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Infiltration, Throughflow, and Overland Flow
Published in Richard J. Chorley, Introduction to Physical Hydrology, 2019
‘Horton overland flow’ is defined as overland flow which occurs when rainfall intensity is so great that not all the water can infiltrate, and is described by Horton [1945]. This type of overland flow is a fairly common phenomenon in semi-arid climatic conditions, but is relatively rare in humid and humid-temperate conditions. The role of vegetation is thought to be a critical cause of this distinction. Vegetation increases the infiltration rate by promoting a thicker soil cover, a better soil texture, and by breaking the impact of raindrops on the surface. Its effect on soil structure is mainly to build up an organic-rich A horizon with a relatively open pore structure and high permeability. If raindrops strike the surface without being impeded by vegetation fine material is thrown into suspension by the impact and is redeposited as an almost impermeable surface skin which can lower infiltration by as much as ten times. Vegetation therefore has a controlling influence on Horton overland flow by increasing both the initial depression storage and the infiltration rate, so that where a dense vegetation cover is established Horton overland flow is very unusual. Soil compaction by animals and vehicles reduces the infiltration rate while increasing depression storage, so that its net influence is problematic.
The role of Canadian research in advancing groundwater hydrology: historical sketches from the past 75 years
Published in Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 2023
Masaki Hayashi, Garth van der Kamp
Freeze’s hillslope flow model (1972a, 1972b), described above, made an important contribution to the major paradigm shift in hillslope hydrology that took place roughly between 1965 and 1980. The prevailing paradigm of hillslope runoff generation until that time was based on the idea that surface runoff occurred when the soil infiltrability was exceeded by rainfall intensity – the process referred to as Horton overland flow. The validity of this idea was questioned by American forest hydrologists making field observations, who noted that the soil covering forested slopes had much higher soil infiltrability than the rainfall intensity of most storm events (Whipkey 1965; Hewlett and Hibbert 1967). These authors identified alternative runoff generation mechanisms, consisting of shallow subsurface flow through saturated highly permeable soil that rapidly delivered water to streams (i.e. subsurface storm flow), and surface flow over the saturated areas formed by the rising water table reaching the ground surface along stream channels, and along ephemeral rivulets in draws and low-lying areas on a hillside (i.e. saturation overland flow). The saturated areas expanded during storms as the water table kept rising (i.e. variable source area concept). These mechanisms were convincingly demonstrated by a PhD student at Johns Hopkins University named Thomas Dunne, who published the results in journal papers while he was working at McGill University (Dunne and Black 1970a, 1970b). Freeze’s (1972a, 1972b) numerical experiments were motivated by the recent development in field-based hillslope hydrology.