The dominating control of terrain relief on the soil water content distribution during wet-dry transitions in headwaters

Abstract:

Redistribution of soil water during and after rainstorms are affected by soil and topography. Understanding how soil-terrain attributes affect soil volumetric water content (VWC) distribution under various wetness states is a prerequisite to accurate hydrological modeling. Herein, we examined the relationships between soil-terrain attributes and soil VWC based on measured soil VWC data in a steep (average 60%), forested, zero-order catchment. Results showed that during the wet state when the scaled wetness [W, relative saturation ratio] was larger than 0.67, topography was the main factor redistributing soil VWC. With the increase in precipitation, the relationship between VWC and topographic wetness index (TWI) increased accordingly (R² = 0.43). However, perhaps the most compelling finding was that as the wetness states switched from transition (W<0.67) to dry (W<0.31) conditions, the relationship between VWC and TWI increased (adjusted R² from 0.09 to 0.31). To illustrate the increase in explanatory power for TWI, we computed water balance at all sites across the landscape. We found that unsaturated lateral flow was required to fully explain the change in VWC, which contradicts the previous understanding that nonlocal control factor (e.g., TWI) is not an important factor in soil water redistribution during the dry periods. We also found that the variability of VWC (CV) at the 40-cm depth (the interface between two soil layers) was higher than that of other depths during the dry state, highlighting that an impeding soil layer might be critical in influencing the movement of unsaturated lateral flow in the dry state. In summary, we concluded that in steep mountain catchments with well-drained soil, topography is the dominant controlling factor of VWC distribution in both wet and wet-dry transition conditions; and the interface of soil horizons may act as an important impeding layer in determining lateral unsaturated flow in the dry state.