On Subgrid-Scale Variations in Root Zone Moisture and Evapotranspiration

Soil moisture varies on a scale much smaller than the finest climate model resolution. Sellers et al. (2007) presents the Bins approach for representing sub-grid variability of soil moisture in a climate model. The Bins approach is shown to improve simulation of ET variations in SiB3 (Baker, Sellers et al. 2017). We present a new stochastic parameterization of hydraulic conductivity, and show the sensitivity of variations in near-surface soil moisture and runoff to variations in permeability of the subsurface. In regions with weathered bedrock, subsurface fractures and macropores could be non-trivial deep stores of moisture. An investigation of the sensitivity of ET to rooting profiles, root water uptake efficiency and hydraulic redistribution shows that it is the 10% of plant root mass below 4m that may access the deep moisture and sustain transpiration through prolonged dry periods. A small negative feedback exists in the root zone, where the depletion of moisture by ET decreases hydraulic conductivity and enhances the retention of moisture.