Wayne Huber has an excellent 90+ page explanation of the history of urban runoff water quality modeling in SWMM including the theoretical background of the buildup-washoff and rating curve equations in SWMM: Huber W.C. (1986), Deterministic modeling of urban runoff quality, NATO ASI Series,Vol. G10, Urban Runoff Pollution, Edited by Torno H.C. Marsalek J., DesbordesM., Springler-Verlag Berlin Heidelberg.

I have a paper copy but some say (from a previous posting on the SWMM List Server) that the book is out of print. If you cannot find the Nato book then look for The SWMM 4 manual (which you can download in PDF from many locations on the web). [SWMM 4 manual from Mitch Heineman's web site http://www.dynsystem.com/netstorm/cdmuswmm.htm]. It has detailed informaton on buildup washoff in the Runoff Chapter and a few of the Appendices. The references in the SWMM 4 manual will lead you to to published thesis by Doug Ammon among others in which the derivation of the buildup and washoff functions are described.
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