Infiltration Excess Overland Flow
Infiltration Excess Overland Flow, also known as Hortonian Flow, occurs when water enters a soil system faster then the soil can absorb or move it, such as when precipitation exceeds the infiltration capacity of the soil. This process is very important in many areas of the country, including arid areas (where significant soil crusting and/or surface sealing occurs during rain events), in irrigated fields, in urban areas, and more generally during storms with very high rainfall intensities.
Hortonian flow was the first mechanism of runoff to be discovered, and was thought to be the main mechanism of all runoff for quite some time. However, Saturation Excess Overland Flow has since been identified as the main runoff mechanism for most of the Northeastern United States.
History: Early Studies in Runoff Generation
For many years, it was believed that the occurrence of surface runoff was primarily controlled by the infiltration characteristics of the ground; whenever rainfall or irrigation water is applied at a higher rate than the soil's infiltration capacity, runoff is generated. Robert E. Horton (1933, 1940) was responsible for some of the early foundational work behind this concept and hence the process is ubiquitously referred to as Hortonian runoff.
Aspects Not Addressed
However, the Hortonian runoff concept does not meaningfully explain storm runoff in many of the humid regions of the US where the infiltration capacity of the ground is typically much greater than average rainfall intensities. Steenhuis and Muck (1988) found that soils of the Northeast, especially the shallow hillside soils maintained in grass and pasture, have infiltration rates that are rarely exceeded by the rainfall rate. This has been confirmed in other studies (Merwin et al., 1994; Dunne and Black; 1970). For example, over 90% of the soils in Delaware County, NY (in the Catskill Mountains) have permeabilities above 3 cm/hr. However, rainfall intensities greater than 3 cm/hr are rare, occurring on average less than once in every two years for summer months; for all the winter months the return periods for these intensities are greater than 50 years (Walter et al. 2002). In contrast, storm responses in streams reflect runoff processes occurring upstream almost every time it rains, which means that there must be some mechanism other than Hortonian Flow generating runoff.