TABLE OF CONTENTS Urban catchment runoff comes mostly from rainfall excess, that is, rainfall minus abstractions. The contribution of prompt subsurface return flow is usually negligible. (This is not necessarily the case for sewers, where leakage through joints and cracked pipes could be considerable.) Among the abstractions, infiltration is by far the most significant. On a rainstorm-event basis, evapotranspiration is relatively negligible. Interception varies with land use and seasons. Depression storage is a matter of definition and subsequent method of estimation. Quantitative information on the initial losses interception and depression storage can be found in, for example, Chow (1964) or Maidment (1993). At any rate, for a heavy rainstorm, the amount of initial losses is relatively small. However, it should be noted that in terms of pollution, or runoff on an annual basis, the contributions of light rainstorms are also significant.
The hydrologic characteristics of urban catchments vary with land uses and seasons. The surface may range from the relatively impervious surfaces such as streets, sidewalks, driveways, parking lots, and roofs to pervious surfaces such as lawns, gardens, bare soil, and parks. Rainfall excess on these surfaces are drained directly or indirectly through adjacent different types of surfaces and gutters into inlet catch basins, and then into sewers or channels (Fig. 14.42)]. The geometric composition of these different types of surfaces in forming a city block or catchment is usually random. This random heterogeneous nature of urban catchment surface imposes great difficulty...
