The Dominant Control of Relief on Soil Water Content Distribution During Wet-Dry Transitions in Headwaters

The redistribution of hillslope soil water during and after rainstorms is affected by soil properties and topography. Therefore, understanding how soil-terrain attributes affect the soil volumetric water content (VWC) distribution under various catchment storages is a prerequisite for accurate hydrological modeling. Herein, the relationships between soil-terrain attributes and soil VWC were examined in a steep (average slope = 60%), forested, zero-order catchment. Detailed topography, soil properties, and runoff, and high frequency (6-min) soil moisture data observed from July 2016 to November 2017 were employed along with resampled VWC data in three time-steps (daily, hourly, and 6-min intervals) for correlation analysis between soil-terrain attributes and measured VWC. The results showed that daily aggregated data overlooked the highly heterogeneous flow conditions under dry-wet transitions, leading to a constantly high correlation between VWC and the topographic wetness index (TWI). In comparison, the 6-min data captured short-lived flow processes, highlighting that, in reality, VWC-TWI correlations fluctuated frequently as a function of catchment storages and precipitation characteristics, with four wetting patterns identified. Under wet-dry transitions, the VWC-TWI correlations increased significantly (from 0.3 to 0.6) as the hillslope gradually drained of lateral subsurface water. Soil water behavior consists of diurnal oscillations superimposed on a declining trend, which demonstrates the existence of unsaturated flow and which contributed to the increase in the strength of the correlations. Finally, through correlation analysis of all drying (n = 4) and wetting (n = 45) periods, we found that soil-terrain attributes and VWC correlations can be used to identify the dominant soil water processes in various catchment storages.

Automated summary

The redistribution of hillslope soil water during and after rainstorms is influenced by soil properties and topography. 6-min data can capture short-lived flow processes, showing VWC-TWI correlations fluctuate frequently, and the correlation significantly increases during wet-dry transitions.