An improved evaporation pan assembly and measurement method comprising adjustable buoyancy floats attached to an outer wave-guard that surrounds the evaporation pan. An anchor system preferably restricts lateral movement while allowing for vertical movement with changing fluid levels. Preferably, the evaporation pan assembly further comprises a baffle system within the evaporation pan to prevent sloshing of the fluid within the pan. The height of the body of fluid being evaluated is preferably measured with a guided float assembly within the evaporation pan.