The Timing of the Onset of Lows of the Summer-Autumn Low Water in the Basin of Upper Don

Considered the timing of the onset (with rounding up to a month) lows of the summer-autumn low water period in the basin of the Upper Don for the periods: from the beginning of the observations in the posts, before 1970, 1971-2000, 2001-2017. Three zones have been identified that differ in generation time of the absolute lows of water flow: the Don itself and the right-bank tributaries of the Don; the left-bank tributaries of the Don; a basin Hoper. It is established that in the current century the absolute minimums of water flow are formed in the selected zones in July – August, August-September, mainly in September, respectively, which is about a month earlier than the dates observed before 1970. The main shifts in the onset dates of the investigated characteristic occurred in 1971-2000, fixed in the current century. In the Hoper basin, there is greater consistency in the onset of absolute lows of water flow than in the river system of the Upper Don. Despite the increase in the duration of high water and summer-autumn low water, as well as the tendency of increasing water content in the period of low runoff, there is a movement of formation dates of low extrema of water content to the beginning of summer-autumn low water. The identified nature of the formation of absolute minimum is apparently the result of intra-annual redistribution of the flow and a response to the positive dynamics of air temperature, which directly or indirectly infruences the hydrological processes in the river catchment area. Earlier summer depletion of rivers can contribute to increased risks of water use during the summer-autumn low-water period, adversely affect the agricultural sectors focused on the use of water for agricultural water supply, significantly change the quality of water in water bodies and create environmental tensions in the basin.