Implication of vegetation response to future climate conditions in current potential evapotranspiration methods – a grassland lysimeter study

Understanding the vegetation response to climate change, especially warming and elevated CO₂, is crucial for a better understanding of the present and future hydrological conditions and processes. Based on recent findings in estimating potential evapotranspiration (PET), the study presents an improved method of estimating PET, that was evaluated with actual evapotranspiration (ET) lysimeter data from a managed alpine grassland.

Research findings from field observations reported reduction in leaf-level stomatal conductance, as higher CO₂ drives partial stomatal closure, consequently reducing ET. Thus, a modified Penman-Monteith (PM) evapotranspiration method (Yang, 2018) was used, that introduces the vegetation response to elevated CO₂ into Penman-Monteiths (PM) formalism, directly targeting the surface resistance (r_s).

Comparing PET values computed with the original PM method with lysimeter data of actual evapotranspiration displayed underestimation of the mean PET. This was also found in a recent study (Schymanski, 2017) that revealed an omission in the Penman-Monteith equation, pointing out that the PM method neglects two-sided exchange of sensible heat by a planar leaf.

This study joined these findings and tested a new method for calculating PET in climate change studies. The proposed PM method accounts for both the plant physiological response to higher CO₂ and two-sided heat exchange of planar leafs. Additionally, other less data consumptive PET methods were evaluated to compare the model performance with the newly derived PET method.

The methods were evaluated and optimized based on lysimeter data of six high precision weighable lysimeters, where each of the grassland lysimeters was subjected to treatment, simulating elevated CO₂ concentrations and warming. The lysimeters are located at the AREC Raumberg-Gumpenstein (Styria, Austria) and are part of an experimental site, which incorporates a CO₂ enrichment technique (+ 300 ppm; miniFACE technique) and infrared heaters (+3° C; T-FACE-Technique). Using the corrected PM equation, that accounts for a two-sided heat exchange, the model performance of the PM equation was improved for both ambient and future conditions. Combining this equation, with the PM method accounting for the plant physiological response to higher CO₂, the corrected method produced much better fit to the lysimeter data compared to the original equation.

The results of this study present an improvement of the PM method that not only enhances the representation of transpiration and sensible heat to changes in atmospheric conditions, but also incorporates the response of elevated CO₂, which make it more suitable for climate change studies.