A framework for scaling of hydrologic conceptualizations based on a disaggregation-aggregation approach

A fundamental question in scale research is how to scale up descriptions of hydrological responses from the small scales at which they are developed to the larger scales at which predictions are required or can be validated, in the presence of spatial heterogeneity of soils, vegetation and topography, and of space-time variability of climatic inputs. One scaling approach is the disaggregation-aggregation approach, which involves disaggregating catchment-scale state variables to point-scale distributions, applying a suitable point-scale physical model using these state variables, and aggregating the resulting point-scale responses to yield a catchment-scale response. Thus, the approach provides a way of linking the catchment-scale state variables with the catchment-scale responses and thereby permits the development of empirical large-scale models that still retain some essence of the small-scale physics. In this paper we illustrate the disaggregation-aggregation approach with two examples that deal with developing catchment-scale conceptualizations of (a) infiltration excess runoff and (b) evapotranspiration, using detailed process-based models available at the small scale. These conceptualizations became the building blocks of the large-scale catchment model, LASCAM.