How do spatial throughfall patterns reflect in soil moisture patterns?

This study investigates whether and how vegetation cover affects the spatial heterogeneity and vertical penetration of water through the Upper Critical Zone (UCZ). We assessed rainfall, throughfall and soil water contents on a 1‐ha temperate mixed beech forest plot in Germany. Throughfall and soil water content in two depths (7.5 cm and 27.5 cm) were measured on an event basis during the 2015 - 2016 growing season in independent high‐resolution stratified random designs. We calculated the increase of soil water content (Δθ) due to the rainfall by the difference between measurements at the beginning (pre-event) and the maximum soil water content after the end of rainfall event (post-event). Since throughfall and soil water content cannot be assessed at the same location, we used kriging to derive the throughfall values at the locations where soil water content was measured. We explore the spatial variation and temporal stability of throughfall and soil water content and evaluate the effects of throughfall, soil properties (field capacity and air capacity), and vegetation parameters (next tree distance) on soil water content variability.

Throughfall patterns were related to canopy density although correlation length decreased with increasing event size. Temporal stability was high, leading to persistently high and lower input locations across rainfall events.

A linear mixed effect model analysis confirmed that the soil water content increase due to precipitation depended on throughfall patterns, in that more water was stored in the soil where throughfall was enhanced. This was especially the case in large events and in both investigated soil depths. However, we also identified additional factors that enhanced or decreased water storage in the soil, and probably indicate fast drainage and runoff components. Locations with low topsoil water content tended to store less of the available water, indicating the role of preferential flow. In contrast in subsoil, locations with high water content, and probably poor drainage, stored less water, indicating lateral flow. Also, distance to the next tree and air capacity modified soil water storage.

Spatial soil water content patterns shortly before a rainfall event (pre-event conditions) seem to be a key factor in soil water content increase, and also explained much of soil water content shortly after the rainfall event. Pre-event soil water content was mostly driven by random local effects, probably microtopography and root water uptake, which were not quantified in this study. The remaining spatial variation was explained by air capacity in both soil layers, indicating the role of macroporosity.

Our findings show at the same time systematic patterns of times and locations where the soil capacity to store water is reduced and water probably conducted quickly to greater depth. Not only soil moisture patterns but also deeper percolation may depend on small scale spatial heterogeneity of canopy input patterns.