Canopy vs. Roots: Production and Destruction of Variability in Soil Moisture and Hydrologic Fluxes

The leaves and stems of forest canopies intercept and redistribute precipitation in space. Many investigations have demonstrated that spatial patterns of throughfall and stemflow are persistent in time, and this produces wet and dry spots in the soil. At the same time, root uptake for transpiration acts to destroy this variability. This homogenization is enhanced by root compensation (extraction at high rates from wet regions) and hydraulic redistribution (transport of water from wet soils to dry via the roots). Because many hydrologic and biogeochemical processes are nonlinear functions of soil moisture, an understanding of the relative strength of the production and destruction of spatial variability is necessary to represent those processes at larger scales. The creation and reduction of spatial variability is investigated through stochastic modeling of soil-moisture dynamics. This work investigated the combined effects of canopy interception and root uptake on the water balance, the localization of recharge, the variability of soil moisture in time and space, and the upscaled relationship between plant uptake and mean soil moisture. Interception and plant uptake counterbalance each other to some extent with respect to the water balance and average hydrologic fluxes, although there may be some conditions for which one process dominates. In contrast, canopy interception has a noticeable effect on recharge localization and the horizontal variability of soil moisture that cannot be undone by root processes. Thus, this variability may need to be accounted for to properly represent biogeochemical processes that are nonlinear functions of soil moisture. In all cases, the particular results depend on the strength of the canopy and root processes, along with the characteristics of climate, soil, and vegetation.