How Much Does Snow Accumulated Over the Winter Affect Spring Floods, and Does Its Contribution Change?

ABSTRACT There is significant evidence that seasonal snow mass, snow cover extent, and snow season duration are consistently decreasing in many lowland rivers in the Northern Hemisphere, often accompanied by a decrease in the magnitude of snowmelt floods (freshets). This paper explores to what extent snow accumulated in a river basin during winter affects subsequent freshet and if the ongoing decrease in runoff is due to a decrease in snow. We analysed maximum annual snow water equivalent (pre‐melt SWE) and freshet runoff data for 20 river basins located in different physiographic conditions within the East European Plain in 1951–2019. For this analysis, we developed a new method for assessing the pre‐melt SWE contribution to freshet runoff as a product of two physically meaningful indicators named ‘variability ratio index’ and ‘elasticity index’. First, we demonstrated that downward trends in both pre‐melt SWE and freshet runoff occurred and were more noticeable in the more southern, less forested, and mid‐area basins. Then, using the developed method, we assessed to what extent the pre‐melt SWE contributes to the detected runoff changes and how this contribution evolves in time. The main conclusions of our study are as follows. It was found that for the entire period, the pre‐melt SWE variation was not the dominant freshet variance contributor (25% on average). Also, we revealed that in a half of the river basins, SWE contribution to runoff has decreased in the recent decades compared with the baseline period (1951–1990), and this decline is mostly associated with a decrease in the elasticity index. Overall, we found that the SWE variability was not the dominant factor in snowmelt runoff changes in the vast territory under consideration over a 69‐year period. The result indicates the ambiguity of the relationship between the warming‐induced reduction in SWE and decrease in snowmelt runoff.