Preferential flow influences the temporal stability of soil moisture in a headwater catchment

The dynamics and controls of soil moisture spatial organization are crucial to modeling water flux and land-atmosphere interactions across landscapes. Despite this importance, the control of subsurface preferential flow on the temporal stability of soil moisture spatial organization (TSSS) has not been thoroughly examined. This study monitored daily soil moisture at multiple depths (5-162 cm) for three consecutive years at 33 sites across a forested headwater catchment spanning 7.9 ha. The standard deviation of the relative difference in soil moisture between each monitoring location and the catchment mean (i.e., SDRD, a measure of TSSS) was calculated to quantify the influence of controlling factors on TSSS. Factors considered included soil moisture monitoring frequency, soil and terrain attributes, soil wetness condition, and preferential flow. The analysis was conducted separately for wet and dry seasons. The results indicated that (1) soil moisture monitoring frequency had a considerable impact on TSSS, particularly in valley and swales during dry seasons. As the monitoring frequency increased from 1 day to 28 days, the SDRD values of areas with varied soil-terrain attributes all decreased, suggesting a more stable TSSS. However, this also implied that coarser monitoring frequencies might not capture short-term (e.g., daily) soil moisture processes and associated TSSS dynamics. (2) The relationship between soil-terrain attributes and TSSS was stronger during dry seasons than wet seasons (R2 of 0.597 vs. 0.301, P < 0.05). (3) TSSS was significantly correlated with preferential flow frequency (r = 0.611, P < 0.05) at the study catchment. Including preferential flow frequency as a regression variable improved the explanation and prediction of TSSS. This study confirms the vital role of preferential flow in regulating TSSS in complex terrain. These findings underscore the need for high-frequency monitoring to accurately characterize soil moisture distributions over time and space in similar environments.