Water enters, remains, and eventually leaves a lake in a variety of ways. A. By measuring the water quantities at each of a lake's inputs and outputs, it can be determined whether water enters the lake mainly from surface or groundwater sources. B. Changes in lake level and volume are caused principally by the amount of evaporation of water into the atmosphere. C. It is sometimes possible to decide whether a lake is surface water dominated or seepage dominated by simple observation at different seasons. D. The average period of time that molecules of water spend in a lake – the residence time – varies from lake to lake and overtime within a particular lake. E. The residence times of surface-water-dominated lakes are usually longer than those of seepage-dominated lakes. F. The residence time of a lake frequently depends on the kinds of organisms to be found in the lake.