ON CHANGE OF REDUCTION FACTOR OF MAXIMUM RUNOFF MODULUS WITH INCREASE OF CATCHMENT AREA
When forming river runoff, as a rule, its maximum modulus decreases (reduction) as the catchment area increases. For catchments with an area of less than 5 km2, reduction of maximum drainage and runoff modules is not expressed, which confirms the accepted assumption of equal drainage and runoff for small catchment areas. This is the first stage of the process of forming flood water runoff on channel-free slopes. At the second stage, catchments with an area of 5 to 10 km2 tend to decrease the maximum runoff modulus with an increase in catchment area. The reason for the decrease in drainage modules is a decrease in the effective catchment area as a result of a change in the snow cover area; for water flow modules - reduction of the effective catchment area and beginning of channel regulation. At the third stage, at catchments with an area of 10 to 100 km2, a decrease in the maximum water flow modulus depending on the catchment area has a stable tendency, determined by channel regulation, a decrease in the existing catchment area, and features of the increase in the catchment area along the length of the main watercourse. At the fourth stage, at catchments with an area of more than 100 km2, the reduction of the maximum module of water flow has a stable appearance and is determined by channel regulation and features of the increase in the catchment area along the length of the main watercourse. For small watercourses, the dependence of the shape coefficient of the catchment (Kf) on its area reflects the process of development of the catchment from slopes in which the width of the catchment is significantly longer, to watercourses where the width of the catchment becomes less than the length of the watercourse. In general, Kf is not sufficiently informative and does not reflect the features of the increase in catchment area along the length of the channel of the main watercourse. The main reason for using Kf in analysis and calculations is the lack of cadastral data on the increase in catchment area along the length of the main watercourse bed for all hydrologically studied rivers
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