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<br />8 <br /> <br />CONCEPT OF BASE FLOW <br /> <br />Base flow is a part of the total flow in a stream. Because base flow <br />cannot be identified and measured in the stream, analytical methods are <br />necessary to separate the base flow from the total flow. Before this sep- <br />aration can be accomplished however, knowledge is needed of the compo- <br />nents of flow. <br /> <br />Components of Flow <br /> <br />Streamflow begins only after the demands of evaporation, transpira- <br />tion and deep percolation have been met and the precipitation exceeds the <br />infiltration rate. Residue water reaching the stream channel may be class- <br />ified into four components of flow6, described herein, that follow four sep- <br />arate paths in their journey. <br /> <br />Channel precipitation.--Some of the basin precipitation falls directly <br />on the stream. The average annual flow of this component may be estimat- <br />ed from the mean annual precipitation and area of the water surface. Ex- <br />cept for streams containing large reservoirs, which are not included in <br />this report, this component is quite small and is generally included with <br />surface runoff. <br /> <br />Surface runoff.--Water that travels on the surface of the ground is <br />termed surface runoff. It travels as sheet flow for a relatively short time <br />and then gathers in small streamlets which join together to form the streams <br />and is the first major component to reach the streams. Surface runoff oc- <br />curs only after the needs of evaporation, transpiration, infiltration, and <br />temporary retention of surface storage have been satisfied. Surface run- <br />off, caused only by storms and snowmelt, is the most important component <br />during floods. <br /> <br />Interflow.--This component is the water that moves in the zone bet- <br />ween the ground surface and the water table, if one exists, until intercepted <br />by a stream channel or returned to the surface by seeps or springs. A rel- <br />atively impermeable soil horizon limits the rate of percolation and forces <br />the water to move laterally above this horizon. Interflow is the remainder <br />of infiltration after soil moisture and deep percolation needs have been <br />satisfied. Because of the varied paths traveled by interflow, some por- <br />tions reach the stream quite rapidly while other portions take a long time. <br /> <br />6 Linsley, R. <br />hydrology: <br /> <br />K., Jr., Kohler, M. A., and Paulhus, J. L. H., 1949, Applied <br />McGraw-Hill Book Co., N. Y. <br />