Influence of soil structure on the spatiotemporal variability of subsurface water flows in a volcanic ash-derived soil

Predicting the spatiotemporal variability of soil moisture dynamics at different scales is a major challenge. Moreover, multimodal soil hydraulic properties resulting from complex soil structures, such as the exhibited by volcanic soils, still lack realistic and dynamic parameterisation. This work aimed to shed light on the spatiotemporal heterogeneity of subsurface water flows and soil water distribution during wet and dry conditions in volcanic ash-derived soil. The volumetric moisture content (VMC) at 10, 20, and 60 cm depth was measured with a set of eight TDR sensors from September 2019 to January 2022 with a 10-minute resolution. These VMC time series were separated into wet (WP) and dry (DP) periods based on the mean VMC. Subsequently, the spatiotemporal variability in moisture content within the soil profile was analysed using spectral analyses. The propagation of periodicities from Prate to the three topsoil VMC time series as well as the time scales in the correlation of the VMC between sensors were described. Finally, the time dependency of wetting slopes (St) on Prate was assessed by the cross-correlation function (CCF). The VMC dynamics vary between WP and DP, related to the water-filled pore space and pore size distribution. The CWT showed that Prate periodicities propagate to VMC, except for periodicities of 3 to 6 months scales. The WC showed that increases in VMC result in an exponential decrease in the minimum time scale of correlation between moisture contents measured within the topsoil. The CCF described a moderate temporal correlation between Prate and St. The soil wetting response to precipitation was notably faster during wet periods, while the cross-correlation lag and the response heterogeneity increased during dry conditions. The heterogeneity of subsurface water flows resulting from complex soil structure dynamics were described by the spatiotemporal variability of soil moisture in volcanic ash- derived soil. VMC response to Prate is faster during wetter conditions than in dry periods. Temporal periodicities within the topsoil suggest that hydraulic properties experience a dynamic shift from a heterogeneous to a homogeneous system. Changes in the temporal correlation of the soil moisture measured within the topsoil, along with an accurate description of the time dependency of St on Prate, can be valuable for further understanding the hysteresis of soil moisture variations in a soil profile.