Modern gully erosion in forest and forest-steppe landscapes of the east of the Russian Plain

The spatial and temporal assessment of modern gully erosion was carried out for a large region (more than 68000 km2 ) of the eastern Russian Plain located at the intersection of forest (subzones of southern taiga, mixed and broad-leaved forests) and forest-steppe landforms within the Republic of Tatarstan. The choice of the territory was caused by the high density of gullies established more than half a century ago, as well as by the availability of multi-temporal cartographic data about the gully network density obtained using a unified methodology. The current gully density was determined by visual interpretation of high and ultra-high resolution satellite images for 2010–2017. A geospatial database was created. For this purpose the system of interpretation features of gully forms was developed. Gully maps were developed based on their talweg type using slope, bank, and bottom characteristics as criteria. Two indicators were used to quantify gully erosion: total length per unit area and density of the gully network, where the basin approach was used as an operational territorial unit. Created geo database compiles gullies characteristics for 1674 basins. A total of 9142 gullies were identified in the study area with an average length of 74 meters. The density of the gully network is currently distributed irregularly over the area and averages 12 m/km2 , reaching a maximum of 405 m/km² . The change in the number of gullies spatially coincides with the distribution of the density of the gully network, being on average 0.2 units/km² , the maximum being 5 units/km² . Among morpho-genetic types, slope gullies dominate (90%), with bank and bottom gullies representing 7% and 3%, respectively. The temporal dynamics of the areal shape and linear growth of active gullies mainly of the slope type was determined by combining each gully shape on two multi-temporal satellite images obtained over a relatively short period (2009–2016). The dynamics were determined for 304 gullies. The average linear growth of gullies is 0.6 m/year, and the average areal growth is 28 m2 /year. The spatial and temporal dynamics of the gully density in the river basins was determined by comparing the data of mapping of the modern gully network with the results obtained by mapping gullies on aerial photographs of 1960–1970s. Overall, a significant decrease in gully density, indicating the slowing down of gully formation processes, was established. The average density of the gully network in all the basins decreased by 230 m/km²  in the study area. Against the background of the general reduction, only in some basins there was a slight increase in gully density. Minimum values of gully density now correlate with the basins with high indices of grassing of the territory. Changes in hydro-climatic conditions (increased winter temperatures, reduced depth of soil freezing and surface snowmelt runoff), reduction in plowed area, gully evolution (transformation from gully to balka stage), planting of protective forest belts has determined the decreasing trend of gully development in the study area.

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