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 represented by the topography of the basin and a negative exponential law linking the transmissivity of the soil with the distance to the saturated zone below the ground level. Although conceptual, this model is frequently 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 and Pacciani (J. Hydrol., 122: 161-219, 1991), a detailed analysis of the TOPMODEL is performed to arrive at a closer understanding of the correspondence of the assumptions underpinning the model with the physical 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 schematisation of the model. The various applications to real situations include the Sieve basin (river Arno tributary), which was used for the comparison of conceptual rainfall-runoff models described in the above-mentioned study by Franchini and Pacciani. This allows that analysis to be extended to the TOPMODEL.