up, model calibration was done in 2012–2017 and the 2018–2020 period was used for validation. The GR4J-Cemaneige model was calibrated at Reola Basin, with a Nash-Sutcliffe Efficiency index of 0.73. The 6 constants of Reola subbasin were transferred to Kalli subbasin for river flow simulation. Net ecosystem exchange (NEE) was measured at the 70 m SMEAR tower with the eddy covariance technique. The balances indicate that the ecosystem at Kalli watershed is slowly becoming a source of carbon and less water is available at the catchment reservoir. NEE has increased from -1.23 μmol m -2 s -1 in 2015 to -0.62 μmol m -2 s -1 in 2020, while the delta water storage decreased from 0.24 mm in 2015 to -0.05 mm in 2020. This behavior may increase soil drying and oxidation, and it will probably release more carbon in the future. This research allows a better understanding of the Järvselja hemi-boreal forest water-carbon dynamics.