Factors affecting the response of small watersheds to precipitation in humid areas

Customary separation of stream hydrographs into overland flow, interflow and base flow has little meaning when applied to most small watersheds. A revised description of runoff processes in forested headwaters relates quick rises in streamflow to variable source areas and subsurface translatory flow, or the rapid displacement of stored water by new rain. Because this makes the classification of hydrograph components difficult, if not impossible, a numerical rating system, the response factor, was developed from precipitation and streamflow records for use in classifying the hydrologic response of small watersheds in humid areas. A simple uniform hydrograph separation method was necessary to make inter-watershed and inter-regional comparison of response meaningful. Long-term hydrograph records from fifteen forested watersheds in eastern United States were separated into quick and delayed flow by computer and ranked according to mean precipitation, quick flow, and the response factors quick flow/precipitation and quick flow/total water yield. Similar data from nine agricultural watersheds allowed comparison of response among twenty-four small basins, systematically revealing important relationships not usually noted. The over-riding prevalence of subsurface flow was indicated and exceptions were pinpointed. Possibilities and advantages of mapping small watersheds or stream networks by a universal response factor were discussed. STREAMFLOW is generated chiefly by processes operating beyond perennial stream channels. This truism alone is justification for detailed study of the means by which precipitation is transformed into streamflow. Classical hydrology has been much concerned with channel and related processes which produce flood volumes and peak stages at critical locations downstream. The main problem on the larger streams has been to relate flood magnitude to flood frequency rather than to source areas. This downstream focus has feft us even today with a poor understanding of source area hydrology, and has contributed to some unnecessary controversy over the source of floods and alternate methods for controlling them. For example, the statement or assumption that all floods are due to surface runoff has persisted in hydrology papers and even in some hydrology textbooks despite much evidence to the contrary in forestry and agricultural research. Virtually all techniques now in general use for estimating flood peaks and volumes, are based on the seldom challenged assumption that a quick rise in streamflow proves that rainfall is failing to infiltrate and is running over the surface of the ground to stream channels. The term interflow was proposed, and is in fairly general use, to refer to subsurface flows which contribute flood waters in some cases, but there has been no general agreement as to what interflow is or how it works. Hursh (1944) suggested long ago that quick subsurface flow was the primary source of stormflow from forest land. But only a few hydrologists have attached any real importance to interflow as a flood phenomenon and it has remained a sort of catch-all term to cover many small watershed processes we do not understand. Despite gradual recognition of the importance of subsurface processes in streamflow, the traditional concept of stormflow as a produce of overland flow has lingered on in virtually all hydrologic