Transpiration is essentially evaporation of Water from plant leaves. Specifically, it is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, called stomata, where it changes to vapor and is released to the atmosphere.
Studies have revealed that about 10 percent of the moisture found in the atmosphere is released by plants through transpiration. The remaining 90 percent is mainly supplied by evaporation from oceans, seas, and other bodies of water (lakes, rivers, streams).
Transpiration rates vary widely depending on weather conditions, such as temperature, humidity, sunlight availability and intensity, precipitation, soil type and saturation, wind, land slope, and water use and diversion by people. However, in general, during a growing season, a leaf will transpire many times more water than its own weight. A large oak tree can transpire 40,000 gallons (151,000 liters) per year.
Check out some simple Transpiration Investigations