 Potential evapotranspiration is expressed in terms of a depth of water, and can be graphed during the year (see figure).Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the soil type for bare soil, and the vegetation.Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally short grass.