Ditch Erosion Processes triggered through Strong Precipitation Events: Implications for Forest Ecosystem Stability and Water Resources Protection

Within the forested karstic alpine water protection zones of various water suppliers in Austria like e.g. those of the Cities of Vienna and Waidhofen/Ybbs, strong precipitation events have triggered erosion processes in ditch sites, causing forest decline and turbidity in the related karstic aquifers. Both processes are intended to be mitigated or avoided, as forest cover and water supply are negatively influenced by these natural disturbances. As response strategy an outstanding precipitation event from the year 2013 on Mount Rax was analyzed both on the level of (A) the meteorological characteristics of this event and (B) it’s impacts on ditch sites and forest cover within the part of the water protection zone affected by this rainfall event (WPZ).

The meteorological characteristics of the event from 9^th of August 2013 define it as outstanding strong precipitation event, yielding 79,2 mm of rainfall within the first 2 hours and 119,2 mm within the whole event (10 hours). It was a thunderstorm event including hail, which became visible by soil temperature analysis. The inclusion of hail lowered the temperature of the water moving in and on the forest soils, by the way increasing its erosive potential through boosting its drag force for any given substances, in our context soil materials. The data stem from high temporal resolution hydro-meteorological stations (10 minutes measurement interval) within the WPZ. The precipitation event caused major impacts on ditch sites, like deep v-notch erosion or erosion of the upper soil substances and forest roads.

Therefore, the ditch sites within the WPZ were analyzed both through interpretation of aerial orthophotos and a field mapping survey leading to the GIS-based processing of the yielded data. It was possible to show that especially ditch sites beneath rock areas showed strong erosion dynamics affecting both forest vegetation cover and soil substances. Deep v-notch erosion occurred in such ditch types. Also totally new ditch sites were created through the analyzed event. In the course of the mapping survey, the soil type, vegetation cover and percentage of the site affected through erosion processes were analyzed.

The only chance to mitigate the impacts of the natural disturbance category “outstanding strong precipitation events” is a stable, vital and dense vegetation cover on ditch sites, including both ground vegetation and tree cover. Especially stable tree species like Sycamore Maple (Acer pseudoplatanus) or European larch (Larix decidua) can facilitate the stability of vegetation cover on ditch sites. The mitigation of the erosion processes has positive effects on forest cover, and above all, on water supply systems, as increased turbidity is one of the main causes for water quality issues in karstic alpine watersheds. Because of this fact it is necessary to avoid processes which damage the natural regeneration dynamics of forest ecosystems, like e.g. browsing damages on tree species. Strategical efforts of forest management have to focus on the stabilization of vegetation cover on such ditch sites. Those are supported through the Forest Hydrotope Model (a forest site model) data base which was elaborated for the WPZ.