Implications for the Missing South Asian Carbon Sink: Hydrologic Coupling of Water and Carbon Balances in a Tropical Catchment

This study applied stable water isotopes (δ 18 O H2O and δ 2 H H2O ) and remote sensing techniques to predict basin‐wide CO 2 uptake in the tropical Deduru Oya River Basin (DOB) in Sri Lanka. It used the fact that water and carbon cycles are linked by transpiration. Between November 2022 and October 2023, the DOB lost 2,563 million m 3 of water by evapotranspiration (ET). This number corresponds to 65.8% of the 3,895 million m 3 of incoming precipitation. Based on the δ 18 O H2O in a revised Craig–Gordon model and normalized difference vegetation index (NDVI), our data showed that ET could be separated into its subcomponents evaporation (E), transpiration (T), and interception (I) with 10.4%, 53.9%, and 1.5% of the annual precipitation, respectively. After translation to carbon uptake via the water use efficiency (WUE), the DOB showed a net primary productivity (NPP) of 1,130 g C m −2 that corresponded to 2,808 × 10 9 g C for the entire basin. After subsequent subtraction of basin‐wide soil respiration (1,737 × 10 9 g C) and surface water degassing (101 × 10 9 g C) back to the atmosphere, our study yielded a net ecosystem production (NEP) of 979 ± 313 × 10 9 g C for the entire basin. These result indicate that the DOB is a carbon sink. Our study is the first to present this technique together with error propagation and with CO 2 evasions from surface waters.