Predicting the timing of overland flow in burned watersheds can help to estimate debris‐flow timing and the location of debris‐flow initiation. Numerical models can produce flow predictions, but they are limited by our knowledge of appropriate model parameters. Moreover, opportunities to test and calibrate model parameters in post‐wildfire settings are limited by available data (measurements of debris‐flow timing are rare). In this study, we use a unique dataset of rainfall and flow‐timing data to test the extent to which model parameters can be generalized from an individual watershed to other watersheds (0.01km2 to >1 km2) within a burned area. Simulations suggest that a single, low, saturated hydraulic conductivity value can be used in post‐wildfire landscapes with reasonable results. By contrast, we found that watershed‐scale effective Manning's roughness parameter values decrease as a power‐law function of basin drainage area. Thus, a Manning roughness parameter calibrated for a single basin within a burned area may not provide adequate results in a different watershed. However, when flow velocity is modeled independently for hillslopes and channels, and different roughness parameters are used for those morphometric units, there is no drainage‐area dependence on the roughness parameters. Moreover, we found that it was possible to use field‐measured grain size data to parameterize the roughness for both hillslopes and channels. Thus, our results show that, employing this generalizable approach, it is possible to use field‐measurements to fully parameterize a model that produces peak flow timing to within a few minutes in storms lasting several hours. Further, we demonstrate how model simulations can be leveraged to identify areas within a watershed that are most susceptible to debris flows. This modeling approach could be used for decision making in hazardous burned areas and would be especially useful in ungaged basins.