E10 SoilVegetationAtmosphere Interaction

Topic

The interaction between soil, vegetation, and the atmosphere describes the bidirectional exchange of energy, water, and carbon among these three components, integrated as a single system. Vegetation is the central element of this entire system: it modifies surface albedo, increases the aerodynamic roughness encountered by the wind, intercepts a portion of rainfall with its canopy, transports water from the soil to the atmosphere via transpiration, and controls the uptake of atmospheric carbon dioxide for photosynthesis by opening or closing its stomata.

Stomatal resistance is precisely the key parameter controlling this transpiration and carbon dioxide exchange: stomata close under water stress (when the soil is dry), under high evaporative demand (when the air is very dry relative to its water-holding capacity), under extreme temperatures, or simply in low light; it is the combination of these four stress factors that determines, at any given moment, the degree to which the stomata close.

Modern models of this soil-vegetation-atmosphere system parameterize stomatal resistance as a multiplicative function of these stress factors, combining the effect of each into a single value.

Canopy interception also retains a portion of rainfall directly on leaf and branch surfaces; this intercepted water evaporates rapidly back into the atmosphere without ever reaching the soil, and can account for between twenty and forty percent of total precipitation falling on a dense forest.

The feedback loop involving soil moisture, evapotranspiration, and precipitation operates at a regional scale over periods ranging from a week to a month, and functions in both directions: evapotranspiration from moist soil increases moisture in the lower troposphere, promoting convective cloud formation and local precipitation—precipitation that, in turn, remoistens the very soil that initiated the cycle. It is precisely this feedback loop that can amplify both droughts—where dry soil produces less evapotranspiration and, consequently, less local precipitation to remoisten it—and wet periods, where saturated soil generates more evapotranspiration and, in turn, more rainfall that keeps it saturated.

The expected behavior of this entire system encompasses the seasonal variation of the leaf area index, which governs both transpiration and interception throughout the year. Thus, spanning from the vegetation with which this document begins to the seasonal variation with which it concludes—and taking in stomatal resistance, canopy interception, and the feedback between moist soil, evapotranspiration, and precipitation—the interaction among soil, vegetation, and atmosphere reveals that vegetation is not merely a passive landscape element. Instead, it acts as an active regulator that amplifies—in either direction—both the droughts and the wet periods imposed upon it by the atmosphere.

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