What must exist for evapotranspiration (ET) to occur?

Evapotranspiration (ET) is a process that involves the transfer of moisture from the land to the atmosphere through both evaporation and plant transpiration. For ET to occur, a relative humidity gradient is essential. This gradient refers to the difference in moisture content between the air surrounding the plant leaves and the moisture content inside those leaves.

When the relative humidity in the atmosphere is lower than that in the leaves, moisture will move from an area of higher concentration (inside the leaves) to an area of lower concentration (the atmosphere), driving the process of vaporization and transpiration. If the relative humidity is high both inside and outside the leaves, little to no water vapor will be released, significantly reducing or halting ET.

While other factors such as temperature gradients or soil moisture are important for overall plant health and soil-water dynamics, they are not direct prerequisites for the process of evapotranspiration. Temperature affects the rate of evaporation, and soil moisture influences plant transpiration, but without a relative humidity gradient, the actual process of water vapor moving from the plant to the atmosphere would not take place effectively.