This sounds like a good science project to work on, and a great introduction to doing experiments. There are lots of things that affect how fast water evaporates, and you can search this site for answers to similar questions concerning water evaporation. I would suggest doing experiments with water to find out which things affect the evaporation rate the most, and by how much. Here are some ideas of things that affect the water evaporation rate:
1) Water surface area
2) Water temperature
3) Ambient humidity of the air
4) Air temperature
5) Wind speed
6) Whether the sun is shining on the container
7) Kind of container the water is held in:
(reason: it takes heat to evaporate water -- think of a clothes dryer -- heat energy must enter the water somehow, and evaporation cools the water off. An insulating container slows heat transfer and may slow evaporation. Also differently colored containers will absorb different amounts of sunlight).
8) Air pressure.
Can you think of more?
Try to do "controlled" experiments. That is, change only one thing at a time when comparing different situations. Otherwise it's hard or impossible to disentangle the effects of several different changes.
(published on 10/22/2007)