Comparing the spatio-temporal variations of soil water content and soil free water content at the hillslope scale

- Whether FWC is better in describing subsurface fast flow than SWC?
- Comparing the spatio-temporal variations and controlling factors of FWC and SWC.
- Thresholds of spatial mean SWC for triggering subsurface fast flow were observed.
- Improved TS analysis was better to identify subsurface fast flow active locations.

The spatio-temporal dynamics of soil water are the key critical zone processes that control hydrological, biogeochemical and environmental processes at various spatial scales. Soil water content (SWC), which has been widely adopted in traditional studies, does not consider the energy state of soil water and thus cannot directly reflect the active status of subsurface fast flow (flux when SWC is above field capacity). By subtracting water content at field capacity (− 33 kPa) from SWC, free water content (FWC) were calculated and used to indicate status of subsurface fast flow. In this study, the spatio-temporal variations and controlling factors of SWC and FWC were compared on a typical bamboo forest hillslope in Taihu Lake Basin, China. An improved temporal stability (TS) analysis replacing the spatial means of SWC in the equation by the field capacity was also proposed to better identify the active locations of subsurface fast flow. Results showed that the SWC and FWC had similar temporal trends and spatial patterns. Thresholds of spatial mean SWCs were found at 10- and 30-cm depths (0.17- and 0.18-m3 m− 3, respectively). Above these thresholds, the spatial means and variances of FWC started to increase with the spatial mean SWCs. This indicated that the subsurface fast flow starts to occur. Below these thresholds, nearly no free water existed and the subsurface fast flow ceased. The active locations of subsurface fast flow determined from the improved TS analysis were not always consistent with the high SWC locations. This indicated that traditional TS analysis was not adequate to interpret the active status of subsurface fast flow. Controlling factors of SWC and FWC spatial variations were generally similar. However, the spatial distribution of FWC was less affected by soil properties and topography. In addition, the influences of controlling factors on FWC were more temporally varied. These findings will be beneficial for identifying the “hot spots” of soil water movement and biogeochemical processes.