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Please allow for direct runoff with no transformation. Even with a Tc of 0, there is still some small amount of lag in applying the rainfall hydrograph. In the absence of custom S-graphs, this would allow the user to conservatively demonstrate no transformation to approving agencies that have specific transformation requirements.
Hi Justin,
many thanks for providing feedback.
The residual lag at Tc = 0 is a side effect of how the engine steps through time in discrete intervals. The first flow value is always set to zero, so the earliest any runoff can appear is at the end of the first timestep. If your timestep is 1 minute, the lag is 1 minute; if it's 5 minutes, the lag is 5 minutes, and so on. To reduce this, the only mitigation mesaure I can think of is using a smaller analysis timestep. That won't remove the lag entirely, but it will reduce it to a negligible amount, which hopefully is enough for your reviewers.