I feel that the reviewers comments are quite positive, specific and relatively easy to address (but see below).I am convinced you will have no problems in accommodating them. In any case, I will wait for your final version and response to comments to decide whether or not to request another round of revisions prior to final publication in HESS.

I'd suggest to emphasize the following issues in your response:
1)variable velocity. All reviewers point it out, and it is relevant in practice. While I understand that it is beyond an analytical solution, all readers would like to know the effect of this simplification. Since you are using comsol for numerical simulations, I'd suggest to expand somewhat the comparison of your analytical solution with numerical simulations. In this case, I'd suggest working with "pore volume" equivalent time(i.e., integral from 0 to t of Q(t)dt divided by Average Q).

2) The issue of inflow (not so much outflows, I believe) mentioned by the reviewers deserves more attention than given in your published reply. As a potential user, I'd like to know how to address the "cooling effect" of cool water inflow into the conduit on a hot water pulse input.

3)I'd also consider relevant the energy balance issue brought up by Ron Green

4) I would add that from a practical point of view, you should be able to provide some suggestion on the conditions under which the test is worth trying (e.g., if the conduit is very long, I'd need a very long heat pulse to be able to observe anything in the output), Can you provide some specific easy-to-follow rule?.

The paper is good! Go ahead!

Good work!
Just a minor technical point. Considering that many readers do not read the body of the paper, but only the conclusions, I'd suggest to define F and tau in the concluding section. Instead, I'd eliminate the mathematical variables that you do not refer to in the conclusions (e.g., rock specific heat or density).