  The terrestrial water cycle links the soil and atmosphere moisture reservoirs
through four fluxes: precipitation, evaporation, runoff and atmospheric
moisture convergence. Each of these fluxes is essential for human and ecosystem
well-being. However, predicting how the water cycle responds to changes in
vegetation cover, remains a challenge (Lawrence and Vandecar, 2015; Ellison et
al., 2017; te Wierik et al., 2021). Recently, rainfall was shown to decrease
disproportionally with reduced forest transpiration following deforestation
(Baudena et al., 2021). Here, combining these findings with the law of matter
conservation, we show that in a sufficiently wet atmosphere forest
transpiration can control atmospheric moisture convergence such that increased
transpiration enhances atmospheric moisture import. Conversely, in a drier
atmosphere increased transpiration reduces atmospheric moisture convergence and
runoff. This previously unrecognized dichotomy can explain the seemingly random
observations of runoff and soil moisture sometimes increasing and sometimes
reducing in response to re-greening (e.g., Zheng et al., 2021). Evaluating the
transition between the two regimes is crucial both for characterizing the risk
posed by deforestation as well as for motivating and guiding global ecosystem
restoration.
