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Robert Elmer Horton (May 18, 1875 - April 22, 1945) was an American ecologist and soil scientist, considered by many to be the father of modern hydrology.

Born in Parma, Michigan, he earned his B.S. from Albion College in 1897. After his graduation, he went to work for his uncle, George Rafter, a prominent civil engineer. Rafter had commissioned a weir study, the results of which Horton analyzed and summarized. In 1900, he was appointed New York District Engineer of the United States Geological Survey.

During his studies of New York streams, Horton determined that the degree to which rainfall could reach the aquifer depended on a certain property of the soil, which he called infiltration capacity. He analyzed and separated the water cycle into the processes of infiltration, evaporation, interception, transpiration, overland flow, etc. Horton was the first to demarcate and label these now-familiar stages of the cycle.

Horton is well-known for his study of maximum runoff and flood generation. His concept of maximum possible rainfall, limiting the effect of rainfall in specific regions, has had a major effect on meteorology. His studies of overland flow aided in the understanding of soil erosion and provided a scientific basis for soil conservation efforts.

Having realized early in his career that the physical character of terrain played a large role in determining runoff patterns, he resolved to isolate the physical factors affecting runoff and flood discharge. He believed these to include drainage density, channel slope, overland flow length, and other less important factors. However, late in his career, he began to advocate a very different mechanism of "hydrophysical" geomorphology, which he believed better explained his prior observations.

Horton detailed his theory in a landmark paper published in 1945, only a month before his death, in the Bulletin of the Geological Society of America. He summarized his conclusions with four laws: the law of stream numbers, the law of stream lengths, the limits of infiltration capacity, and the runoff-detention-storage relation. His results demonstrate that the salient factor in aqueous soil erosion is the minimum length of overland flow necessary to produce enough runoff to effect erosion. This seminal work may be considered the founding of modern stream chemistry modeling, since it was the first comprehensive set of mathematical models to link basin hydrology with a water pollutant, namely sediment.

Horton is the namesake of the Robert E. Horton Medal, awarded by the American Geophysical Union to recognize outstanding contributions to the field of hydrological geophysics.

• Surface runoff

• Biography
• Robert E. Horton's perceptual model of infiltration processes