Climate Change Projections of Potential Evapotranspiration for the North American Monsoon Region

Abstract We assessed future projected changes in terrestrial evaporative demand by calculating Potential Evapotranspiration (PET) for the North American Monsoon region at the Southwest U.S. and Mexico. The PET projections were calculated using the daily Penman-Monteith equation and the terrestrial meteorological variables needed for the equation (i.e. minimum and maximum daily temperature, specific humidity, wind speed, incoming shortwave radiation, and pressure) were available from the North American – CORDEX initiative. We used six dynamically downscaled projections of three CMIP5 GCMs forced with RCP8.5 emission scenarios (i.e. HadGEM2-ES, MPI-ESM-LR, and GFDL-ESM2M), each was dynamically downscaled to ~ 25 km by two RCMs (i.e. WRF and regCM4). All terrestrial annual PET projections showed a statistically significant increase when comparing 1986–2005 to 2020–2039 and 2040–2059. The regional spatial average of the six climate models projected an increase in the annual PET of about + 4% and + 8% for 2020–2039 and 2040–2059, respectively. The projected average 20-year annual changes over the study area range respectively for the two projection periods were + 1.4%-+8.7% and + 3%-+14.2%. The projected annual PET increase trends are consistent across the entire region and for the 6 climate models. Higher annual changes are projected in the northeast part of the region, while smaller changes are projected along the pacific coast. The main drivers for the increase are the projected warming and increase in the vapor pressure deficit. The projected changes in PET, which represent the changes in the atmospheric evaporative demand, are substantial and likely to impact vegetation and the hydrometeorological regime in the area.