Certified Crop Advisor Practice Exam

Which of the following must exist for ET to occur?

1. high soil temperature

2. relative humidity gradient

3. excess moisture

4. low wind speed

The correct answer is: relative humidity gradient

Evapotranspiration (ET) occurs when water is transferred from the land to the atmosphere, involving both evaporation from soil and other surfaces and transpiration from plants. For this process to take place, a relative humidity gradient is essential. A gradient occurs when there is lower humidity in the atmosphere compared to the moisture content in the soil and plant tissues. This difference drives the movement of water vapor from the moist surfaces into the drier air, facilitating the evaporation process. While high soil temperature, excess moisture, and low wind speed can influence the rate of ET, they are not prerequisites for the occurrence of ET itself. High soil temperatures can increase evaporation rates, but ET can still occur at lower temperatures. Similarly, while excess moisture may enhance ET, it is not a requirement for it to happen. Low wind speeds could slow down evaporation by reducing air movement, but ET can still take place under varying wind conditions if the humidity gradient exists. Therefore, the presence of a relative humidity gradient is essential for creating the driving force necessary for water vapor to move from the soil and plant surfaces into the atmosphere.