Rainfall-runoff reaction controlled by soil moisture thresholds in a small Alpine catchment

Since 2009, we have continuously monitored soil moisture in the Eastern Alpine torrent catchment of the Brixenbach (Tyrol, Austria). The measurement network is one of the rare with a high spatial resolution and long temporal coverage and consists of eight sites with three frequency-domain (FD) sensors 10 cm below soil surface. The resulting data allowed us to analyse the precipitation-runoff reaction of the catchment depending on the antecedent soil moisture content. In Meißl et al. (2020) we found:

• The site-specific soil moisture medians correlate with altitude, but don’t correlate with sites’ slope, the topographic index nor the specific upslope area.
• In contrast to the results of other authors who analysed much shorter time series, the scatter plot of the spatial standard deviation of soil moisture against the spatial mean does not show a convex shape. We found that progressive drying during rainless periods leads to increasing spatial variability of soil moisture contents at mean soil moisture values<40 vol%. Above about 42 vol% the spatial variability of soil moisture contents decreases.
• The most exceptional out of the 547 analysed rainfall-runoff events took place at rainfall event types with high precipitation sum and long duration, but low intensity or at events with medium precipitation sum, short duration, but high intensity.
• 244 precipitation events triggered a significant increase in soil moisture (≥ 0.5 vol%) and a total runoff of at least three cubic metres. During these events, the Brixenbach catchment showed a clear threshold behaviour: Discharge coefficients above 0.23 were only observed when the spatial mean soil moisture exceeded 43.5 vol% at the eight sites. Looking at the individual sites, this threshold is also more or less clearly visible, but at different levels. The level of the spatial mean of all sites thus depends strongly on the number and local characterstics of the sites used.
• If we define the relative soil moisture as proportion of the maximum soil moisture content of the site during the whole measurement period, the threshold ranges between 0.65 and 0.80 with the sites’ mean of 0.72, which can be interpreted as saturation deficit of 0.28.
• At moist conditions, event streamflow peaked prior to soil moisture, which can be explained by increased surface flow volumes at higher soil moisture as well as already initialized subsurface flow paths.

The analyses of the long-term soil moisture time series provide a valuable insight into the hydrological system of the Brixenbach catchment and may help to identify critical conditions, which may lead to floods, also under changed conditions in future.

References: Meißl G., Zieher Th., Geitner C. (2020): Runoff response to rainfall events considering initial soil moisture – Analysis of 9-year records in a small Alpine catchment (Brixenbach valley, Tyrol, Austria). Journal of Hydrology: Regional Studies, August 2020, 100711. https://doi.org/10.1016/j.ejrh.2020.100711