Overestimation of evapotranspiration across India if not considering the impact of rising atmospheric CO2

Evapotranspiration (ET), a key component of the hydrological cycle, responds to and influences climate change, making accurate estimation of reference ET (ET o ) critical for long-term impact assessments. The widely applied FAO Penman–Monteith (FAO-PM) equation for calculating ET o does not account for rising atmospheric CO 2 , which reduces vegetation stomatal conductance and can lead to systematic overestimation of ET o . We derived a modified FAO-PM equation incorporating CO 2 effects on stomatal behavior. Using projections from five global circulation models, we compared spatiotemporal average of ET o estimates for India from the original and modified equations under SSP5-8.5 and SSP1-2.6. Differences were 0.11–1.29 mm day −1 (2021–2030), 0.09–1.90 mm day −1 (2051–2060), and 0.17–3.14 mm day −1 (2091–2100) under SSP5-8.5, with slightly lower values under SSP1-2.6. Seasonal differences between the predicted ET o from the two equations peaked during the pre-monsoon, reaching 3.90 mm day −1 (SSP5-8.5) and 1.74 mm day −1 (SSP1-2.6). Neglecting stomatal responses to CO 2 could lead to ET o overestimation of ~29% under SSP5-8.5 by 2100, potentially biasing projections of droughts, heatwaves, and water demand. By contrast, overestimation is moderate (~13%) under SSP1-2.6. Incorporating the impact of CO 2 into ET o estimation is therefore essential for robust climate change impact assessments.