Evaporation is greatest when the differences between air and water temperature are greatest. The timing of this peak varies by lake. Because the lakes are so large, their surfaces release a great deal of water into the air. The lakes vary a great deal in this respect, however. A shallow, warm lake, like Lake Erie, experiences a great deal more evaporation than a deeper, colder lake, like Lake Superior.
Water also evaporates from vegetation, soil, streams, ponds, and smaller lakes.
The water vapour in the atmosphere falls into the Great Lakes Basin as rain, snow, hail or sleet, either falling directly onto the surface of the lakes or being added to the lakes as runoff from the drainage basin. Once water falls to the surface, it can take a number of different paths. The water may quickly vaporize, be absorbed by plant roots, or simply fall to the surface waters of the Great Lakes. If the water falls to the land of the Great Lakes Basin, it will either percolate down through the soil and become groundwater or will become surface runoff, eventually making its way to a stream or lake. A number of factors determine whether the water that falls will become groundwater or runoff.