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

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 Correspondence to: A. M. Makarieva ([email protected]) 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. 1 Is it a trivial problem, to keep land moistened? Liquid water is an indispensable prerequisite for all life on Earth. While in the ocean the problem of water supply to living organisms is solved, the landmasses are elevated above the sea level. Under gravity, all liquid water accumulated in soil and underground reservoirs inevitably flows down to the ocean in the direction of the maximum slope of continental surfaces. Water accumulated in lakes, bogs and mountain glaciers feeding rivers also leaves to the ocean. So, to accumulate and maintain optimal moisture stores on land, it is necessary to compensate the gravitational runoff of water from land to the ocean by a reverse, ocean-to-land, moisture flow. When soil is sufficiently wet, productivity of plants and ecological community as a whole is maximized. With natural selection coming into play, higher productivity is associated with higher competitive capacity. Thus, evolution of terrestrial life forms should culminate in a state when all land is occupied by ecological communities functioning at a maximum possible power limited only by the incoming solar radiation. In such a state local stores of soil and underground moisture, ensuring maximum productivity of terrestrial ecological communities, should be equally large everywhere on land irrespective of the local distance to the ocean. Being determined by the local moisture store, local loss of water to river runoff per unit ground surface area should be distanceindependent as well. It follows that in the stationary state the Published by Copernicus GmbH on behalf of the European Geosciences Union. 1014 A. M. Makarieva and V. G. Gorshkov: Biotic pump of atmospheric moisture amount of locally precipitating moisture, which is brought from the ocean to compensate local losses to runoff, should be evenly distributed over the land surface. In the absence of biotic control, air fluxes transporting ocean-evaporated moisture to the continents weaken exponentially as they propagate inland. The empirically established characteristic scale length on which such fluxes are damped out is of the order of several hundred kilometers, i.e. much less than the linear dimensions of the continents. Geophysical atmospheric ocean-to-land moisture fluxes cannot therefore compensate local losses of moisture to river runoff that, on forested territories, are equally high far from the ocean as well as close to it. This means that no purely geophysical explanation can be given to the observed existence of highly productive forest ecosystems on continentscale areas of the order of tens of millions square kilometers, like those of the Amazonia, Equatorial Africa or Siberia. To ensure functioning of such ecosystems, an active mechanism (pump) is necessary to transport moisture inland from the ocean at a rate dictated by the needs of ecological community. Such a mechanism originated on land in the course of biological evolution and took the form of forest – a contiguous surface cover consisting of tall plants (trees) closely interacting with all other organisms of the ecological community. Forests are responsible both for the initial accumulation of water on continents in the geological past and for the stable maintenance of the accumulated water stores in the subsequent periods of life existence on land. In this paper we analyze the geophysical and ecological principles of the biotic water pump transporting moisture to the continents from the ocean. It is shown that only intact contiguous cover of natural forests having extensive borders with large water bodies (sea, ocean) is able to keep land moistened up to an optimal for life level everywhere on land, no matter how far from the ocean. The paper is structured as follows. In Sect. 2 the exponential weakening of precipitation with distance from the ocean is demonstrated for non-forested territories using the data for five terrestrial transects of the International Geosphere Biosphere Program (Sect. 2.1); it is shown that no such weakening occurs in natural forests, which points to the existence of the biotic pump of atmospheric moisture (Sect. 2.2); how the water cycle on land is impaired when this pump is broken due to deforestation is estimated in Sect. 2.3. In Sect. 3 the physical principles of the biotic pump functioning are investigated. The non-equilibrium vertical distribution of atmospheric water vapor associated with the observed vertical lapse rate of air temperature (Sect. 3.1) produces an upward directed force, termed evaporative force, which causes the ascending motion of air masses (Sect. 3.2), as well as the horizontal air motions from areas with low evaporation to areas with high evaporation. This physical principle explains the existence of deserts, monsoons and trade winds; it also underlies functioning of the biotic moisture pump in natural forests. Due to the high leaf area index, natural forests maintain powerful evaporation exceeding evaporation from the oceanic surface. The forest evaporation flux supports ascending fluxes of air and “sucks in” moist air from the ocean. In the result, forest precipitation increases up to a level when the runoff losses from optimally moistened soil are fully compensated at any distance from the ocean (Sect. 3.3). Mechanisms of efficient retention of soil moisture in natural forests are considered in Sect. 3.4. In Sect. 4 it is discussed how the continent-scale biotic pump of atmospheric moisture could be produced by natural selection acting on individual trees. In Sect. 5, based on the obtained results, it is concluded that the long-term stability of a terrestrial water cycle compatible with human existence is unachievable without recovery of natural, selfsustaining forests on continent-wide areas. 2 Ocean-to-land moisture transport on forested versus non-forested land regions 2.1 Moisture fluxes in the absence of biotic control Let F be the horizontal moisture flux equal to the amount of atmospheric moisture passing inland across a unit horizontal length perpendicular to the stream line per unit time, dimension kg H2O m−1 s−1. With air masses propagating inland to a distance x from the ocean (x is measured along the stream line), their moisture content decreases at the expense of the precipitated water locally lost to runoff. Thus, change of F per unit covered distance is equal to local runoff. In the absence of biotic effects, due to the physical homogeneity of the atmosphere, the probability that water vapor molecules join the runoff, should not depend on the distance traveled by these molecules in the atmosphere. It follows that the change dF of the flux of atmospheric moisture over distance dx should be proportional to the flux itself: