The derivation of f‐curves of infiltration‐capacity from natural rainfall has been undertaken fey only a few hydrologists. The use of the infiltrometer does not supplant the necessary for f‐curves on large or small areas based on natural rainfall. Infiltrometer f‐curves, excepting fc‐values, do not agree with natural rainfall f‐curves.
In a previous paper entitled “stream‐flow from rainfall by unit method” (Eng. News‐Record, April 7, 1932), the writer defined a unit graph as the hydrograph of runoff from a given area, due to a one‐inch runoff‐depth applied in one day or in any other convenient unit of time. A procedure for deriving the unit graph from observed rainfall‐ and runoff‐records was presented. After the unit graph has been derived a runoff‐record for the given area, for any rain or series of rains, may be computed by a simple summation process. Figure 1, from the aforesaid paper, graphically illustrates the summation process and it will not be described further here.
During the past two years, the writer has supervised a research and study of infiltration by the Illinois State Planning Commission. Among the results was a procedure whereby infiltration‐capacities can be computed from observed average loss‐rates “fav”. The average loss‐rates, sometimes called “average areal infiltration‐capacities,” are fully described by Dr. Robert E. Horton in the Transactions of the American Geophysical Union. Their derivation is now a standard practice in hydrology. Based on our work, we make the following comments.
When average loss‐rates are plotted against time of duration, they show little or no correlation excepting the few, relatively small, loss‐rates which are derived from long durations or with initially wet soils. The average loss‐rate bears little resemblance to either initial infiltration‐capacity, fo, or ultimate infiltration‐capacity, fc.
This paper gives data on certain observed rates of soil‐infiltration and other rainfall‐losses during the progress of a storm. Infiltration takes place only during the period of rainfall and surface‐runoff (excepting cases like the dry Western stream‐beds). R. E. Horton (Surface‐runoff phenomena, Bull. 101, Horton Hydrologic Laboratory, Edwards Brothers, Pub. Ann Arbor, Mich.) has presented a method whereby the hydrograph of surface‐runoff may be derived from the observed stream‐flow hydrograph. With this hydrograph of surface‐runoff and with the application of M. M. Bernard's “pluviograph” (An approach to determine stream‐flow, Trans. Amer. Soc. C. E., No. 100, 1935), it is possible to find the infiltration and other losses during the storm‐period, on a drainage‐basin, provided good records of rainfall and of stream‐flow are available.
The objective of this paper is to furnish a working procedure to engineers whereby the principles of the infiltration‐theory may be utilized to estimate the volumes of infiltration or surface‐runoff from natural basins. While adhering to the basic principles of the infiltration‐theory, the authors have used some approximations and averages in order to simplify the problem and reduce the time and labor otherwise involved.
This work has been made possible through the courtesy of the United States Corps of Engineers and the United States Department of Agriculture permitting the use of their data on rainfall, runoff, and their derived average loss‐rates (fav) on basins from 10 to 3,000 square miles in Oklahoma and Mississippi.
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