A network-indexbased version of TOPMODEL for use with high-resolution digital topographic data

This paper describes the preliminary development of a network index approach to modify and to extend the classic TOPMODEL. Application of the basic Beven and Kirkby (1979) form of TOPMODEL to high resolution (2.0 m) laser altimetric data (based upon the U.K. Environment Agency’s Light Detection and Ranging (LIDAR) system) to a 13.8 km(2) catchment in an upland environment identified large areas of saturated areas that remained unconnected from the drainage network even during an extreme flood event. This is shown to be a particular problem with using high resolution topographic data especially over large spatial scales. To deal with the hydrological consequences of disconnected areas, we present a simple network index modification in which saturated areas only connect when the topographic index is sufficient for there to be zero or negative saturation deficits along a complete flow path. This is combined with an enhanced method for dealing with the problem of pits and hollows which is shown to become more acute with higher resolution topographic data. The paper concludes by noting the implications of the research as presented for both methodological and substantive research that is currently under way.