Drought-driven Changes in Terrestrial Water Use Efficiency Impact Lateral Carbon Transport

The intensification of hydrological conditions, including drought, drives changes in net ecosystem exchange (NEE). Plant water-use efficiency (WUE) mediates these changes through the coupling of GPP and evapotranspiration (ET) and can increase as a physical adaptation in response to water stress, altering the magnitude of globally significant carbon (C) fluxes. Since WUE impacts both runoff and ecosystem carbon content, changes in WUE will likely impact lateral C transport (LCT) from terrestrial to aquatic ecosystems, a historically unaccounted for component of NEE. Previously, we found that LCT is sensitive to model vegetation parameters, and particularly water-use efficiency (WUE) due to its impacts on both C and water cycling. However, more research is needed to determine whether accounting for short-term drought-driven changes in WUE can improve predictions of LCT at the watershed scale. Here, we apply a coupled terrestrial-aquatic C and hydrology process model at the watershed scale and use a data assimilation approach to quantify the impact of dynamic WUE on LCT estimates compared to a traditional static WUE parameter. We calculate dynamic WUE from Eddy covariance flux tower data collected from the Duke Forest AmeriFlux site and compare terrestrial and aquatic model outputs to MODIS and USGS stream gauge data, respectively. We run two model scenarios for each of the three vegetation sites present at Duke Forest (grassland, hardwood forest, and pine plantation). In scenario 1, we use a static PFT-specific WUE parameter. In scenario 2, we use dynamic WUE data. Study results suggest that using dynamic WUE data is valuable for modeling LCT response to drought, and that predictability is improved yet highly depends on vegetation type. Therefore, the study indicates that dynamic WUE data has an important role in terrestrial-aquatic models for accurately quantifying and predicting carbon fluxes in response to future environmental stress and change.