干旱区生态环境知识资源中心

Investigating the role of forests for maintenance of the water cycle on land is critically important in the current situation of rapid global elimination of the natural vegetation cover. In this paper we contribute to the on-going discussion of the issue with two aspects. (1) Theoretical consideration of the water cycle on land reveals the importance of correct identification of independent and dependent terms in the water budget with respect to changing vegetation cover for understanding possible scenarios of water cycle change under anthropogenic impact. An important controlling influence of the vegetation cover is imposed on the outgoing fluxes of atmospheric moisture A(-) from land to the ocean, which is maximized in deserts and minimized in forested areas, while the dependencies for runoff and precipitation are the reverse. (2) Physical mechanisms allowing for efficient water retention and minimization of A(-) in forest ecosystems are investigated. Atmospheric water vapor is in aerostatic equilibrium when the temperature lapse rate is less than G = 1.9 K km(-1) and out of aerostatic equilibrium when G > 1.9 K km(-1). In the former case there are no vertical upward fluxes of the evaporated water. It is shown that the temperature profiles developed under the closed canopies of natural forests keep water vapor in aerostatic equilibrium preventing soil moisture loss to A(-), in contrast to the situation in open ecosystems like grasslands. The analyzed evidence allows one to conclude that an intensive water cycle on land can be restored after recovery of natural, self-sustained closed canopy ecosystems on continent-wide areas.