ИСТИНА

Gully erosion is an important soil degradation process in the Southern half of the Russian Plain, because of high proportion of cultivated lands (>30% from total area). Vaytsko-Kamskoe interfluve area is located within in the East-Northern part of the Volga river basin and it is area of the Udmurt Republic, Russia (56°N; 52°E). Two approaches were used for evaluation of gully head retreat rates. High resolution aerial photographs (flights 1959, 1970 and 1980) were used for evaluation mean annual gully head retreat for periods 1959-1970 and 1970-1980. Field monitoring of 168 gully heads located in 28 locations within the different parts of the study area was organized since 1978 and it is continued until now. Number of gully heads under the monitoring were changing because of stabilization of some gullies and including in monitoring new active gullies. The different types of gullies including slope, bank and bottom gullies with mostly cultivated catchments were included in monitoring system. Measurements of gully head retreat are made once per year for the most part of sites. But retreat of 34-40 gully heads located in the Eastern part of the study area with high proportion of the arable lands are measured twice per year (after snow-melting in May and after rain-storm season in October-November). It is necessary to underline that period of monitoring of gully head retreat was coincided with climate warming, which began since the second half of 1970th. According the information collected from the aerial photographs the mean linear headcut retreat decrease from 2,4 m yr-1 in 1959-1970 to 1,9 m yr-1 in 1970-1980 for the entire study area. However such changes can be explained by inter-annual fluctuations and partly by stabilization parts of gullies because of critical reduction of their catchment area. Similar situation is observed until the middle of 1990th (Fig.1A), but in this time in addition some gullies stabilized because of abandoned of arable lands during 1991-1996 and partly because of part of gully have been levelled. Based on the results of first stage of monitoring (1978-1996) it is possible to conclude that 81% of gully linear headcut retreat occurred during period of snow-melting in March- April with the remainder during the warm season. The second period of monitoring (1997-2015) of the gully head retreat is characterized by the sharp decline of the mean linear headcut retreat rate (0,3 m yr-1) (Fig.1A). Such tendency is observed for the all gully types. But clear trend of increasing gully head retreat since minimum values in 2006 is observed for the bottom gullies (Fig.1B). The main reason of the serious reduction of the gulling since 1996 is increasing of winter air temperatures as a consequence of global warming. In the result serious reduction of surface runoff from the slope occurred during snow-melting in the years with frozen soil depth below 40-50 cm. It is confirmed by reduction of snow-melting input in annual gully head retreat to 53% with relatively high variation from year to year.