Guessing a bit, because you don't tell us what pf and tf are, but assuming they are water height and time vectors, I suspect that it may help to change the first line of code to
[~, pkindex] = max(pf);
This is because the first result from max (in this case 1.8268) is the maximum value stored in the vector pf. What you need is not, I think, that value, but the index at which it is stored, which is the second result from max.
