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Sheet and rill erosion plays an important role in the transformation of agricultural slopes in the Carpathians. It is an episodic process of short duration which can take place both in the winter and summer hydrological half-years as a result of meteorological short-term and high-intensity events like short downpours, intense continuous rainfall and rapid thaws. The climate of the Carpathians is characterized by the variability of its parameters caused by the growing height of the mountain belts from the north to the south as well as by the considerable latitudinal extent of the mountain range. The summer season is crucial to annual precipitation totals. Maximum daily precipitation totals are the result of intense continuous rainfall (mainly in June and Juty) and short local downpours (mainly in May). In the flysch Polish Carpathians the role of sheet and rill erosion on agricultural slopes is different in areas characterized by mountain relief (Beskidy Mountains) and foothill relief, because the two regions have different water circulation patterns and weathered material displacement patterns. Another crucial factor that helps account for the differences in the effectiveness of sheet and rill erosion is vegetation cover. The lack of sufficiently long measurement series or diversification between regions of the Carpathians makes it difficult to determine the frequency of extreme sheet and rill erosion and their recurrence, which means it is not possible to define the quantitative criteria for establishing extreme sheet and rill erosion processes for the whole area of the Polish Carpathians. The aim of this paper is to examine the criteria on the basis of which extreme sheet and rill erosion on agricultural slopes has so far been determined as well as the effects and role of the process in the transformation of the slopes.
The study on soil wash was conducted on the experimental slope within the Scientific-Research Station of the Institute of Geography and Spatial Organization (Polish Academy of Sciences) in Szymbark in years 1990-96. Five experimental plots installed on the slope were 2m wide, had lengths of 2, 4, 8, 16, and 32m and were under black fallow. Measurements of overland flow and soil wash reveled relationships between the processes and lengths of the plots and sums and intensity of rainfall events. The volume of the surface flow increased with increasing length of the plots, from 3352 dm³ (2m) do 12153 dm³ (32m), but on the other hand this was accompanied by decrease in the flow index. The index value was highest on the shortest plot (2 m) - almost 5 times greater than on the 32m-long plot. The observed relationship is typical for the river catchments: the smaller the catchment, the higher the flow index is (pertaining to 1 km² of the area). The soil wash occurred to be more differentiated with respect to the length of the plots (from 131,6kg to 1166,4kg). The relationship between maximum intensity of rain and soil wash on all plots has a character of polynomial equations of the second degree, described by negative correlation coefficients. As the plot's length increases, the relationship is stronger. During rainfall events of a very high intensity, over 1,5 mm min⁻¹ (high kinetic energy), the soil wash is limited by aggregating soil particles due to action of the rain droplets. Agrotechnical treatments significantly influence the process of soil relocation (weeding, ploughing). Observed soil wash directly following such treatments was much more intensive in comparison with conditions of slightly compressed soil.
The paper presents the results of slopewash studies carried out in hydrological years 2007-2008 on 7 runoff plots located within the marginal part of the Carpathian Foothills near the Research Station of the Institute of Geography and Spatial Management in Łazy. All the plots were 2 m wide. Four of them were 22.1 m long while the remaining three were 11.1, 5.5 and 2.8 m in length. All the plots were closed with 2 m Gerlach troughs. Eroded material was collected in tanks placed by every plot, while runoff was registered by means of lymnigraphs. The inclination of the plots' surface was 8°. Studies were carried on silty pseudogley soil (Stagnic Luvisols). Precipitation totals amounted to 814, l mm (2007) and 585,3 mm (2008) and these values significantly ditfered from the mean precipitation total for 1987-2008 hydrological years (661,5 mm). In 2007 the rainfall erosivity factor (EI₃₀) was 1628.9 MJ mm ha⁻¹h⁻¹ and turned out to be almost two times as much as it was for 2008 (667.0 MJ mm ha⁻¹h⁻¹). In the summer halfyear of 2007 hydrological year there were 13 slopewash events whereas in 2008 there were none. Mean annual soil erodibility [K] reached the value of 0.0377 Mg ha h MJ⁻¹mm⁻¹ha⁻¹, which is significantly lower than the value calculated on the basis of the USLE model (0.0738). Runoff varied considerably in the summer half-year. Maximum values of the runoff coefficient amounted to 39.2% for the potato, 34.2% for the fallow land, 16.3% for the winter wheat and 12.0% for the meadow. Mean annual slopewash values amounted to 31.4 kg ha⁻¹ for winter wheat, 41.9 kg ha⁻¹ for meadow, 43,396.0 kg ha⁻¹ for potato and 47,340.2 kg ha⁻¹ for fallow. The C factor value for the whole vegetation period of the potato was 0.92, and 0.0008 for the winter wheat.*
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