Drainage density and effective precipitation

Abstract

Since the work of Horton in 1945, several geomorphologists have made attemps to explain the factors controlling the regional variations of drainage density. One of the most important contributions to this endeavour was made by Melton (1957) who demonstrated that drainage density has a significantly high negative correlation with Thornthwaite's P/E index. Melton linterpreted this as a result which shows the efficacy of the natural vegetative cover in controlling erosional processes. Recently the author, in a study of the morphometry of dissection in the central hills of Sri Lanka, found that the relationship between drainage density and effective precipitation in this area is significantly positive. This result when considered in combination with Melton's finding could be interpreted as an indication of the fact that above a certain critical level of effective precipitation (i.e. 80–90) the relationship between drainage density and P/E index turns positive. Such a conclusion is in agreement with the results of recent work by Langbein and Schumm (1958) and Hadley and Schumm (1961), who demonstrated that sediment yield reaches a maximum under grassland conditions and possibly reaches another peak where the impeding effect of vegetation cannot be increased by further increase of precipitation.