M4 Evapotranspiration and Plant Control

Topic

Evapotranspiration is the transfer of water from the Earth's surface—both from the soil and from the vegetation itself—into the atmosphere in the form of vapor; it combines two distinct processes within a single flux: direct evaporation from the soil and open water surfaces, and transpiration by plants through their leaves. It is the second-largest flux in the hydrological cycle, surpassed only by precipitation itself: on a global scale, evapotranspiration returns to the atmosphere nearly sixty percent of all precipitation falling on land surfaces.

Actual evapotranspiration is always equal to or less than the potential evapotranspiration the atmosphere could sustain: when the soil has sufficient water and conditions are favorable, actual evapotranspiration fully matches potential evapotranspiration; however, as soon as the soil begins to dry out or plants close their stomata, actual evapotranspiration falls below that potential rate.

The Penman-Monteith equation serves as the physical framework for calculating both potential evapotranspiration—assuming the resistance offered by the plant canopy is virtually zero—and actual evapotranspiration, by incorporating a positive canopy resistance that reflects the control exerted by stomata. The atmospheric demand placed on this system is determined by net available radiation (after accounting for heat flux into the soil) and the air's vapor pressure deficit—the difference between the vapor pressure the air would have if saturated and the vapor pressure it actually contains. The greater this vapor pressure deficit, the higher the evaporative demand the atmosphere imposes on the surface.

The water supply sustaining evapotranspiration travels from the soil to the plant roots via matric flow and capillarity; However, when soil moisture drops below the permanent wilting point—a suction threshold of around 1,500 kilopascals—that supply is no longer sufficient to meet atmospheric demand; it is precisely this insufficiency that creates the gap between actual and potential evapotranspiration.

Stomatal control is the central biological mechanism governing this entire relationship: as the soil water deficit increases, the leaf's stomata close to prevent the leaf from drying out. While this closure reduces transpiration, it also causes a corresponding reduction in the plant's carbon fixation. The resistance stomata offer to the passage of water vapor increases exponentially as soil water stress intensifies, temperatures become extreme, and the air's vapor pressure deficit rises. Models that jointly represent the soil, vegetation, and atmosphere parameterize this stomatal control as a multiplicative function that combines precisely these stress factors.

The expected behavior of this system includes a distinct diurnal cycle in evapotranspiration—peaking around midday and reaching a minimum at night—as well as seasonal variation, with a summer peak when evapotranspiration is high and the soil remains moist, followed by an abrupt drop in evapotranspiration once the soil dries sufficiently. Thus, from the initial transfer of water to the atmosphere to the eventual abrupt reduction—and encompassing the relationship between actual and potential evapotranspiration, the Penman-Monteith equation, soil water supply, and stomatal control—the interplay of evapotranspiration and plant regulation ensures that the rate at which the land returns water to the atmosphere depends, at any given moment, on the degree to which the vegetation's stomata are open or closed.

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