Scale dependent coupling and transfer limits of ecosystem carbon and water use efficiency across an inland river basin.

Sustainable management of inland river basins requires joint assessment of carbon retention and water loss, yet the relation between these efficiencies across sites and within sites remains unclear. The study utilized data from 17 eddy covariance observation sites in the Heihe River Basin from 2008 to 2024 to reconstruct the carbon and water records. The study employed two indicators of ecosystem carbon use efficiency (CUE) and water use efficiency (WUE), and conducted a comprehensive analysis through gradient analysis, mixed models, specific scale coupling, XGBoost-SHAP, and year-site validation methods. The results showed that first environmental component explained 63.0% of the variance among site environments. Mean CUE was 0.578, 0.589, and 0.413 upstream, midstream, and downstream, respectively. The corresponding WUE means were 1.51, 2.26, and 1.22 g C/kg H2O respectively. Water supply and atmospheric demand accounted for 57 to 66% of absolute CUE model attribution across reaches, while energy variables accounted for 53 to 61% of absolute WUE attribution. When complete years were excluded, basin prediction R2 was 0.377 for CUE and 0.689 for WUE. When complete sites were excluded, R2 was -0.158 and 0.459, respectively. Annual CUE and WUE deviations were positively associated downstream, but spatial transfer of downstream WUE was poor. Joint assessment of both efficiencies therefore informs monitoring, while predictions at unmonitored sites require local validation.