Space-time scaling properties of soil moisture and evapotranspiration in water-limited ecosystems

The spatial and temporal scaling properties of soil moisture and evapotranspiration in water-limited ecosystems are investigated. These scaling properties are critical for efforts to measure, model, and predict fundamental ecohydrologic processes. Simulation studies yield insight into the dependence of space-time scaling on soil, vegetation, and climate characteristics. Soil moisture and transpiration are first examined at the plant scale to improve predictions through a better understanding of the temporal scaling of rainfall. Simulation results demonstrate that rainfall data resolved at the daily level allow accurate prediction of soil moisture and transpiration dynamics for smaller time resolutions. Furthermore, ecohydrologic dimensionless numbers are shown to be accurate indicators for assessment of rainfall-data resolution.; Soil-moisture scaling is then investigated to advance the understanding of the relationships among soil-moisture values integrated over different soil depths. This analysis is motivated by recent research that attempts to improve hydrologic models through incorporation of soil-moisture values inferred from satellite measurements. A detailed model reveals that a non-unique, dynamic relationship exists between instantaneous soil-moisture values averaged over different depths, and it evolves in time as a function of the hydrologic inputs and soil and vegetation characteristics. While the structure of the relationship is evident with high sampling frequency, its multi-valued functional form is unknown. Statistical measures can overcome this limitation, and a framework is presented that predicts the mean of soil moisture as a function of averaging depth.; The final research component concerns spatial scales for which ecohydrologic properties and climatic forcings vary over the land surface. The effects of spatial heterogeneity in precipitation and vegetation are examined with results demonstrating that relationships between spatially averaged variables are non-unique due to hysteresis. These complex relationships evolve with increasing averaging area based on the characteristics of the heterogeneity. A methodology is then presented to upscale functional relationships controlling the soil-water balance based on simulation data. A threshold scale for soil moisture and evapotranspiration is also identified, beyond which the non-unique relationships will vary only slowly as averaging area becomes larger. This threshold scale is then related to parameters that characterize the scale of the dominant heterogeneity.