Hydraflow Hydragraphs Extension
1. Is there a way to model a pond with a tailwter that varies over time?
2. Is there a way to export a reservoir's resulting elevation vs. time data?
Not to my knowledge there is. The only type of variable tailwater analysis that Hydraflow Hydrographs has is the interconnected ponds, routines. Otherwise, you can just put in a static tailwater in the outlet parameters.
Rather than trying to update a structure to mimic a variable tailwater in hdyraflow, I'd recommend modeling anything with a variable tailwater condition in SSA. With SSA you can create a channel section, run a flow through it, set the Tc, etc. and it will essentially build a rating curve. It also can serve to determine if you increase the peak at this downstream point. Main thing you have to do if you use SSA with hydrodynamic routing is just make sure you understand and read up on the parameters, and check all your link time series to see that they look correct and don't show oscillations in them. There are several threads regarding parameters for hydrodynamic modeling.
Also, in SSA you can set up a time series, hydrograph, with time vs. elevation to an outlet. I have used this to correlate a seperate detention model with my pipe network.