Spatiotemporal dynamics of soil moisture and the occurrence of hysteresis during seasonal transitions in a headwater catchment

Abstract

Spatiotemporal dynamics of soil moisture and its hysteresis behavior are crucial for the integration of soil, vegetation, and hydrological processes in headwater catchments. While extensive research has been conducted on soil moisture patterns within distinct dry or wet seasons, the transitions between these seasons have received relatively little attention. Therefore, exploring the variability in soil moisture and its hysteresis characteristics during these transitional periods is imperative. This study delves into the relationship between spatial mean $\overline{\theta }$ and standard deviation σ_θ (σ_θ($\overline{\theta }$)) of soil moisture over both time and space, utilizing three years of daily monitoring data from 33 sites within a 0.08 km² headwater catchment in Pennsylvania, USA, with a focus on the seasonal transition periods. Through empirical orthogonal function (EOF) analysis, key factors that influence the occurrence of the hysteresis cycles in the σ_θ($\overline{\theta }$) relationship were identified. The findings of this study reveal a linear trend in the σ_θ($\overline{\theta }$) relationship over time during transitional periods, contrasting with behavior observed within the dry and wet seasons. Notably, hysteresis cycles in the σ_θ($\overline{\theta }$) relationship are present in each season, predominantly during wetter periods. These cycles are categorized into two types: those without reorganization, featuring wetting-up and drying-down phases, and those with reorganization, which also include a reorganization phase. The first two EOFs explain 75.3 % of the variation in hysteresis cycles and exhibit significant correlations with soil characteristics, topography, and vegetation. More specifically, topography and soil properties primarily influence soil moisture patterns during the transition from wet to dry seasons, whereas vegetation and topography are more influential during the transition from dry to wet. The observed spatiotemporal variability and the occurrence of hysteresis cycles in soil moisture supplement the behaviors and mechanisms of soil moisture in seasonal transitions in forested headwater catchments.