Much of Earth's surface is covered by a weathered cover of regolith. Whether forming a true soil with well-developed horizons or a weakly developed detrital cover, the regolith is composed of a mix of mineral particles, organic fragments, and pore space. Commonly, much of the pore space is filled with air and to a lesser extent water. When large amounts of precipitation are received in a region, the pore space fills with water as the input of water from precipitation exceeds the output of water from the soil column to the water table. Decreases in infiltration lead to increases in runoff. The lag time between the precipitation event and the arrival of water to stream channels decreases significantly when soil saturation occurs. As a result, peak discharge increases significantly and the likelihood of overbank flow is high (Smith and Ward, 1998). Spatially, soil saturation may occur over large-scale basins, which leads to flooding in large areas. The peak discharge flows downstream and becomes concentrated in higher order streams causing flooding. In many cases saturation follows a period of high amounts of precipitation over a prolonged time period, possibly weeks or months (Wolman and Gerson, 1978; Ward and Robinson, 1990).
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