Sustained evaporation requires three things:
- a supply of energy,
- a supply of water, and
- a transport mechanism.
The substantial energy requirement for evaporation is due to one of water’s peculiar properties—its unusually high latent heat of vaporization. The latent heat of vaporization is the energy input required to overcome the molecular forces of attraction between water molecules in liquid form. The term “latent heat” here refers to the energy that is absorbed by water during the phase change apart from any change in temperature. The value of the latent heat of vaporization is temperature dependent and at 15°C is 2.5 x 106 J kg-1. Recall from Fig. 1‑1 that in the SI system, J stands for joule, which is the basic unit of work or energy. Prior to adoption of the SI system, energy was often measured in calories (cal), where a calorie was the amount of energy required to raise the temperature of one gram of water by one degree Celsius at atmospheric pressure. One joule is approximately 4.18 calories. A different unit, the large calorie or kilogram calorie or food calorie (Cal), is still often used to report the energy content of foods. One food calorie is approximately 4,180 joules.
When you think of evaporation, you likely think of the energy supply for evaporation as coming from the sun. And, that is typically the primary energy source. But, it is important to recognize that the energy supply can also be drawn from the body undergoing evaporation, e.g. the soil, or from its surroundings, e.g. the air. That is why sweat evaporating from our body helps to cool us down. That is also why evaporation can occur even during the nighttime.
In addition to a supply of energy, sustained evaporation also requires a supply of water. For soil, this means that liquid water must be transported from within the soil to the location where vaporization is occurring, which is typically at or near the soil surface. For this reason, the soil hydraulic properties which influence water flow and retention can also influence the evaporation process. The final requirement for sustained evaporation is a water vapor transport mechanism. The concentration of water vapor in the atmosphere must be less than concentration of water vapor at the evaporating surface. This concentration gradient drives diffusion of water vapor into the atmosphere. Once in the atmosphere, the vapor transport is often dominated by advection, i.e. vapor movement by bulk air flow. Thus, higher wind speeds can increase the rate of evaporation.
The above-ground supply of energy and the capacity of the atmosphere to transport vapor away from the land surface are often lumped together and described as the evaporative demand. The rate of evaporation from the soil is then limited by either the evaporative demand or by the soil itself, by its ability to transport water to the location of vaporization.