  The terrestrial water cycle links the soil and atmosphere moisture reservoirs
through four fluxes: precipitation, evaporation, runoff, and atmospheric
moisture convergence (net import of water vapor to balance runoff). Each of
these processes is essential for human and ecosystem well-being. Predicting how
the water cycle responds to changes in vegetation cover remains a challenge.
Recently, changes in plant transpiration across the Amazon basin were shown to
be associated disproportionately with changes in rainfall, suggesting that even
small declines in transpiration (e.g., from deforestation) would lead to much
larger declines in rainfall. Here, constraining these results by the law of
mass conservation, we show that in a sufficiently wet atmosphere, forest
transpiration can control atmospheric moisture convergence such that increased
transpiration enhances atmospheric moisture import and resulting water yield.
Conversely, in a sufficiently dry atmosphere increased transpiration reduces
atmospheric moisture convergence and water yield. This previously unrecognized
dichotomy can explain the otherwise mixed observations of how water yield
responds to re-greening, as we illustrate with examples from China's Loess
Plateau. Our analysis indicates that any additional precipitation recycling by
additional vegetation increases precipitation but decreases local water yield
and steady-state runoff. Therefore, in the drier regions and early stages of
ecological restoration, the role of vegetation can be confined to precipitation
recycling, while once a wetter stage is achieved, additional vegetation
enhances atmospheric moisture convergence and water yield. Evaluating the
transition between regimes, and recognizing the potential of vegetation for
enhancing moisture convergence, are crucial for characterizing the consequences
of deforestation and for motivating and guiding ecological restoration.
