Global estimation of effective plant rooting depth: Implications for hydrological modeling

Plant rooting depth ( Z r ) is a key parameter in hydrological and biogeochemical models, yet the global spatial distribution of Z r is largely unknown due to the difficulties in its direct measurement. Additionally, Z r observations are usually only representative of a single plant or several plants, which can differ greatly from the effective Z r over a modeling unit (e.g., catchment or grid‐box). Here, we provide a global parameterization of an analytical Z r model that balances the marginal carbon cost and benefit of deeper roots, and produce a climatological (i.e., 1982–2010 average) global Z r map. To test the Z r estimates, we apply the estimated Z r in a highly transparent hydrological model (i.e., the Budyko‐Choudhury‐Porporato (BCP) model) to estimate mean annual actual evapotranspiration ( E ) across the globe. We then compare the estimated E with both water balance‐based E observations at 32 major catchments and satellite grid‐box retrievals across the globe. Our results show that the BCP model, when implemented with Z r estimated herein, optimally reproduced the spatial pattern of E at both scales (i.e., R 2  = 0.94, RMSD = 74 mm yr −1 for catchments, and R 2  = 0.90, RMSD = 125 mm yr −1 for grid‐boxes) and provides improved model outputs when compared to BCP model results from two already existing global Z r data sets. These results suggest that our Z r estimates can be effectively used in state‐of‐the‐art hydrological models, and potentially biogeochemical models, where the determination of Z r currently largely relies on biome type‐based look‐up tables.