Aggregation and scaling of spatially variable hydrological processes: Local, catchment-scale and macroscale models of water and energy balance

This thesis addresses aggregation and scaling issues in hydrological modeling while presenting water and energy balance models at the local, catchment, and macroscales. A local water and energy balance model is presented first. Local hydrologic fluxes are modeled by coupling simple representations of atmospheric forcing, vertical soil moisture transport, plant control of transpiration, and lateral subsurface flow. The sensitivity of evapotranspiration and runoff are investigated with respect to variable soil properties, vegetation characteristics, and initial root zone soil moisture.; The local model is aggregated up to the catchment scale using a spatially-explicit geographic information system (GIS) approach. A catchment is discretized into a number of grid elements. A GIS is coregistered with the catchment grid elements. The local model is applied at each grid element of the catchment. Explicit spatial variability is incorporated into the model by allowing all model parameters, inputs, and outputs to vary between grid elements of the GIS. Catchment grid elements are coupled to each other using simple expressions for lateral subsurface flow. The catchment-scale hydrologic fluxes are simply the average of the individual grid-element fluxes. Modeled hydrologic fluxes are compared to those observed during the First ISLSCP Field Experiment (FIFE).; A statistical aggregation procedure is also presented. Large-scale fluxes are determined by aggregating local fluxes with respect to a statistical distribution of combined soil and topographic parameters. The resulting statistical-dynamical model incorporates spatial variability in topography and soils, and thus soil moisture, canopy resistance, and the hydrologic fluxes. This model is proposed for use as a grid-scale land-hydrology parameterization in regional and global atmospheric models, as well as for off-line studies of water and energy balance. Model computed evapotranspiration and streamflow is compared to observed water and energy balance data collected at FIFE.; The interrelationship between the aggregation and scale problems is explored. The scales at which spatially-variable hydrological processes must be represented explicitly, using the GIS approach (explicit aggregation), or statistically, using the statistical-dynamical approach (statistical aggregation), are investigated.