ИСТИНА

Mapping gullies activity is crucial for studying its impacts including land degradation, sediment production and influence on the surface water pollution. Aerial photos and satellite imagery is widely used for remote sensing-based gully mapping over large areas. Detailed spatial-temporal assessment of active gully spatial distribution was undertaken for the Srednee Povolzie region (central part of the Volga River basin, including 5 large administrative regions) for few time intervals, beginning from end of 1950th. High quality aerial photos (linear scale 1:17 000) were used for identification of active gully length for individual small river watersheds (the mean area 37,5 km2) in 1959 (the entire area), 1970 and 1991 (N-E of study area). High resolution space images (GeoEye-1) are using for identification of active gully length and gully head density for some typical river watersheds in 2009-2014. The total area of mapping is 171,7 x103 km2 with total number of watersheds is 4575 (including inter-basin space). The gully density values were classified into seven categories (Fig.1A).The most part of active gullies is concentrated along the dry valleys and river valleys and they had catchments with high proportion of arable lands (Fug.1B). Watersheds with high and extremely high gully density are located within uplands and in areas with very high relative heights. The more detail assessment of gully density dynamic was undertaken for North-Eastern part of study area, where comparison of gully density was done for two times intervals: 1959-1970 and 1970-1991. Weak decrease trend (2-3%) of gully density was found for both time intervals, but both increasing and declining trends was found for different river watersheds during period 1970-1991. New active gullies had appeared mostly in areas with relatively short duration of ploughing. The significant reduction of gully density is observed for the most parts of watersheds of the Srednee Povolzie since 1991 because of several reasons, including climate changes, leading to a decrease of a surface runoff from slope during snow-melting, abandoned relatively steep cultivated slopes and reduction of a catchment area of the gullies because their growth. The comprehensive analysis of temporal-spatial changes of gully density and reasons of changes for the Srednee Povolzie region is the main content of the presentation.