Drainage density

Drainage density is defined as the cumulative length of all stream channels in a basin divided by the contributing area ( D = ZLu / Au ). It is a useful measure of

topographie texture or linear scales of landforms in fluvially eroded landscapes. Since it was first invented by Horton (1945), it has been extensively utilised in many hydrological studies, and is still considered by many as the best available index to describe a particular stream network. Drainage density is essentially a measure of dissection that reflects the competing effectiveness of overland flow and infiltration (Patton, 1988). Horton also recognised that drainage density is an approximate measure o f the length of overland flow, as one-half its inverse is the distance between a stream channel and the top of the adjacent divide.

Drainage density has always been regarded as the parameter most appropriate to express the character o f the drainage network. As a parameter in catchment studies, drainage density can be used in three main ways (Gregory and Walling, 1968). First, it is related to watershed or physiographic characteristics such as relief ratio, rock type and basin shape. Secondly, it is related to input and output o f the drainage basin system. Thirdly, drainage density may be useful in relation to past conditions and prediction of future responses to climatic forcing.

Drainage density was one of the first morphometric properties used for providing a measure o f topographic texture in fluvially eroded landscapes. The major control of drainage density D, at the macroscale, is climate. It is in general believed that D is low in arid areas due to lack of runoff; increases as precipitation increases to reach a maximum in semiarid areas, decreases as vegetation exceedingly impedes runoff to a minimum in humid areas; and again increases to a possible second maximum in superhumid areas (e.g. Gregory and Gardiner, 1975). If the relationships between present drainage density and its controlling variables are understood, then deductions may be made about the development of this particular basin characteristic in the future. At the continent-wide scale, the dominant control o f D is climate. Abrahams and Ponczynski (1985) showed that drainage density varies inversely with precipitation in all hut desert climates. Moglen et al. (1998) also concluded that maximum drainage density occurs at an intermediate annual precipitation depth with lower drainage densities produced by changes to either drier or wetter climates. A number of studies have already identified ways in which the drainage density varies

in a single catchment (e.g. Gregory and Walling, 1968; Morgan, 1972; Blyth and Rodda, 1973). It is also essential to know how that particular representation o f the network which is a static distribution on a map of a specific date relates to the network o f streams which can potentially function in a particular basin (Ovenden and Gregory, 1980). The network shown on a particular topographic map will provide a single value for the range of densities between the extremes of short term drainage net expansion and contraction.

Gregory and Gardiner (1975) underlined the fact that, within a single drainage basin, drainage density may be interpreted in several ways including the basic network composed o f perennial streams; the maximum network embracing ephemeral and intermittent streams, and the rate of expansion influenced by the precipitation intensity characteristics of individual storms. Day (1978), in an analysis o f flow length graphs suggested that stream behaviour during rainfall events is more complex than previously realised, with individual channels expanding or contracting, thus indicating a dissimilar response o f a drainage network to equal rainfall amounts. In the light o f all o f the above, drainage density measurements should only be taken as indicative and certainly not conclusive o f the recent evolution of this particular catchment.

Using a polar planimeter, Whitendale River catchment’s area was found to be 12.51 km^. The drainage density of the network was calculated (Tables 3.11-3.13) from all three OS map editions, considering only the watercourses as these are shown on the topographic maps, for consistency. It should be noted that the values for that specific catchment are considered in general low, in agreement with the fact that the Whitendale river occupies an area that receives an annual rainfall in excess o f 1500 mm and in most cases over 2000 mm. There is very little variation observed in drainage density values between the 1951 and the 1978 editions o f OS maps, and a small increase from the first edition. However, one could simply claim that the increase quite simply reflects the fact that recent maps depict a greater number of first-order tributaries than older maps. It becomes apparent that drainage density

alone can not provide enough information for the possible diachronic evolution dynamics of the channel network.

The calculations for drainage density were repeated for the degraded network (Tables 3.14-3.16). The results of the degraded network are almost identical to the previous findings. There is a considerable increase in drainage density between the 1850 and 1951 editions and a smaller one between the 1951 and 1978 editions. In general, results demonstrate that first-order streams do not greatly influence the outcome o f drainage density calculations in this particular catchment.