Influence of the dry event induced hydraulic redistribution on water and carbon cycles at five AsiaFlux forest sites: A site study combining measurements and modeling

Abstract

Recent studies have incorporated hydraulic redistribution (HR) schemes into Earth system models to explore its effects on the hydrological, biogeochemical, and ecological processes of terrestrial ecosystems. However, the influence of HR on ecosystems during increasingly frequent extreme drought events, and the performance of different HR schemes at sites with contrasting ecosystem types and climate regimes, remains unclear, especially among ecologically fragile forest ecosystems in Asia. In this study, we incorporated two different HR schemes into Community Land Model version 4.5 (CLM4.5). The model generally improved predictions of soil moisture, evapotranspiration (ET), and carbon (C) fluxes at five AsiaFlux sites with contrasting climate regimes. Upward HR stimulated surface soil moisture, and increased ET by 0.29–0.68 mm H2O day−1, and net ecosystem C uptake by 1.03 × 10–6 to 1.64 × 10–5 g C m−2 s−1, at the five forest sites during dry events. Remarkable downward HR was found only in a semi-arid ecosystem located at the northeastern margin of the Qinghai–Tibet Plateau. Incorporating of a simple empirical HR scheme into CLM4.5 yielded better performance than incorporating a physically based scheme into the model for ecosystems with seasonal frozen soil layers. Combined with previous studies at AmeriFlux sites, it can be concluded that climate zones with overlapped dry season and growing season showed clear hydraulic lift during dry season; climate zones with overlapped wet season and growing season showed clear hydraulic descent if precipitation was limited (e.g. ≤ 400 mm a−1), and did not showed clear influences of HR if the precipitation was unlimited (e.g. ≥ 800 mm a−1).