Soil evaporation measurements from bare soil and shaded soil under an agroforestry tree canopy were used to construct a model to predict soil evaporation with and without tree shade. It was found that a simple daily time step model based on the Ritchie (1972)approach was unable to predict daily soil evaporation accurately, but was capable of providing good estimates of cumulative soil evaporation over hydrologically significant periods (weeks–months). This model was used to show how trees could reduce annual soil evaporation directly beneath their canopy by an average of 35% (compared to completely bare soil), equivalent to 21% of rainfall. In sparse agroforestry tree canopies the area average saving is smaller, depending on tree leaf area index (LAI). The model also demonstrated how annual saving in soil evaporation due to a tree canopy might vary with rainfall, with a maximum of around 180 mm being achieved once rainfall exceeded 1000 mm year1. This saving in soil water is very significant and will help offset the enhanced evaporative losses associated with tree canopies due to interception and re-evaporation of rainfall or as tree transpiration
Tag: evaporation
The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence
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 exponentially 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 findings 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 explains 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 moisture recycling due to additional vegetation increases precipitation but decreases local water yield and steady-state runoff. Therefore, in the drier regions/periods and early stages of ecological restoration, the role of vegetation can be confined to moisture recycling, while once a wetter stage is achieved, additional vegetation enhances atmospheric moisture convergence. Evaluating the transition between regimes, and recognizing the potential of vegetation for enhancing moisture convergence, are crucial for characterizing the consequences of deforestation as well as for motivating and guiding ecological restoration.
How transpiration by forests and other vegetation determines alternate moisture regimes
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.