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.