Roger

You may not have overland flow pre-se, since the runoff would concentrate in the furrows. In the prairies, runoff from tilled fields is often nil for rainstorms less than the 5 year extreme even. Even in areas with relief, contour plowed furrows typically run perpendicular to the natural ground slope (to prevent erosion) and the furrows may intercept all of the flow if the furrow depth exceeds the depth of rainfall. I have often seen furrows overtopped and a easily-defined erosion channel following extreme rainfalls, but this would not be applicable for general modelling.

Manning 'n' values for tilled farm fields in the flood plain are available, but would not be applicable since these would be based on deeper flood plain flow and not runoff.

You may wish to cook something up using the SCS/NRCS Curve Numbers from TR 55 (i.e. ratio the flow from a known area CN by the estimated CN for your tilled field). See table 2.2b for Curve Numbers for Fallow ground, row crops, grain crops, etc. Also note that sheet flow would turn into shallow concentrated flow (after 300 feet of sheet flow, using TR 55 rules of thumb).

The TR 55 manual is available on-line . . . http://www.il.nrcs.usda.gov/engineer/enggsoft.htm <http://www.il.nrcs.usda.gov/engineer/enggsoft.htm>

Regards,

Kirby McRae, P.Eng.
kmcrae@UMAGROUP.COM