  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.
