Comment on hess-2022-230

Abstract

Almost no hydrological model takes into account that changes in evapotranspiration are affected by how the vegetation responds to changing CO2 and climate. This severely limits their ability to quantify the impact of climate change on evapotranspiration and thus water resources. We developed a simple approach for mimicking, in hydrological models, the impact of active vegetation on potential evapotranspiration (PET) under climate change. This approach can be applied for climate change impact studies by hydrological models that compute PET as a function of net radiation and temperature only, i.e., using the Priestley-Taylor (PT) equation. Our approach is based on the work of Milly and Dunne (2016) (MD), which compared the change of non-water-stressed actual evapotranspiration (NWSAET) as computed by an ensemble of global climate models (GCM) with various methods for computing PET change. MD proposed to estimate the impact of climate change on PET as a function of only the change in net energy input at the land surface, i.e., to neglect changes in other climate variables. The new mimicking approach (PT-MA) for application in hydrological models retains the impact of temperature on daily to interannual as well as spatial PET variations but removes the impact of the long-term temperature trend on PET such that long-term changes in future PET are driven by changes in net radiation only. We implemented the PT-MA approach in the global hydrological model WaterGAP 2.2d and computed daily time series of PET between 1901 and 2099 using the bias-adjusted output of four GCMs for RCP8.5. With PT-MA, increases of GCM-derived NWSAET between the end of the 20th and the end of the 21st century are simulated well by WaterGAP, while severely overestimated with the standard PT. Application of the mimicking approach in WaterGAP results in smaller future decreases or larger future increases in renewable water resources (RWR) as compared to neglecting active vegetation, except in a small number of grid cells where increased inflow from upstream due to increased upstream runoff leads to enhanced evapotranspiration from surface water bodies or irrigated fields. On about 20 % of the global land area, the mimicking approach leads to an increase of RWR that is more than 20 % higher than when neglecting the active vegetation, while on more than 10 % of the global land area, the projected RWR decrease is lowered by more than 20 %. We recommend applying the mimicking approach to assess climate change hazards by any hydrological model that does not include stomatal conductance when calculating PET.