<P> High Spatial Resolution WorldView - 2 Imagery for Mapping NDVI and Its Relationship to Temporal Urban Landscape Evapotranspiration Factors * (3) </P> <P> Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired by a specific crop or ecosystem if there were sufficient water available . This demand incorporates the energy available for evaporation and the ability of the lower atmosphere to transport evaporated moisture away from the land surface . Potential evapotranspiration is higher in the summer, on less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy for evaporation . Potential evapotranspiration is also higher on windy days because the evaporated moisture can be quickly moved from the ground or plant surface, allowing more evaporation to fill its place . </P> <P> Potential evapotranspiration is expressed in terms of a depth of water, and can be graphed during the year (see figure). </P> <P> 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 . This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient . In agriculture, this is called a crop coefficient . The difference between potential evapotranspiration and precipitation is used in irrigation scheduling . </P>

Which of the following could be a cause of decreased evaportranspiration