Combined measurement and modeling of the hydrological impact of hydraulic redistribution using CLM4.5 at eight AmeriFlux sites

Authors: FU, C. - University Of Connecticut; WANG, G. - University Of Connecticut; GOULDEN, M.L. - University Of Connecticut; Scott, Russell - Russ; BIBLE, K. - University Of Washington; CARBON, Z.G. - Marine Biology Laboratory

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2016
Publication Date: 5/17/2016
Citation: Fu, C., Wang, G., Goulden, M., Scott, R.L., Bible, K., Carbon, Z. 2016. Combined measurement and modeling of the hydrological impact of hydraulic redistribution using CLM4.5 at eight AmeriFlux sites. Hydrology and Earth System Sciences. 20:2001-2018. https://doi.org/10.5194/hess-20-2001-2016.
DOI: https://doi.org/10.5194/hess-20-2001-2016

Interpretive Summary: Many plants have been shown to move water through their roots from wetter regions of the soil to drier ones and vice versa. While many field studies have shown that this process has important hydrological and ecological consequences on the functioning of ecosystems, the current generation of earth system models do not include this process of water redistribution by plants. This study incorporated this effect into a commonly-used earth system model and examined the ability of the resulting hybrid model to capture the observed ecosystem evaporation and soil moisture storage across an ecologically broad selection of sites in North America. Our comparison showed that the hybrid model increased simulated evaporation, particularly at mid-range levels of soil moisture. This comparison also revealed that plant water redistribution is particularly important for ecosystems that have a pronounced dry season but are not too dry that sparse vegetation and very low soil moisture limit its effects.

Technical Abstract: Effects of hydraulic redistribution (HR) on hydrological, biogeochemical, and ecological processes have been demonstrated in the field, but the current generation of standard earth system models does not include a representation of HR. Though recent studies have examined the effect of incorporating HR into land surface models, few (if any) have done cross-site comparisons for contrasting climate regimes and multiple vegetation types via the integration of measurement and modeling. Here, we incorporated the HR scheme of Ryel et al. (2002) into the NCAR Community Land Model Version 4.5 (CLM4.5), and examined the ability of the resulting hybrid model to capture the magnitude of HR flux and/or soil moisture dynamics from which HR can be directly inferred, to assess the impact of HR on land surface water and energy budgets, and to explore how the impact may depend on climate regimes and vegetation conditions. Eight AmeriFlux sites with contrasting climate regimes and multiple vegetation types were studied, including the Wind River Crane site in Washington State, the Santa Rita Mesquite savanna site in southern Arizona, and six sites along the Southern California Climate Gradient. HR flux, evapotranspiration (ET), and soil moisture were properly simulated in the present study, even in the face of various uncertainties. Our cross-ecosystem comparison showed that the timing, magnitude, and direction (upward or downward) of HR vary across ecosystems,and incorporation of HR into CLM4.5 improved the model-measurement matches of evapotranspiration, Bowen ratio, and soil moisture particularly during dry seasons. Our results also reveal that HR has important hydrological impact in ecosystems that have a pronounced dry season but are not overall so dry that sparse vegetation and very low soil moisture limit HR.