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Miguel Ponce conversation with Victor Mockus

Dr. V. Miguel Ponce is a professor at the San Diego State College of Engineering, specializing in engineering hydrology, surface-water and environmental hydrology, sedimentation engineering, watershed management, and web-based engineering. Click here for his website.

Victor Mockus was a long-time engineer with the Soil Conservation Service who, in the 1950's and 60's developed many of the techniques now known as the "SCS Method" or "Runoff Curve Number Method". Although several engineers were involved in this work, Mockus is considered the originator. He tried to verify his techniques with Agricultural Research Service data. However, neither the notes of documentation that Mockus may have made concerning the development of his methods, nor the specific ARS datasets used, have been saved.

Dr. Ponce engaged in a bit of investigative hydrology when he interviewed Mockus in 1996. His interview is on his website here and reproduced below:

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 never completed.

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


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