I. INTRODUCTIONRunoff, or surface runoff, refers to all the waters flowing on the surface of the earth, either by overland sheet flow or by channel flow in rills, gullies, streams, or rivers. Stream flow refers to the flow in natural streams. Runoff and stream flow are continuous processes by which water is constantly flowing from higher to lower elevations by the action of gravitational forces. In this way, overland flow concentrates into small streams, which in turn combine to form larger streams and rivers. Eventually, rivers flow into oceans completing the hydrologic cycle.Runoff is usually expressed in terms of either volume or flow rate. The usual units of runoff volume are cubic meters, cubic feet, or acre-feet. Flow rate or discharge is usually expressed in cubic meters per second (m 3 /s) or cubic feet per second (cfs). Discharge at a cross-section or gaging station usually varies in time; therefore, its value at any time is the instantaneous or local discharge. Instantaneous values of discharge can be integrated over a period of time to give the runoff volume for the entire period.Runoff is also expressed in terms of depth units, by dividing the runoff volume by the catchment/watershed/basin area to obtain an equivalent spatially averaged runoff depth. In certain applications of flood hydrology runoff can also be expressed as: a) peak discharge per unit drainage area (m 3 /s/km 2 ; cfs/mi 2 ), b) peak discharge per unit runoff depth (m 3 /s/cm; cfs/in), and c) peak discharge per unit drainage area per unit runoff depth (m 3 /s/km 2 /cm; cfs/mi 2 /in).This introductory section sets the stage for the remainder of the chapter. It contains two subsections: 1) Description of Physical Processes and 2) Variability of Runoff. Description of Physical Processes seeks an understanding of the runoff processes as a framework for the detailed calculations that will follow. Variability of Runoff describes the aspects inherent to the study of runoff, namely its local, spatial, temporal, seasonal, regional, and geographical variability.
A. Description of Runoff ProcessSurface runoff comprises all the waters flowing on the surface of the earth in response to precipitation. Surface runoff eventually goes on to constitute stream flow; however, stream flow comprises both surface and subsurface flow. The subsurface flow component of stream flow consists of exfiltration from interflow and ground water flow.Surface runoff occurs when water originating in precipitation (rainfall and snow) flows freely on the surface of the earth, driven by gravitational forces. Typically, the first manifestation of surface runoff is 2. Stream Flow Generation Stream flow generation differs from overland flow generation in that it includes both surface and subsurface runoff. Classical hydrology defines stream flow in terms of three components: a) surface runoff, b) interflow, and c) ground water flow. Notwithstanding classical hydrology, recent theories of stream flow generation have emphasized the timing, rather than the path, of the components ...