I still don’t believe that disclosure is an issue. If there is a capture barrel that exceeds 2500, it would be personal property, not a fixture to the property. The registration with the Division of Water Rights would be under the individual’s name, not the property per se.
However, if the owner had some sort of capture device that was affixed to the property, then yes, there might be some disclosure requirement if it exceeded the 2500 gallons.