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item SMITH, R.
item Goodrich, David - Dave

Submitted to: Encyclopedia of Hydrological Sciences
Publication Type: Book / Chapter
Publication Acceptance Date: 8/1/2004
Publication Date: 7/1/2005
Citation: Smith, R.E., Goodrich, D.C. 2005. Rainfall excess overland flow. In: Anderson, M. G., editor. Encyclopedia of Hydrological Sciences. John Wiley & Sons, LTD. P. 1707-1718.

Interpretive Summary: Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. Detailed study of water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions. A primary method in which runoff is produced in these regions is termed 'infiltration excess' where the rainfall rate is greater than ability of the soil to accept the rain water. This chapter provides a review of the primary issues affecting runoff generation and methods to treat them mathematically and in models that can be used to make runoff and flood prediction. The review concludes that we have a number of modeling tools that are appropriate for small watersheds. However, the biggest challenge in using these models is estimating how basic watershed properties such as soils, how rapidly these soils absorb rainfall and land use and land cover vary over watersheds.

Technical Abstract: Runoff generated from storm rainfall is largely determined by local interaction of the properties of the rainfall, ground cover, land use, and soil. Here we discuss the physical dynamics of those cases where rainfall generates surface runoff at the point where it falls. Infiltration excess occurs when the rainfall comes at a rate higher than the rate at which the otherwise unsaturated soil can absorb water. The runoff or surface water flow resulting from infiltration limitation is called infiltration excess runoff, and the (variable) limiting soil intake rate is called the soil infiltrability. The predominance of relatively short high intensity storms in most subhumid and semi-arid zones means that these areas are more prone to infiltration excess runoff. Increasingly, human activity (e.g. urbanization, compaction, etc.) results in an overall decrease of soil infiltrability resulting in globally increasing areas of infiltration excess runoff generation. Infiltration excess runoff is one of the more dynamic or rapidly changing fluxes in the hydrologic cycle, and requires knowledge of the temporal pattern of rainfall rates to characterize it accurately.