A Taxonomy of Small Watershed Rainfall-Runoff

Abstract

A study of over 11,000 event rainfall and associated direct runoff events from 100 small watersheds was done, in a search for distinct patterns of runoff response and/or association with land type. The results show unexpected variety in the geometry and scale of the rainfall -runoff response. Groupings of similar response type and magnitude were made, and the associations with vegetative cover were tested. Five separate response groups were identified as follows: 1) Inactive, characterized by no recorded responses to any rainstorm in an extended period of record; 2) Complacent, characterized by a very small part of the rainfall (ca 0.1 to 3 percent) being converted to direct runoff, often as a linear response; 3) Standard behavior, the expected "textbook" response common to agricultural lands and humid sites, and in which the runoff slope increases with increasing rainfall, and the scale of runoff far exceeds the complacent response; 4) Violent behavior, in which an abstraction threshold of 2 -6 cm clearly precedes a sudden high response; and 5) Abrupt response in which a very high portion of the rainfall is converted to event runoff without appreciable abstraction, as typified by extensively urbanized drainages. The responses and the group identifications were parameterized by a simple broken -line linear rainfall-runoff equation, and a dichotomous key based on coefficient values is proposed. Only mild associations between response type or coefficient values and the four vegetative covers (Forest, Range, Agriculture, and Urban) were found. The variety of hydrologic behavior on forested watersheds encompassed that of the other three land types.