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OOHIJo 9 Troxell7 subdivides interflow into storm subsurface runoff and a part of the seasonal subsurface runoff. The amount of interflow depends on hori- zontal and vertical permeability of the soil and the depth of water table, if any, from the ground surface. In some areas interflow is the largest part of the total annual runoff. Hertzler8 interpreted that interflow averaged 85 percent of the total annual runoff in part of North Carolina. While this may not be typical, it does show that interflow can be an appreciable com- ponent. Ground-water flow.--Water flowing in the zone of saturation is termed ground-water flow. It is the most delayed portion of precipitation, because the precipitation from a given storm may take many months to appear as streamflow. Ground-water flow contributes to streamflow through springs and through seepage into streamlets and stream channels. Meaning and Uses of Base Flow and Direct Runoff Separation of hydro graphs of total flow into the four components pre- viously described, is somewhat inexact and hypothetical because the phy- sical identification of the sources of flow is virtually impossible. There- fore a knowledge of these four components is applied to separate total flow into two parts, namely base flow and direct runoff. The concept and appli- cation of base-flow and direct-runoff data are explained in the following paragraphs. Meaning of base flow.--Base flow has had many different definitions, each definition depending upon the uses or needs of the base-flow data. Mayboom9, classifies base flow as only the ground-water component, Kun- klelO and Barnesll include portions of the interflow component. If base flow were only ground-water flow, a large part of the sustained flow pro- vided by the interflow component would be ignored. On the other hand, if all of the interflow were included in the base flow an excessive amount of rapid runoff would be included. 7 Troxell, H. C., 1953, The influence of ground-water storage on runoff in the San Bernardino and eastern San Gabriel Mtns. of Southern California: Trans. Am. Geophys. Union, V. 34. 8 Hertzler, R, A., 1939, Engineering aspects of the influence of forests on mountain streams: Civil Eng., V. 9, pp 487-489. 9 Mayboom, P., 1961, Estimating ground-water recharge from stream hydrographs: Jour. Geophys. Research, V. 66. 10 Kunkle, G. R., 1962, Baseflow-duration curve, ground-water discharge from a drainage basin: V, 67. 11 Barnes, B. S., 1959, Consistency in unitgraphs: Proc. Am. Soc. Civil Engineers, V. 85, No. HY8. a technique for the study of Jour, Geophys. Research,