PHYSICAL INTERPRETATION AND SENSITIVITY ANALYSIS OF THE TOPMODEL

The TOPMODEL is a variable contributing area conceptual model in which the predominant factors determining the formation of runoff are repre sented by the topography of the basin and a negative exponential law l inking the transmissivity of the soil with the distance to the saturat ed zone below the ground level. Although conceptual, this model is fre quently described as a 'physically based model' in the sense that its parameters can be measured directly in situ. In line with the analysis of various conceptual rainfall-runoff models conducted by Franchini a nd Pacciani (J. Hydrol., 122: 161-219, 1991), a detailed analysis of t he TOPMODEL is performed to arrive at a closer understanding of the co rrespondence of the assumptions underpinning the model with the physic al reality and, in particular, the role that topographic information ( expressed by the topographic index curve) and the nature of the soil ( expressed by saturated hydraulic conductivity and its decay with soil depth), have within the model itself. Also investigated is the extent to which the model parameters actually reflect the physical properties to which they refer and how far their values offset the inevitable sc hematisation of the model. The various applications to real situations include the Sieve basin (river Arno tributary), which was used for th e comparison of conceptual rainfall-runoff models described in the abo ve-mentioned study by Franchini and Pacciani. This allows that analysi s to be extended to the TOPMODEL.