Critical soil moisture detection and water-energy limit shift attribution using satellite-based water and carbon fluxes over China

Abstract. Critical soil moisture (CSM), a tipping point of soil moisture (SM) when evapotranspiration (ET) begins to suffer from water limitation, is essential for the vegetation state and corresponding land‐atmosphere coupling. The water and energy-limited regime in various biomes and climates shifts under global climate change. However, detecting CSM and attributing water-energy limit shifts to climate and ecosystem variables are challenging as in-situ observations of water, carbon fluxes, and SM are sparse. In this study, CSM derived from two satellite-based methods were assessed over China: the difference between the correlation between SM and ET and the correlation between vapor pressure deficit (VPD) and ET (correlation-difference method) using four satellite-based ET products; the covariance between VPD and gross primary production (GPP) (VPD-GPP-SM method) using four satellite-based GPP products. The ET and GPP products were Penman-Monteith-Leuning (PML) ET and GPP, Global LAnd Surface Satellite (GLASS) ET and GPP, Collocation-Analyzed Multi-source Ensembled Land Evapotranspiration (CAMELE) ET, Surface Energy Balance Algorithm for Land evapotranspiration in China (SEBAL) ET, Two-Leaf light use efficiency model based (TL) GPP, and SIF-based GPP (GOSIF). At flux sites, satellite-based ET and GPP were evaluated by the eddy covariance technique, and CSMs derived from the site-level GPP and ET data were evaluated by the evaporative fraction-soil moisture (EF-SM) methods. Our study revealed that the performance of ET, GPP, and CSM at the site scale demonstrated reliable results and applicability to regional scales. The intercomparison of CSM from multi-source ET and GPP datasets across China indicated their consistency and robustness. Generally, CSM decreased from southern to northern regions of China and decreased with increasing layer depth, particularly in the Tarim Basin and Haihe River Basin. Areas characterized by clay-rich soils (e.g., 0.39 m3/m3 using GOSIF GPP and 10 cm depth SM), and forests (e.g., 0.37 m3/m3 using GOSIF GPP and 10 cm depth SM), and located within the Pearl River Basin and Southeastern River Basin displayed a relatively high CSM. Decreased energy limitations in western and southern regions in June–September over the period 2001–2018 were associated with increasing ET and decreasing precipitation, respectively. The findings highlight the variability in CSM and its primary determinants, offering valuable insights into the potential water limitation on both ET and GPP processes under comparable SM circumstances.