Drainage Network Density

the ratio of the sum of the lengths of all rivers (including intermittent streams) in a basin or any given area (expressed in linear kilometers) to the area of the basin (expressed in square kilometers—that is, D = ∑ L/F. where D is the drainage density; ∑L, the length of all the rivers; and F, the basin area.

The drainage network density is an index of the development of surface runoff in the territory under consideration. Its value depends on the geological structure and relief of the locality, the climate, and the plant and soil cover. The drainage density is greater in mountainous regions than on plains, both because the amount of precipitation is greater in the mountains and because permeability decreases there. In forested regions the drainage density is found to be somewhat less than in treeless areas, because of more favorable water filtration conditions. In the USSR drainage densities vary widely—in the European part the density ranges from 0.15 to 0.25 km/km², in Southern Siberia it is over 0.40: on the northwest slopes of the Glavnyi Caucasus Range, to 1.49: and in Ciscaucasia, 0.05.

Determination of the value of the drainage density depends largely on the scale of the initial topographical maps, the method of measuring the length of the rivers, and the validity of the reduction coefficient. Spatial differences in drainage density can be represented on a map by isograms or, more often, on a cartogram.