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

Abstract

Abstract. 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 the passive geophysical 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 hydrostatic 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 transpiration 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 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.

Macrophysiology for a changing world

“…Forests constitute a biotic pump, moving atmospheric water into the interior of continental systems (Makarieva & Gorshkov 2007). In areas dominated by non-forested landscapes, ocean to land moisture transport ceases several hundred kilometres inland, which leads to a rapid decline in precipitation as a consequence of the absence of sufficient evaporative water loss.…”

“…It is the concatenated evapotranspiration of water from many closely packed individual trees that gives rise to this biotic pump. Removal of the trees, especially along coastal fringes, shuts down the pump, and even in areas where rainfall is considerable it is predicted that deforestation will lead to a 10-fold reduction in mean continental precipitation and run-off ( Makarieva & Gorshkov 2007). This novel biologically based physical principle is perhaps one of the most significant discoveries for understanding the probable impacts of ongoing deforestation across the globe.…”

Le rôle des forêts et des arbres dans l’adaptation sociale à la variabilité et au changement climatiques

“…Bien que l'évapotranspiration des forêts réduise les débits totaux dans un bassin versant, elle renvoie également de l'eau dans l'atmosphère, ce qui peut augmenter les précipitations dans la région 80 . Les forêts peuvent également pomper l'humidité atmosphérique et envoyer l'air humide des océans vers les terres 81,82 , mais ce rôle des forêts dans les processus hydrologiques à l'échelle régionale fait débat 83 …”

The effect of grass transpiration on the air temperature

“…Makarieva & Gorshkov (2007) showed that transpiring plants work as a biotic pump driving hydrological cycle on land. The activity of this pump is controlled by plant stomata (Saito et al 2003;Wallach et al 2010).…”