ABSTRACT
The processes governing water movement in natural fields are influenced by different factors such as rainfall intensity and duration, infiltration rates, catchment area, slope, and soil cover. Among these, a key role is played by infiltration, the process of water entering the soil (see Chapter 7). Hillslope hydrologic processes are strongly influenced by the patterns of infiltration rate through space and time (Freeze, 1980). The Hortonian mode of overland flow (Horton, 1933; Rubin and Steinhardt, 1963) is generated at a point on a hillslope when the rainfall intensity exceeds the infiltration rate, and surface ponding is initiated. A second mechanism, as described by Dunne (1978), requires surface saturation to occur from a rising water table over a shallow impeding layer; ponding and overland flow occur when no further soil moisture storage is available. The Horton mechanism is more common on upslope areas whereas the Dunne mechanism is generally observed in near-channel wetlands. While several other streamflow
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generation mechanisms exist (see Chow et al., 1988; Dingman, 2002), the focus of this chapter is on Hortonian runoff generation.
