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How does soil moisture influence evaporation from land?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Soil moisture is a critical factor in the hydrologic cycle, directly influencing the rate at which water returns to the atmosphere from land surfaces through evaporation and transpiration, collectively known as evapotranspiration.12

The influence of soil moisture on evaporation occurs through several key mechanisms:

Direct Evaporation: Water held in the soil diffuses upward toward the surface to replace water lost to the atmosphere. The rate of this evaporation depends on the speed at which moisture is supplied to the surface via capillary suction within the soil pores. As the soil dries, this supply rate decreases, limiting further evaporation.13
Transpiration: Plants extract moisture from the soil through their root systems. The availability of water within the root zone is a primary determinant of transpiration rates; when soil moisture is depleted, plants may close their stomata, significantly reducing the amount of water vapor released into the atmosphere.3
Energy Balance: Soil moisture levels affect the thermal properties of the ground. In periods of high moisture, energy is consumed by the phase change of water, which can regulate surface temperatures and influence the overall water balance.14

Ultimately, soil moisture acts as a reservoir that modulates the exchange of water between the land and the atmosphere, with depletion often occurring during periods of high solar radiation and peak plant growth.14