The SCS method (SCS National Engineering Handbook, 1972) of estimating direct runoff from storm rainfall is based on methods developed by SCS hydrologists in the last three decades, and it is in effect a consolidation of these earlier methods. The hydrologic principles of the method are not new, but they are put to new uses. Because most SCS work is with un-gauged watersheds (not gauged for runoff) the method was made to be usable with rainfall and watershed data that are ordinarily available or easily obtainable for such watersheds.
The principal application of the method is in estimating quantities of runoff in flood hydrographs or in relation to flood peak rates. These quantities consist of one or more types of runoff. An understanding of the types is necessary to apply the method properly in different climatic regions. The classification of types used in this handbook is based on the time from the beginning of a storm to the time of the appearance of a type in the hydrograph. Four types are distinguished: Channel runoff occurs when rain falls on a flowing stream or on the impervious surfaces of a streamflow-measuring installation.
It appears in the hydrograph at the start of the storm and continues throughout it, varying with the rainfall intensity. It is generally a negligible quantity in flood hydrographs, and no attention is given to it except in special studies. Surface runoff occurs only when the rainfall rate is greater than the infiltration rate. The runoff flows on the watershed surface to the point of reference. This type appears in the hydrograph after the initial demands of interception, infiltration, and surface storage have been satisfied. It varies during the storm and ends during or soon after it.
Surface runoff flowing down dry channels of watersheds in arid, semiarid, or subhumid climates is reduced by transmission losses, which may be large enough to eliminate the runoff entirely. Subsurface flow occurs when infiltrated rainfall meets an underground zone of low transmission, travels above the zone to the soil surface downhill, and appears as a seep or spring. This type is often called “quick return flow” because it appears in the hydrograph during or soon after the storm. Base flow occurs when there is a fairly steady flow from natural storage.
The flow comes from lakes or swamps, or from an aquifer replenished by infiltrated rainfall or surface runoff, or from “bank storage”, which is supplied by infiltration into channel banks as the stream water level rises and which drains back into the stream as the water level falls. This type seldom appears soon enough after a storm to have any influence on the rates of 2 the hydrograph for that storm, but base flow from a previous storm will increase the rates.