THE TOPKAPI MODEL by E. TODINI and L. CIARAPICA Department of Earth and Geo-Environmental Sciences University of Bologna, Bologna, Italy ABSTRACT: The paper introduces a new distributed rainfallrunoff model derived upon the assumption that the horizontal flow at a point in the soil and over the surface can be approximated by means of a kinematic wave model. The point assumption is then integrated up to a finite pixel dimension, thus converting the original differential equation into a non-linear reservoir equation based upon physically meaningful parameters, which solution can be found numerically. The catchment behaviour is finally obtained by aggregating the non-linear reservoirs into three cascades, representing the soil, the surface and the drainage network, following the topographic and geomorphologic elements of the catchment. The main advantage of this approach lies in the possibility of deriving the model structure and the parameters on the basis of digital elevation maps, soil maps, land use maps etc.. and in its capability of being applied at increasing spatial scales without loosing model and parameters physical interpretation. These properties make the model suitable for land-use and climatic change impact assessment; for extreme value analysis, given the possibility of its extension to ungauged catchments; and last but not least as a promising tool for the General Circulation Models (GCMs).
1. INTRODUCTION The idea of developing the new rainfall-runoff model arises from the experience gained in years of applied research as well as from the need of finding a model which could be easy to calibrate from available maps and valid at different spatial scales. Due to the generally large computer time requirements of distributed models, in the past, lumped and semi-distributed (a cascade of lumped) models (Todini, 1989) have been widely