Storm‐discharge of streams in the upper Mississippi Valley consists of surface‐flow and storm‐seepage. The total runoff, resulting from a storm, of either of these components does not bear a fixed relation to the runoff of the other. The two are influenced by different sets of conditions. The two components of the storm‐flow can be separated by graphical analysis. A reversal of the process has been found useful in determining the hydrograph of ground‐water flow.

It is generally recognized that the nomenclature of subsurface‐flows is not satisfactory. We may expect that a more suitable terminology will not be adopted until the nature of quick subsurface‐runoff is better understood. There are several gaps in our knowledge to be bridged before this type of discharge can be established as a simple flow resulting from a known set of conditions, or as the total of a number of flows showing similar behavior but produced by different causes. The writer has found that the recession‐characteristics of quick subsurface‐runoff are of great practical value in the separation of ground‐water flow from the hydrograph of stream‐discharge. Various studies, especially those of HORTON, HORNER and FLYNT, and R. T. ZOCH, have shown that in headwater‐areas where evaporation from stream‐surfaces and transpiration from the banks is relatively small, the depletion‐curves of overland‐flow and of ground‐water discharge may both be defined by equations of the form where Qo and Q are values of the discharge separated by a time‐interval t. Hydrographs of natural stream‐flow cannot usually be divided into two elements having this form of recession, but most of them may readily be separated into three such elements. The recession‐characteristics of the middle or intermediate element are highly useful in the graphical separation of ground‐water flow. In this procedure it is not necessary to separate the element of overland‐runoff, because the value of k for the intermediate element is determined during periods when the overland‐runoff is negligible, The writer has used the term “interflow” for the intermediate element in order that its recession‐slope can be used as a tool without inviting a controversy regarding its source.

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