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Robert Horton demonstrated in his seminal 1945 paper that physically based quantitative models for landscape evolution can be constructed by using predicted overland flow in a sediment transport equation for sheetwash. He envisioned drainage network evolution by infiltration-limited overland flow as a process of channel incision, network growth, and then abstraction to a stable channel network fed by hillslopes too short for channel initiation. Not until the work of Tom Dunne in the late 1960s in the Sleepers River watershed, Vermont, was it realized that overland flow, and consequently hillslope evolution, could occur by an entirely different mechanism than that proposed by Horton. Dunne showed that in certain predictable zones of the landscape, exfiltration from saturated grounds adds to precipitation on the soil surface to form what he later called saturation overland flow. Many researchers have since found that this form of overland flow occurs in humid and semiarid landscapes throughout the world. So clear is Dunne's contribution to defining this process that some refer to it as the “Dunne mechanism” to distinguish it from “Horton overland flow.” His work also documented unquestionably the applicability of the partial area concept in explaining runoff generation. Because of this work, his research in snowmelt runoff, and his subsequent authorship with Luna Leopold of the widely used book entitled Water in Environmental Planning, Dunne has established himself as a leader of process hydrology.