Seasonal dynamics of surface runoff in mountain grassland ecosystems differing in land use

Better understanding of surface runoff quantity for distinct hydrological units becomes increasingly important as many rainfall-runoff models use static surface runoff coefficients and neglect key factors affecting ecohydrological dynamics, e.g. land cover and land use. Especially in small-scale alpine catchments, surface runoff and its contribution to mountain torrent runoff is frequently underestimated. In our study, the seasonal variability of surface runoff on abandoned areas and pastures in the alpine catchment ‘Kaserstattalm' (Stubai Valley, Austria, Eastern Alps) was analyzed using a rain simulator along with soil water content (SWC) and soil water tension (SWT) measurements. Additionally, seasonal variability of soil physical and soil hydraulic properties were assessed. Analyzing more than 30 rainfall simulations on 10m² plots at a rate of 90mmh⁻¹ (equivalent to convective precipitation events with 100years return period) revealed a mean surface runoff coefficient of 0.01 on abandoned areas and 0.18 on pastures. Regarding seasonal variability, relevant surface runoff was limited to pastures in autumn with a maximum runoff coefficient of 0.25. The field capacity (Fc) of all soils was found to be stable throughout the season. However, for pastures, cattle trampling led to a significant increase of dry bulk density (BD) of up to +0.33gcm⁻³ (p ⩽0.01) in the top 0.1m of the soil which is attributed to a compaction of macropores. Although measured infiltration rates decreased by more than 60%, BD could ‘recover' during the winter season presumably due to freezing-and-thawing cycles and bioturbation processes decreasing soil compaction. This study highlights that impacts of land-use changes on soil physical properties make surface runoff difficult to model. Moreover, dynamic and interactive behaviour of soil parameters have to be considered in order to make realistic assessments and accurate predictions of surface runoff rates. Finally, land-use effects on runoff formation in general and significance at catchment scale are discussed.