Basin Characteristics

Surface characteristics influence infiltration and runoff rates (Roberts, 1989; Kuhnle et al., 1996). Impervious surfaces such as exposed bedrock or a paved road accelerate surface runoff, thus decreasing lag time between the precipitation event and entrance of water into a nearby channel. Urbanized areas, therefore, with large percentages of impervious surface such as roofs, streets, and parking lots coupled with an engineered drainage system designed to move water quickly to stream channels greatly increase the chances that some flooding will occur after a significant precipitation event (Wolman, 1967; Hammer, 1972; Roberts, 1989; Newson, 1992). Conversely, rural areas with large areas of soil, natural vegetation, and the potential for a faster infiltration rate are less likely to have significant flooding resulting from a single precipitation event. Removal of as much as half the forest cover and a decrease of marsh land along the Yangtze River in China has led to increased flooding. Half a billion people, or 45% of China's total population, reside on the banks or floodplains of the Yangtze and the area produces about 42% of China's gross domestic product. In 1998, 79.6 million people in three Chinese provinces were affected by repeat flooding on the Yangtze. The floods killed more than 3000 people. Fourteen million people were evacuated and 21 million were made homeless (Weather.ou.edu/spark/AMON/v2_n3/News/DR_980819Chinal2.html).

Topography

Topography will influence the rate at which precipitation will be incorporated as stream discharge (Patton, 1988}. Steep, rocky canyon walls have low infiltration rates as well as a great deal of potential gravitational energy that leads to the concentration of discharge during a short period of time (Strahler. 1964). Alluvial plains usually have a much longer lag time between a precipitation event and the introduction of runoff water into a stream channel. When land cover on steeper slopes is affected by perturbations such as wild fire or building-related oversteepen-ing of slopes, the likelihood of mass movement events is greatly increased. These events are usually related to unstable regolith on steep slopes, which is susceptible to failure when sufficient precipitation is received. For example, see Figure 1.

High Amounts of Precipitation

Flooding is created by the delivery of larger than normal amounts of runoff into stream channels (Smith and Ward, 1998, p. 67). Periods of above-average precipitation lead to floods. !n some cases seasonal variability leads to great fluctuations in

Figure 1 (see color insert) Quebrada San Julián upstream of Caraballcda showing evidence of rcccnt debris flows and flash floods. Note the high slope angles, large numbers of debris flow scars, and abundance of new alluvium and colluvium in the channel bed and fan surface. See ftp site for color image.

Figure 1 (see color insert) Quebrada San Julián upstream of Caraballcda showing evidence of rcccnt debris flows and flash floods. Note the high slope angles, large numbers of debris flow scars, and abundance of new alluvium and colluvium in the channel bed and fan surface. See ftp site for color image.

stream discharge. Wet-dry subtropical or monsoonal climates with distinctive seasons of precipitation lead to fluctuations from dry stream channels to potential flooding events. These cyclical events are related to large-scale atmospheric circulation patterns that operate through an annual or longer period. In the midlatitudes, the annual migration of subtropical high pressures and the polar front lead to distinct precipitation patterns. In the tropics, monsoonal flow can lead to large precipitation events (Milne, 1986). On longer time scales El Niño and La Niña events are persistent over several years and can lead to wet or dry conditions over large areas of Earth's surface from the Equator to the midlatitudes (Waylen and Caviedes, 1987; Pearce, 1988; Ely, et al., 1994).

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