Understanding ecosystem gross primary productivity, evapotranspiration, and water use efficiency of maize using ISBA-A-g s land surface model over temperate and tropical semi-arid climates

Water use efficiency (WUE) links carbon assimilation and water loss and is an important indicator of ecosystem functioning under changing hydroclimatic conditions. Process-based land surface models (LSMs) provide a framework for simulating carbon-water interactions, but their ability to reproduce ecosystem-scale WUE across contrasting climates remains insufficiently evaluated. This study assesses the performance of the ISBA-A-g s land surface model, implemented within the SURFEX platform, in simulating gross primary productivity (GPP), evapotranspiration (ET), and WUE (GPP / ET) for maize under temperate conditions in France (FR-Lam) and tropical semi-arid conditions in India (Ind-IITH). The model was driven by site-specific meteorological and vegetation data across six growing seasons at FR-Lam under sprinkler irrigation and two growing seasons at Ind-IITH, representing monsoon and winter crops under alternate furrow irrigation (AFI). Model calibration indicated contrasting controls on carbon and water exchanges between the two sites. At FR-Lam, relatively higher cuticular conductance and stronger atmospheric control on stomatal behaviour were identified, whereas at Ind-IITH, mesophyll conductance and soil moisture stress thresholds were important calibration parameters. At FR-Lam, the model simulated mean seasonal cumulative GPP, ET, and WUE of 1039±20 g C m −2 , 610±31 kg H 2 O m −2 , and 1.70±0.10 g C kg −1 H 2 O, respectively, compared with measured values of 1026±30 g C m −2 , 562±42 kg H 2 O m −2 , and 1.82±0.11 g C kg −1 H 2 O. At Ind-IITH, simulated values were 766±15 g C m −2 , 567±30 kg H 2 O m −2 , and 1.35±0.11 g C kg −1 H 2 O, compared with measured values of 793±11 g C m −2 , 522±20 kg H 2 O m −2 , and 1.51±0.12 g C kg −1 H 2 O. Diagnostic analysis based on underlying water use efficiency (uWUE) further indicated that the model reproduced the relationship between carbon assimilation and transpiration-driven water loss. Overall, ISBA-A-g s captured the seasonal dynamics of maize carbon and water exchanges across the two contrasting climates, although differences between simulated and measured ET and WUE remained. The results highlight the potential of process-based modelling to assess carbon-water coupling across diverse hydroclimatic conditions and provide insights into the physiological controls governing crop water use.