Comparative analysis of conditions for snowmelt runoff formation in representative lowland (Kasmala) and low-mountain (Maima) river basins of the Upper Ob

The paper deals with main factors of flood-induced runoff formation in two river basins-indicators of the Upper Ob basin, i.e. the low-mountain (Maima) and the lowland (Kasmala) ones. The basins are characterized by similar zonal conditions of heat supply being at the same time different in geological-geomorphological structure and evolution of landscapes that brings to differences in parameters of the cold period and main characteristics of flood. The results of the analysis of characteristics for the cold and flooding periods as well as calculations of principal indicators of floods for a long-term period are given. The average annual discharge (1998-2017) for the Maima ranges from 4.32-11.90 m³/s (average 8.41 m³/s), while for the Kasmala (1998-2017) - 0.43-3.04 m³/s (1.78 m³/s). The duration of the cold period at rivers Maima and Kasmala is 109-155 and 113-162 days, respectively. The study rivers are contrasting by major indicators of high water. Generally, the volume of snowmelt runoff on the Maima is 1.5 and the average discharge is three times greater than on the Kasmala. However, maximum discharges are higher at the Kasmala river that indicates the difference in snowmelt conditions in these river basins. Flood on the Maima usually begins in the third decade of March (in 60 % of cases during the observation period), less often - in the second decade of March (25 %) and the first decade of April (15 %), whereas on the Kasmala - in the third decade of March (60 %), less often - in the second decade of March (15 %) and in the first (15 %) - the second (10 %) decades of April. Maximum discharges (20.60-110.00 m³/s) on the Maima river usually occur on day 24 and on the Kasmala (3.50-150.00 m³/s) - on day 18, after the flood onset. For these basins, the difference in dependences of snowmelt runoff volume on physical and geographical features are established. In contrast to the low-mountain river Maima, the lowland Kasmala river with its larger length and catchment area is distinguished by a smaller volume and runoff depth during a high water period. This is due to higher annual and winter moisture content of low mountains and larger stream gradients responsible for a shorter water lag time. Along with the specific geological structure, larger gradients provide greater runoff from the Maima basin and shorter flood duration as compared to the Kasmala one. The study rivers show mutually contradictory trends characterizing the relationship between the runoff coefficient during high water and the annual water content coefficient, and the relationship between the total precipitation for the cold period and the runoff depth during high water. The annual water content of the Kasmala with a predominantly snow-alimentation type is more dependent on floodwater than that of the Maima River with its mixed type of alimentation. For the latter, runoff volume changes in proportion to total precipitation during the cold period that is due to larger gradients of the catchment area and less Quaternary deposits cover as compared to the Kasmala River. With a flood period onset, maximum discharges from the Kasmala River are formed earlier than on the Maima. This is explained by a considerable arrival of total solar radiation to the underlying surface, less forest cover and frozen soils in the Kasmala basin.

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