On the Ratio of Climatic and Anthropogenic Factors in the Change of the Volga River Runoff

Two variants (methods) were used to calculate the contribution of anthropogenic and climatic factors to the change in the annual and seasonal runoff of the Volga River at Volgograd during periods of significant anthropogenic impact on waters (1930–2020 and 2011–2020) compared with the conditionally natural period of runoff formation (1879–1929). The first is based on comparing the anthropogenic change in runoff due to the influence of reservoirs and irretrievable water consumption with the total change in runoff for the comparison periods. In the second variant, the reconstructed conditionally natural runoff during periods of significant anthropogenic impact is compared with the total change in runoff, calculated by the relationship between the Volga River runoff and the runoff of rivers that are indicators of climatic conditions. It is assumed that climatic changes are mainly of natural origin. It is revealed that the influence of anthropogenic and climatic factors on the overall decrease in the annual runoff of the Volga River in comparison with the conditionally natural period is unidirectional and comparable. Both variants of the calculation give similar values for the ratio of anthropogenic and climatic factors in the total change in runoff in the 1930–2020 period. There are more significant differences in the calculations for the 2011–2020 period and in the assessment of seasonal changes, most noticeably manifested in the spring flood and in the winter. Anthropogenic and climatic factors act in the same direction, reducing the flow of the spring flood and increasing it in the winter, mainly due to anthropogenic factors. The existing differences in the ratio of factors can largely be explained by landscape transformations in the Volga River basin, including in river basins that are indicators of climatic conditions. The impact of such transformations on runoff is not taken into account in water management statistics, although in recent decades different types of economic activities in watersheds have largely offset each other in their impact on runoff. The share of anthropogenic factors in the Volga River runoff changes can vary widely depending on the type of water resources use which is be not always rational.