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Biotic pump of atmospheric moisture as driver of the hydrological cycle on land

DOI:10.5194/hessd-3-2621-2006
Authors:
Anastassia Makarieva at Petersburg Nuclear Physics Institute
Anastassia Makarieva
Victor Gorshkov at Petersburg Nuclear Physics Institute
Victor Gorshkov
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Abstract and Figures

In this paper the basic geophysical and ecological principles are jointly analyzed that allow the landmasses of Earth to remain moistened sufficiently for terrestrial life to be possible. 1. Under gravity, land inevitably loses water to the ocean. To keep land moistened, the gravitational water runoff must be continuously compensated by the atmospheric ocean-to-land moisture transport. Using data for five terrestrial transects of the International Geosphere Biosphere Program we show that the mean distance to which air fluxes can transport moisture over non-forested areas, does not exceed several hundred kilometers; precipitation decreases exponentially with distance from the ocean. 2. In contrast, precipitation over extensive natural forests does not depend on the distance from the ocean along several thousand kilometers, as illustrated for the Amazon and Yenisey river basins and Equatorial Africa. This points to the existence of an active biotic pump transporting atmospheric moisture inland from the ocean. 3. Physical principles of the biotic moisture pump are investigated based on the previously unstudied properties of atmospheric water vapor, which can be either in or out of aerostatic equilibrium depending on the lapse rate of air temperature. A novel physical principle is formulated according to which the low-level air moves from areas with weak evaporation to areas with more intensive evaporation. Due to the high leaf area index, natural forests maintain high evaporation fluxes, which support the ascending air motion over the forest and "suck in" moist air from the ocean, which is the essence of the biotic pump of atmospheric moisture. In the result, the gravitational runoff water losses from the optimally moistened forest soil can be fully compensated by the biotically enhanced precipitation at any distance from the ocean. 4. It is discussed how a continent-scale biotic water pump mechanism could be produced by natural selection acting on individual trees. 5. Replacement of the natural forest cover by a low leaf index vegetation leads to an up to tenfold reduction in the mean continental precipitation and runoff, in contrast to the previously available estimates made without accounting for the biotic moisture pump. The analyzed body of evidence testifies that the long-term stability of an intense terrestrial water cycle is unachievable without the recovery of natural, self-sustaining forests on continent-wide areas.
Geography of the regions where the dependence of precipitation P on distance x from the source of moisture was studied. Numbers near arrows correspond to regions as listed in Table 1. Ar- rows start at x = 0 and end at x = x max , see Table 1 for more details.
Geography of the regions where the dependence of precipitation P on distance x from the source of moisture was studied. Numbers near arrows correspond to regions as listed in Table 1. Ar- rows start at x = 0 and end at x = x max , see Table 1 for more details.
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Dependence of precipitation P (mm year − 1 ) on distance x
Dependence of precipitation P (mm year − 1 ) on distance x
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Water vapor partial pressure and evaporative force in the terrestrial atmosphere. (a) Saturated partial pressure of water vapor p H 2 O (z) , Eq. (11), and weight of the saturated water vapor
Water vapor partial pressure and evaporative force in the terrestrial atmosphere. (a) Saturated partial pressure of water vapor p H 2 O (z) , Eq. (11), and weight of the saturated water vapor
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The physical principle that the low-level air moves from areas with weak evaporation to areas with more intensive evaporation provides clues for the observed patterns of atmospheric circulation. Black arrows: evaporation flux, arrow width schematically indicates the magnitude of this flux (evaporative force). Empty arrows: horizontal and ascending fluxes of moisture-laden air in the lower atmosphere. Dotted arrows: compensating horizontal and descending air fluxes in the upper atmosphere; after condensation of water vapor and precipitation they are depleted of moisture. (a) Deserts: evaporation on land is close to zero, so the low-level air moves from land to the ocean year round, thus “locking” desert for moisture. (b) Winter monsoon: evaporation from the warmer oceanic surface is larger than evaporation from the colder land surface; the low-level air moves from land to the ocean. (c) Summer monsoon: evaporation from the warmer land surface is larger than evaporation from the colder oceanic surface; the low-level air moves from ocean to land. (d) Hadley circulation (trade winds): evaporation is more intensive on the equator, where the solar flux is larger than in the higher latitudes; low-level air moves towards the equator year round; seasonal displacements of the convergence zone follow the displacement of the area with maximum insolation. (e) Biotic pump of atmospheric moisture: evaporation fluxes regulated by natural forests exceed oceanic evaporation fluxes to the degree when the arising ocean-to-land fluxes of moist air become large enough to compensate losses of water to runoff in the entire river basin year round.
The physical principle that the low-level air moves from areas with weak evaporation to areas with more intensive evaporation provides clues for the observed patterns of atmospheric circulation. Black arrows: evaporation flux, arrow width schematically indicates the magnitude of this flux (evaporative force). Empty arrows: horizontal and ascending fluxes of moisture-laden air in the lower atmosphere. Dotted arrows: compensating horizontal and descending air fluxes in the upper atmosphere; after condensation of water vapor and precipitation they are depleted of moisture. (a) Deserts: evaporation on land is close to zero, so the low-level air moves from land to the ocean year round, thus “locking” desert for moisture. (b) Winter monsoon: evaporation from the warmer oceanic surface is larger than evaporation from the colder land surface; the low-level air moves from land to the ocean. (c) Summer monsoon: evaporation from the warmer land surface is larger than evaporation from the colder oceanic surface; the low-level air moves from ocean to land. (d) Hadley circulation (trade winds): evaporation is more intensive on the equator, where the solar flux is larger than in the higher latitudes; low-level air moves towards the equator year round; seasonal displacements of the convergence zone follow the displacement of the area with maximum insolation. (e) Biotic pump of atmospheric moisture: evaporation fluxes regulated by natural forests exceed oceanic evaporation fluxes to the degree when the arising ocean-to-land fluxes of moist air become large enough to compensate losses of water to runoff in the entire river basin year round.
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... Makarieva and Gorshkov (2007) , proposed that condensation induced atmospheric volume reduction at a higher rate over regions with more intensive evapotranspiration. Regions with low air pressure will draw water vapor from high pressure region around, which leads to a net transfer of atmospheric water vapor to the regions with the highest ET ( Makarieva and Gorshkov, 2007 ). Generally, the "acceptor " regions have greater ET, which implies greater condensation of water vapor, and induces a reduction in atmospheric pressure. ...
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