Vegetation regulates streamflow intra-annual variability by adapting to climate variations

Abstract:

This study aims to provide a mechanistic explanation of the empirical findings on the emergent patterns of streamflow intra-annual variability reported in a companion data-driven study across the contiguous United States. A mathematical extension of the Budyko formula with explicit accounts for the soil moisture storage change is introduced with a focus on the intra-annual variability of streamflow. The mathematical extension is then used to systematically examine the relative contributions of the intra-annual variability of precipitation, potential evaporative energy, soil water storage change and their co-variances. It is shown that, apart from aridity index whose importance has been recognized before, the variance of soil water storage change, its covariance with precipitation and the coupling of the seasonality in precipitation and potential evaporation are relatively more influential than the other factors. More importantly, the vital role of vegetation affecting streamflow intra-annual variability through regulating soil moisture storage is revealed under different climate conditions by using Leaf Area Index as an indicator of the above ground biomass directly and plant root system indirectly. In Mediterranean catchments where the water and energy cycles are out of phase, more persistent vegetation types (i.e. evergreen forests) are established with advanced root system to maximize the usage soil moisture. While in less humid catchments where precipitation and potential evaporation are more synchronous, recurrent vegetation types (i.e. deciduous forests, pastures) take over the dominance. Moreover, the two emerging classes in how vegetation correlates to the synchronism of EP and P suggest the recurring influence of aridity index: vegetation in arid catchments inclines to be more efficient in water usage to maintain a more persistent above ground gross production.