This paper presents the model development component of a body of research which addresses aggregation and scaling in multiscale hydrological modeling. Water and energy balance models are developed at the local and catchment scales and at the macroscale by aggregating a simple soil-vegetation-atmosphere transfer scheme (SVATS) across scales in a topographic framework. A spatially distributed approach is followed to aggregate the SVATS to the catchment scale. A statistical-dynamical approach is utilized to simplify the large-scale modeling problem and to aggregate the SVATS to the macroscale. The resulting macroscale hydrological model is proposed for use as a land surface parameterization in atmospheric models. It differs greatly from the current generation of land surface parameterizations owing to its simplified representation of vertical process physics and its statistical representation of horizontally heterogeneous runoff and energy balance processes. The spatially distributed model formulation is explored to understand the role of spatial variability in determining areal-average fluxes and the dynamics of hydrological processes. The simpler macroscale formulation is analyzed to determine how it represents these important dynamics, with implications for the parameterization of runoff and energy balance processes in atmospheric models.

This paper explores the effects of spatial variability and scale on areally averaged evapotranspiration. A spatially distributed water and energy balance model is employed to determine the effect of explicit patterns of land surface characteristics and atmospheric forcing on areally averaged evapotranspiration over a range of increasing spatial scales. The analysis is performed from the local scale to the catchment scale. The study area is King's Creek catchment, an 11.7 km² watershed located on the native tallgrass prairie of Kansas. It is shown that a threshold scale, or representative elementary area (REA) exists for evapotranspiration modeling. It is shown further that the dominant controls on the scaling behavior of catchment-average evapotranspiration, and thus the size of the REA, depend on the dominant controls on its components (bare-soil evaporation, wet canopy evaporation, and dry canopy transpiration) and whether evapotranspiration is occurring at potential rates or soil- and vegetation-controlled rates. The existence of an REA for evapotranspiration modeling suggests that in catchment areas smaller than this threshold scale, actual patterns of model parameters and inputs may be important factors governing catchment-scale evapotranspiration rates in hydrological models. In models applied at scales greater than the REA scale, spatial patterns of dominant process controls can be represented by their statistical distribution functions. It appears that some of our findings are fairly general and will therefore provide a framework for understanding the scaling behavior of areally averaged evapotranspiration at the catchment and larger scales. Our results may have further implications for representing subgrid-scale land surface heterogeneity in hydrological parameterizations for atmospheric models.

The models presented in the previous paper (Famiglietti and Wood, this issue) are applied at their appropriate scales for evapotranspiration modeling at the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) site. The local soil-vegetation-atmospheric transfer scheme is applied at five flux measurement stations in the northwest quadrant of the FIFE site. Simulations were performed for three of the four FIFE “golden (cloud-free) days” with good results. The spatially distributed model was applied at the 11.7-km² King's Creek catchment, also located in the northwest quadrant of the FIFE site, during FIFE Intensive Field Campaigns (IFCs) 1–4. Simulated catchment average evapotranspiration was compared to an average of observations made at the five aforementioned measurement stations with good results. The macroscale formulation was applied to both the King's Creek catchment and the entire 15-km FIFE site for evapotranspiration simulations. Macroscale model simulations for King's Creek were nearly identical to the spatially distributed results, implying that at this location and at this scale, the assumptions invoked in the development of the macroscale formulation are reasonable. The macroscale model was also employed to simulate evapotranspiration from the entire 15-km site for the four golden days. Simulated evapotranspiration rates show reasonably good agreement with the 22-station average of observations. However, it is suggested that at 15-km and larger scales, simulation error may arise as a result of the macroscale assumptions of areally averaged atmospheric forcing, vegetation parameters, soil parameters, and the methods by which these data and other flux observations are aggregated. A methodology to combat these problems at larger scales is reviewed.

A catchment scale water balance model is presented and used to predict evaporation from the King's Creek catchment at the First ISLSCP Field Experiment site on the Konza Prairie, Kansas. The model incorporates spatial variability in topography, soils, and precipitation to compute the land surface hydrologie fluxes. A network of 20 rain gages was employed to measure rainfall across the catchment in the summer of 1987. These data were spatially interpolated and used to drive the model during storm periods. During interstorm periods the model was driven by the estimated potential evaporation, which was calculated using net radiation data collected at site 2. Model-computed evaporation is compared to that observed, both at site 2 (grid location 1916-BRS) and the catchment scale, for the simulation period from June 1 to October 9, 1987.