I met with Victor Mockus on July 12, 1996, at his home.
I arrived at 3:00 pm and left at 4:45 pm.
He said that he had spent considerable time at Coshocton, Ohio, in the 1930's,
and was very familiar with the data at that site. He said that SCS developed the
runoff curve number method for small basins, less than 400 square miles. The
objective was to have a method which would evaluate the "before" and "after"
hydrologic response from events. The hydrologic data collection was started in
various U.S. sites in 1928. The method was based on data encompassing 10 to 20
years of field research.
He said that he had nothing to do with the hydrologic soil groups (A, B, C, and
D). That it was Muskgrave's idea to classify all soils (60 or 70 of them at the
time) into four HSG's. The method had to be simple enough to be used by people
that had little experience with hydrology. He said he had converted the
S (potential storage) into something simpler and more manageable (the CN's).
The relation between Ia and S was a tough problem.
In an attempt to avoid this
problem, he said he preferred to use P - Ia in the abscissa,
but was overuled by
his superiors. They zeroed into 0.2 for the
ratio Ia/S because that value appeared
to be at the center of their data.
The hydrologic condition (good, fair, poor) was his contribution. It was
developed with data at Hastings, Nebraska, and at Waco, Texas. The watersheds
varied in scale from 0.1 acre to 10 square miles. The method was not supposed to
account for indirect runoff (interflow and groundwater flow); only direct runoff.
He had been working on a method to predict indirect runoff, but this work was
He saw no problem with changing the adopted 0.2 to 0.1 or 0.3,
or any other value, if the data under consideration warranted it.
He said that the method was developed for events, but it was based on daily data,
because that was the only data available in large quantities. The method was not
supposed to be a predictor of the rate of infiltration, but rather of the total volume
of infiltration for a given event. He said the method was supposed to predict an
average trend, and not the response of individual storms, which could deviate
from the average trend. He said that the criticism in Maidment's book (Chapter 9,
page 26) is unwarranted because the variability of the method is supposed to be
accounted for in the antecedent moisture condition (AMC).
He said he arrived at the equation (P - Q)/S = Q/P one evening after dinner,
seeing that it fitted the data very well, and after having tried many other alternative
relations. He said that that was the relation that best fitted the data.
He said that the
AMC envelopes were a reasonable maximization, for which they used all the
available data; still, this is a statistical function.
He said that the method should apply to arid conditions, with some qualifications.
If there was crusting, most likely it would not apply. Saturation overland flow
was the most likely runoff mechanism to be simulated by the method, and not
necessarily Hortonian overland flow or crusting.
He said that he saw no limit to a basin-scale application of the runoff curve
number equation, other than that which is imposed by the requirements of spatial rainfall uniformity.
He said that there were three wise men in the 1930's hydrology in the U.S.:
Robert Horton, L. K. Sherman, and W. W. Horner, who perhaps unduly influenced the
development of hydrologic thought. He said that Sherman did not develop the
unit hydrograph concept; that he took his ideas from publications of the Boston
Society of Civil Engineers.
He said that in his experience, people were hesitant to go to the field to perform
measurements. He said that there was a lot of politics among the agencies, with
the Corps (of Engineers) trying to dictate hydrology, and the ARS being too
theoretical, even though it was funded largely through SCS.
He said Ven T. Chow was fond of appearances; that he appeared in a picture in
Life magazine flanked by a rainfall generator and a computer. He said that
anybody who knew would immediately realize you could not put both in the same
room, since the computers of that day required precisely controlled ambient
moisture, which could not be guaranteed in the presence of a rainfall generator.
He said that he was 83 years old, and that his last name (Mockus) was a
modification of the Lithuanian original.
Victor Miguel Ponce
San Diego, California
July 15, 1996