Is the Clark Tc the same as the Tc from TR-55 ? Is it possible to calculate the Clark Tc using the TR-55 methodology (sheet + shallow conc. + channel) ?

I think you're referring to the Clark Unit Hydrograph procedure. This is a method for generating a unit hydrograph, based on the Tc and storage coefficient R. The Clark procedure does not calculate the Tc - it uses the Tc as input. The Tc is calculated separately by the procedure of your choice, such as the TR-55 worksheet.

Try a Google search for additional details.

Peter Smart
HydroCAD Software
www.hydrocad.net

Thank you,
What got me confused is this paper, http://prr.hec.gov.pk/Chapters/775S-3.pdf , it states:
"The definition of time of concentration in Clark model is somewhat different than that used elsewhere in hydrology. Clark’s Tc is time of travel required by last drop of rainfall excess at the hydraulically most remote point in the catchment to reach the channel network, in contrast to the outlet that is used elsewhere (Straub et al., 2000)."

I find that statement peculiar and not particularly accurate. Also a candidate for the Engineering Language / Grammar Skills subforum.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

Akel - Peter hit right on with his reply. Tc, quite often, is a regional equation or method. So I would ask why Clark; is it regionally required? Why not just use TR55?

Clark (1945) defined Tc as the time from the end of effective rainfall over the watershed to the inflection point on the recession limb of the surface runoff hydrograph. In practice, for ungaged watersheds this time is usually estimated by empirical equations since runoff hydrographs from the watershed are not often available. See Papadakis and Kazan (1987) for an empirical equation for time of concentration.

Papadakis, C. N., and Kazan M. N., "Time of Concentration in Small Rural Watersheds," Proceedings of the 1987 ASCE Engineering Hydrology Symposium, Williamsburg, Virginia, pp.633-638.

@gbam
We're working in Jordan on a sub-basin that drains into the dead sea. The catchment lies in a semi-arid to arid region and our objective is to build small desert dams to address the water deficiency issue in the country. The last study on the sub-basin was done in the early 1990's and mainly used SCS methodologies. Time has shown that the results are unsatisfactory, mainly because the SCS methods were developed for humid regions; very different from what we have here. The Clark method will be more suitable for our region given its flexibility.

agree that SCS is not an ideal method for desert, Clark might be more suitable if the drainage area is not too large (5 - 10 square miles maximum for a single basin)

Why do you suggest such a small drainage area ? I've seen the method used for subbasins measuring (100 - 600) km² in area. My particular subbasin is about 450 km². Any suggestion as to what Tc calculation method would be most suitable for such area and an arid climate?.

Well, you could subdivide your watershed into smaller sub-basins, generate the hydrographs for each sub-basin and route and combine them using HEC-1. Or, if information is available, you could do a regression analysis of watersheds in the area and develop a regional regression equation. We typically use S-graphs that are developed based on regional data. This method is described in the Design of Small Dams (USBR)

Thank you for your help.