Drainage Network Volumes and Precipitation in Britain

Abstract

Drainage networks are analysed in eleven basins representing a range of climatic locations in England. Field survey of 328 channel cross-sections allows the volume of the channel network to be established by relating channel capacity to total stream length. The relation between volume of the channel network and mean annual flood discharge for eight basins is better than that between drainage area and mean annual flood. Channel network volume is also significantly related to precipitation amount and to a precipitation intensity index. Further refinement of the measure of stream network volume has to take place by analysing the volume of different stages of network expansion. The maximum network volume for the East Dart basin is I 29 times the volume of the mapped network. THE drainage network is the most sensitive feature of the drainage basin which links effective precipitation input to output of discharge and associated sediment. The sensitivity of the network derives from two things: the density of the watercourses should reflect the availability of moisture and therefore of precipitation amount and intensity, and the density and arrangement of the drainage network components are directly related to hydrograph characteristics, especially the rising limb and the peak value of the hydrograph. The parameter used most frequently to express the character of the drainage network has been drainage density. It has been argued (Cotton, 1964) that drainage density should reflect precipitation character and Melton (1957) derived an inverse relation between drainage density and Thornthwaite's P/E index. More recently, Madduma Bandara (1974) has demonstrated that this relation becomes positive above a critical level of effective precipitation. The drainage network cannot be regarded as a static drainage basin characteristic and a number of studies have identified the way in which the drainage density varies in a single catchment (Gregory and Walling, 1968; Morgan, 1971; Blyth and Rodda, 1973). Whereas Stoddart (1968) had earlier identified two alternative hypotheses for the explanation of drainage density, it is now possible (Gregory, 1976a), in the light of our knowledge of network dynamics, to reconcile these two hypotheses. Thus drainage density will increase with precipitation as a greater amount of rainfall per unit area will require a greater density of channel per unit area and this applies to the basic, unexpanded perennial network. The alternative hypothesis, that drainage density decreases with increasing rainfall because vegetation density, transpiration, moisture retention and other factors increase, applies to the maximum expanded drainage network.