Characterization of soil moisture response patterns and hillslope hydrological processes through a self-organizing map

Abstract

Abstract. Hydrologic events can be characterized as particular combinations of hydrological processes on a hillslope scale. To configure hydrological mechanisms, we analyzed a dataset using an unsupervised machine learning algorithm to cluster the hydrologic events based on the dissimilarity distances between the weighting components of a self-organizing map (SOM). The time series of soil moisture was measured at 30 points (at 10 locations with three different depths) for 356 rainfall events on a steep, forested hillslope between 2007 and 2016. The soil moisture features for hydrologic events can be effectively represented by the antecedent soil moisture, soil moisture difference index, and standard deviation of the peak-to-peak time between rainfall and soil moisture response. Five clusters were delineated for hydrologically meaningful event classifications in the SOM representation. The two-dimensional spatial weighting patterns in the SOM provided more insights into the relationships between rainfall characteristics, antecedent wetness, and soil moisture response at different locations and depths. The distinction of the classified events could be explained by several rainfall features and antecedent soil moisture conditions that resulted in different patterns attributable to combinations of hillslope hydrological processes, vertical flow, and lateral flow along either surface or subsurface boundaries for the upslope and downslope areas.

Responses of soil moisture at different topographic positions to rainfall events along a steep subtropical forested hillslope

“…Tian et al (2019) used soil moisture monitoring data from a long-term large-scale network in the Qilian Mountains in China to quantitatively estimate the profile distribution of soil moisture increments based on soil wetting events. Lee and Kim (2021) Prior studies also revealed strong control of topography on soil moisture regimes (Burt & Butcher, 1985;Francis et al, 1986). Key hillslope-scale topographic characteristics affecting the lateral or vertical redistribution of soil water include slope shape, slope gradient, slope position, slope length, relative wetness, and soil properties (Tromp-van Meerveld & McDonnell, 2006;Western et al, 1999).…”

“…These various indices describe one or the other aspects of soil moisture response separately. However, only a few studies have focused on the combination of different indices to provide a more nuanced understanding of soil moisture dynamics (Jin et al, 2018; Lee & Kim, 2021; Tian et al, 2019). For instance, Jin et al (2018) analysed the wetting front regimes of rainwater through soil profile on two contrasting land use hillslopes in the Loess Plateau of China.…”

“…Tian et al (2019) used soil moisture monitoring data from a long‐term large‐scale network in the Qilian Mountains in China to quantitatively estimate the profile distribution of soil moisture increments based on soil wetting events. Lee and Kim (2021) examined the lag time between the time of maximum soil moisture response and the time of peak rainfall intensity in a steep hillslope. Within this context, there remains a need to use multiple response indices to highlight how rainfall affects different aspects of soil moisture response (e.g., response amplitude, response time and rate), and how we integrate these indices to obtain a comprehensive understanding of the dynamic response patterns in steep and humid hillslopes.…”

Response of soil moisture to prewinter conditions and rainfall events at different distances to gully banks in the Mollisol region of China

“…After the freeze–thaw period in seasonally frozen regions, rainfall events can evoke different soil moisture responses throughout the soil profile of the gully bank. The response of soil moisture to rainfall events has been investigated for various topographic positions, depths, and land cover conditions (Lee & Kim, 2021; Zhu et al, 2014). Generally, the soil moisture in the surface soil layer (0–40 cm) increases rapidly after precipitation (Cheng et al, 2020; Wang et al, 2013).…”

“…The extent of near-saturation zones on gully banks may affect the position and dynamics of soil slumping during the next rainy season. Thus, gully bank erosion processes are closely related to prewinter freezing soil moisture conditions (Fouli et al, 2013;Wen et al, 2023) and subsequent rainfall events (Lee & Kim, 2021;Poesen et al, 2003;Zhu et al, 2014). Generally, an increase in soil moisture on gully banks is directly correlated with rainfall events (Danielewska et al, 2015).…”