PA-28 pilot reported loss of Engine RPM after takeoff. The pilot executed an air turn back and landing at departure airport. It was deduced that water had contaminated the fuel and had not been properly sumped out during pre-flight.

Narrative

Upon departing from Runway XX at ZZZ; I experienced a sudden loss of 400-500rpm after reaching 500ft msl. full power returned after 2 seconds; and then dropped again by 400-500rpm 5 seconds after that. After the 2nd drop in RPM I notified Tower of the issue and requested to return to land as soon as possible. Tower [gave priority handling] for me and cleared me for Runway XX or XY; and I took YY. No further issues occurred after the second drop in RPM and I was able to land and taxi back with no issue. After talking to ground ops we both suspect the issue came from the fuel system; and as it rained the night before and was still cold and damp the morning of; we agreed the most likely issue was fuel contaminated with water. Although I did sump the fuel before the flight; I only had 6 hours of sleep prior and was still shrugging off tiredness from the morning and I suspect that a small quantity of water had been missed during the sump. I'll definitely be a little more thorough on inspection of fuel sumps in the future to mitigate future contamination occurrences (assuming fuel contamination was the issue on this particular flight).