Aim of the study The use of underground water for drinking water supply has increased over the recent years, due to intensive pollution of local and transboundary rivers in the Greater Caucasus region of Azerbaijan. In addition to relatively deep groundwaters, underground flow of rivers is also widely used for the purpose. The purpose of the present study is to analyze the condition of underground flow of rivers in the region, and to determine their territorial distribution patterns. Material and methods The possibilities of using the minimum monthly water discharge have been investigated to determine the annual rates of underground flow to rivers. The study was conducted in 4 options, and in each option the values obtained based on the minimum monthly discharge of underground flow have been compared to values obtained based on hydrographs separation drawn up for typical years. Results and conclusions The underground flow coefficient, which is a quantitative indication of the recharge of groundwaters by infiltration in the Greater Caucasus region, is affected by the physical and geographical factors. As the average altitude increases, the underground flow coefficient also increases. Thus, the average altitude of catchment area is an integral indicator of the physical and geographical factors affecting the recharge of groundwaters by infiltration. The difference between the average multi-year values of the Greater Caucasus rivers baseflow coefficient and the coefficient of recharge by groundwater is about 0.25. In the area under investigation, the coefficient of recharge by groundwater for the studied rivers is on average 63.85% in dry years, 55.2% in medium years, and 59.5% in wet years.

The article is devoted to the analysis of modern changes in annual runoff, its main components – underground and surface runoff, as well as the minimum winter and summer-autumn runoff rivers of the Greater Caucasus within Azerbaijan. A brief review of previous studies on flow changes in the country is given. It is noted that in these works the method of geographical comparison and linear trend analysis were used. It is concluded that the changes in the surface and under ground runoff of the rivers of the Greater Caucasus due to climate change have not yet been analysed. The data on the runoff of 17 hydrological observation points covering 1934–2017 were analysed. All these hydrological observation points are located in the mountainous part of the river basins, i. e. runoff indices characterise the natural or conditionally natural regime of rivers. The annual values of the underground flow were determined as the arithmetic average of the monthly average minimum winter and summer-autumn water discharges. The annual values of surface runoff are calculated as the difference between annual and underground runoff. The method of geographical comparison is used. Calculations and generalisations of the results obtained are performed for different periods, according to the recommendations of the World Meteorological Organisation. It was revealed that, for 1981–2010 and 2001–2017 surface runoff of the rivers of the studied region decreased compared to runoff for the base period (1961–1990), due to a decrease in the amount of snow precipitation and a decrease in the volume of spring flood. However, there was a more significant increase in the underground flow of rivers and, therefore, an increase in annual flow was observed throughout the region. The dynamics of changes in the minimum river flow, especially in the winter season, is also positive, since over the past decades the snow cover has been melting earlier than usual, and favorable conditions are being created for the formation of groundwater that feeds the rivers during periods of minimal runoff. It is noted that the revealed nature of changes in various indices of river flow in the studied region is associated with climate change.