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

Sruthi Surendran¹Nandhana Sunil²Tanushri Pahari^1,3Yufeng He⁴Deepak Jaiswal^1*

• ¹Environmental Sciences and Sustainable Engineering Centre, Indian Institute of Technology Palakkad, Palakkad, India
• ²Department of Computer Science and Engineering, Indian Institute of Technology Palakkad, Palakkad, India
• ³Department of Biotechnology, National Institute of Technology Raipur, Raipur, Chhattisgarh, India
• ⁴University of Illinois at Urbana-Champaign, Champaign, Illinois, United States

The widely used FAO Penman-Monteith (FAO-PM) equation does not account for rising atmospheric CO2, which is known to reduce stomatal conductance that can lead to overestimation of reference evapotranspiration (ETo) in climate impact studies. We proposed a modified version of the FAO-PM equation that considers the impact of CO2 concentration on stomatal behavior and, consequently, on ETo. Comparing ETo estimates from the original and modified equations for India under SSP5-8.5 showed differences of 0.11-to-1.29 mm day -1 (2021-2030), 0.09-to-1.9 mm day -1 (2051-2060), and 0.17-to-3.14 mm day -1 (2091-2100). Under SSP1-2.6, differences were 0.12-to-1.26 mm day -1 , 0.12-to-1.47 mm day -1 , and 0.11-to-1.38 mm day -1 , respectively.Seasonal impacts were highest in the pre-monsoon, reaching 0.12-to-3.90 mm day -1 under SSP5-8.5 and 0.14-to-1.74 mm day -1 under SSP1-2.6. Ignoring CO2 effects could overestimate ETo by ~29% under SSP5-8.5 by 2100, leading to questionable predictions of droughts, heatwaves, and water demands. However, impact of CO2 was moderate under SSP1-2.6 resulting in ETo overpredictions of only 13%.