Robert E. Horton (1875–1945)

Robert Elmer Horton was born May 18, 1875, in Parma, Michigan, and received a B.S degree from nearby Albion College in 1897. His professional work then began under the direction of his uncle, George Rafter, a prominent civil engineer who had earlier worked on the Erie Canal and was at the time concerned with its proposed deepening into a seaway. For better streamflow measurements, Rafter had commissioned Cornell laboratory weir studies for which Horton analyzed and summarized the results. This initial work was then considerably extended upon Horton’s becoming New York District Engineer of the U.S. Geological Survey in 1900. The resulting publications became the standard work on the subject.

Subsequent extensive stream gaging then directly led to his earliest work on the low flow or base flow of New York streams, recognizing that groundwater was the major component of this runoff, but how much incident rainfall could reach the aquifer depended on what Horton termed the infiltration capacity of the soil. Thus began his development and utilization of techniques for systematic separation into the several now familiar components of infiltration, evaporation, interception, transpiration, overland flow, etc., all of which owe their refinement and quantification primarily to Horton.

Throughout his career, Horton was also concerned with maximum runoff and flood generation. Indeed, he is best known to meteorologists as an early advocate of a “maximum possible rainfall,” or limiting storm specific to each region. This results in a maximum flood, as best expressed in his own words: “There is for each drainage basin a certain finite rate of flood discharge which Nature is incapable of transcending…” He went on to show that storms approaching this envelope had already been experienced. Horton’s studies of infiltration and overland sheet flow provided a basis for analyzing soil erosion and for devising strategies for soil conservation. This work led in turn to a long series of investigations concerned with the process of drainage basin development. He had realized very early in his work that physical characteristics were important for determining runoff. He had listed those factors important to runoff and flood discharge, including drainage density, channel slope, overland flow length, and other factors. However, he now proposed the converse idea of “hydrophysical” geomorphologic-erosional processes responsible for these same observed stream patterns and drainage properties.

After two decades of refinement, this masterful idea appeared in final form in a 95-page landmark paper just 1 month before his death on April 22, 1945. Horton summarized his results in four laws: the law of stream numbers, the law of stream lengths, limiting infiltration capacity, and the runoff-detention-storage relation. He showed that the most important factor for aqueous erosion was the minimum length of overland flow required to produce sufficient runoff to initiate erosion. While Horton had always considered himself to be a hydraulic engineer, it is interesting to note that this important paper was in fact published in the Bulletin of the Geological Society of America. However, in the words of a contemporary reviewer: “This important and valuable contribution by Horton could have come only from a man of his varied and extensive experience and with his great breadth of vision.”

Upon reviewing Horton’s accomplishments, one is struck by the gradual evolution of his ideas. Rarely does a novel idea emerge full-blown; instead, most had precursors in discussions of his own earlier papers or the work of his colleagues. Because Horton was very active in several professional societies, many of his important contributions were made in just this way. Furthermore, the ultimate emergence of his seminal ideas was the result of two innate abilities: his continual thinking across disciplinary lines and his continual interplay between engineering practice and scientific curiosity.

In recognition of his outstanding effectiveness as an engineer and scientist and to serve as an inspiration to workers in the field, the American Geophysical Union continues to honor Horton through its Robert E. Horton Medal, which recognizes outstanding contributions to the geophysical aspects of hydrology. The first recipient was Walter Langbein in 1976. The Hydrology Section of AGU has attached Horton’s name to its service award and to a generous research grant program for Ph.D. candidates in hydrology or water resources. The American Meteorological Society also honors him by its Horton Lectureship, first given by Luna Leopold in 1974.

Henry M. Paynter

—National Academy of Engineering Pittsford, Vermont