Drained, dampened, delayed: Deep soil moisture dynamics

Soil moisture is most often measured in-situ only to depths of about 50 cm. This is due to either the larger effort or challenges in installation or at some sites due to the presence of weathered bedrock. It is furthermore often assumed that with the top 50 cm of the soil we already capture the main part of the root zone and thus the major processes of infiltration, evaporation and transpiration should all be reflected in these soil moisture observations. However, due to lack of data we cannot be sure that this is really the case.

In this study we are reviewing soil moisture dynamics measured in the field at depths ranging between 70 and 450 cm. This includes more than 100 sensors at depth >70 cm and more than 60 at depth >100 cm. These sensors are installed in sandy soil in 14 different forest stands of the TERENO observatory in north-eastern Germany. We examine both seasonal and event responses. We furthermore compare the responses in the unsaturated zone also to the responses observed in shallow and deep groundwater. Using simple uncalibrated 1D Hydrus simulations we then put our observations into the context of those simulated by the model under pure matrix flow conditions, thus ignoring any preferential flow. The above described setup allows us to investigate the effects of infiltration, percolation, preferential flow, deep drainage, and transpiration at depths usually not accounted for in standard monitoring networks.