Relationship between precipitation, river runoff and evaporation over the European Russia and its variability

The changes in annual precipitation, evaporation, river runoff, and potential evapotranspiration for 1946– 1979 and 1980–2021 were considered for 225 catchments located in European Russia (ER) north of 48°N and without large reservoirs within their boundaries,. The ER is generally characterized by higher values of all water balance components in 1980–2021 compared to 1946–1979. A slight increase in river runoff was found for the northern part of the ER, within the Arctic rivers basins and the basins of theUpper and Middle Volga River and the Baltic Sea. A decrease prevailed in the southern part, within the basins of Don, Ural and the Lower Volga rivers. The most notable change was an increase in runoff in the central part of the region. At the same time, its increase for most catchments exceeded the increase in precipitation – the median coefficient of runoff for 225 basins increased from 0,38 to 0,40. The modified Budyko’s equation made it possible to calculate the dependence of the runoff coefficient on the degree of climate aridity for each basin in terms of ω, which shows the part of precipitation spent on evaporation under a given value of the aridity index. The dependence did not change for the northern ER. The transformation of precipitation into runoff has become more effective in the central part, while in the southern part it became less effective. The spatial distribution of parameter ω showed close relationships (determination coefficient more than 0,76) between the degree of synchrony of the maximum potential evapotranspiration and precipitation events, the proportion of forest cover and bare soil in the catchment, air temperatures in January-February, and the fraction of solid precipitation relative to the annual total. However, applying the derived dependence to estimate changes in ω has led to unsatisfactory results, suggesting that the spatial parameters which determine ω values do not control its temporal variability. Based on the estimated ω value for the period of 1980–2021 and climatic scenarios of precipitation and evaporation changes the runoff change for the 21st century was calculated. It is expected that most of the rivers in the ER will increase their runoff in the 21st century regardless of climate change scenarios. On average, 76% of catchments will experience the same direction of river flow changes as in 1946–2021. The forecasted degree of changes will be close to observed during that period.

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