Does consideration of water routing affect simulated water and carbon dynamics in terrestrial ecosystems?

Abstract. The cycling of carbon (C) in terrestrial ecosystems is closely coupled with the cycling of water. An important mechanism connecting ecological and hydrological processes in terrestrial ecosystems is lateral flow of water along landscapes. Few studies, however, have examined explicitly how consideration of water routing affects simulated water and C dynamics in terrestrial ecosystems. The objective of this study is to explore how consideration of water routing in a process-based hydro-ecological model affects simulated water and C dynamics. To achieve that end, we rasterized the regional hydro-ecological simulation system (RHESSys) and employed the rasterized RHESSys (R-RHESSys) in a forested watershed. We performed and compared two contrasting simulations, one with and another without water routing. We found that R-RHESSys was able to correctly simulate major hydrological and ecological variables regardless of whether water routing was considered. When water routing was considered, however, soil water table depth and saturation deficit were simulated to be greater and spatially more heterogeneous. As a result, water (evaporation, transpiration, and evapotranspiration) and C (forest productivity, soil autotrophic and heterotrophic respiration) fluxes also were simulated to be spatially more heterogeneous compared to the simulation without water routing. When averaged for the entire watershed, the three simulated water fluxes were greater while C fluxes were smaller under simulation considering water routing compared to that ignoring water routing. In addition, the effects of consideration of water routing on simulated C and water dynamics were more apparent in dry conditions. Overall, the study demonstrated that consideration of water routing enabled R-RHESSys to better capture our preconception of the spatial patterns of water table depth and saturation deficit across the watershed. Because soil moisture is fundamental to the exchange of water and C fluxes among soil, vegetation and the atmosphere, ecosystem and C cycle models therefore need to explicitly represent water routing in order to accurately quantify the magnitude and patterns of water and C fluxes in terrestrial ecosystems.