Evaporation is an interesting poetry topic idea because of the ideas that it can lead to.
For example, starting with evaporation, a poet can examine the idea of loss. They could write about a lake that evaporates or water from a glass. They could examine the process or look it at it as it completes (for example, when the last of the water is gone from the lake). This could be used either directly or metaphorically.
From another point of view, evaporation could be viewed as leading to growth. Water that evaporates from one place, can be rained down on another.
To take a less direct approach, a poet could examine the evaporation of something other than water. They could look at subjects as wide spread as a chemical in an industrial process to a liquid on another planet.
Additionally, to take a less obvious approach, a poet could examine evaporation from the point of view of the change of state: a liquid turning to a gas. This could be used as a metaphor for many transitions.
A poet also might look at evaporation from the point of view of the cooling it can provide, and write a poem about relief.
In addition to the ideas above, there are many other ideas that the poetry topic idea of evaporation can lead to.