Effects of Spatial Heterogeneity in Rainfall and Vegetation Type on Soil   Moisture and Evapotranspiration
  Nonlinear plant-scale interactions controlling the soil-water balance are generally not valid at larger spatial scales due to spatial heterogeneity in rainfall and vegetation type. The relationships between spatially averaged variables are hysteretic even when unique relationships are imposed at the plant scale. The characteristics of these hysteretic relationships depend on the size of the averaging area and the spatial properties of the soil, vegetation, and rainfall. We upscale the plant-scale relationships to the scale of a regional land-surface model based on simulation data obtained through explicit representation of spatial heterogeneity in rainfall and vegetation type. The proposed upscaled function improves predictions of spatially averaged soil moisture and evapotranspiration relative to the effective-parameter approach for a water-limited Texas shrubland. The degree of improvement is a function of the scales of heterogeneity and the size of the averaging area. We also find that single-valued functions fail to predict spatially averaged leakage accurately. Furthermore, the spatial heterogeneity results in scale-dependent hysteretic relationships for the statistical-dynamic and Montaldo & Albertson approaches. 
Michael J. Puma, Michael A. Celia, Ignacio Rodriguez-Iturbe, Jan M.   Nordbotten, Andrew J. Guswa, and Dmitri Kavetski
physics.geo-ph