Investigating the impact of active layer thickening on vertical soil moisture distribution in the Tibetan Plateau

Permafrost degradation caused by climate warming has potentially large impact on the hydro-eco environment in the Tibetan Plateau (TP) through affecting soil water redistribution, and it is critical to investigate the soil moisture changes and estimate their response to future climate conditions. In this study, we first analyzed the in-situ soil temperature and moisture data to examine the impact of active layer thickening on soil moisture redistribution. There is generally a “water-rich zone” around the bottom of the active layer at sites with the active layer thickness (ALT) greater than ~2 m, and a relative low soil moisture zone occurs approximately between the bottom of the root zone (~ 0.4 m) and the bottom of the active layer. However, at shallower-ALT sites (e.g., ALT< 2 m), a “soil water rich zone” occurs at the upper active layer rather than at the bottom of the active layer, and soil moisture at the deeper active layer generally shows a decreasing trend along soil depth. We used a process-based permafrost hydrology model to represent the above effects of active layer thickening on soil moisture redistribution through modifying the soil hydraulic profile. Model sensitivity runs indicate that soil moisture redistribution with active layer thickening is largely due to dramatic changes of hydraulic conductivity between the root zone and deeper layers (>~ 1m). The saturated hydraulic conductivity tends to increase a little in the root zoon and then show a sharp exponential decline along soil depth, while the pedo-transfer functions that are commonly used in models cannot reproduce this process well.

Our results indicate that shallower ALT helps to retain soil moisture in the soil root zone; however, when ALT increases to a certain depth, the root-zone soil layer tends to lose water because of little recharge from deeper (>~1m) soils due to the dramatical decreases in soil hydraulic conductivity. Therefore, active layer thickening may exacerbate soil drying in the root-zone, which will have negative impacts on the vegetation growth and performances of ecosystem functioning. We will further investigate the soil moisture changes under different climate scenarios in order to better project the future hydro-eco response in the TP permafrost region.