Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 13 12 2009 10.5194/hess-13-2399-2009 http://www.hydrol-earth-syst-sci.net/13/2399/2009/ http://www.hydrol-earth-syst-sci.net/13/2399/2009/hess-13-2399-2009.html http://www.hydrol-earth-syst-sci.net/13/2399/2009/hess-13-2399-2009.pdf 2399 2412 2009-12-14 Reducing scale dependence in TOPMODEL using a dimensionless topographic index A. Ducharne agnes.ducharne@upmc.fr Laboratoire Sisyphe, CNRS/UPMC, Paris, France This paper stems from the fact that the topographic index used in TOPMODEL is not dimensionless. In each pixel i in a catchment, it is defined as xi=ln(ai/Si), where ai is the specific contributing area per unit contour length and Si is the topographic slope. In the SI unit system, ai/Si is in meters, and the unit of xi is problematic. We propose a simple solution in the widespread cases where the topographic index is computed from a regular raster digital elevation model (DEM). The pixel length C being constant, we can define a dimensionless topographic index yi=xi-lnC. Reformulating TOPMODEL equations to use yi instead of xi helps giving the units of all their terms and emphasizes the scale dependence of these equations via the explicit use of C outside from the topographic index, in what can be defined as the transmissivity at saturation per unit contour length T0/C. The term lnC defines the numerical effect of DEM resolution, which contributes to shift the spatial mean x of the classical topographic index when the DEM cell size C varies. A key result is that both the spatial mean y of the dimensionless index and T0/C are much more stable with respect to DEM resolution than their counterparts x and T0 in the classical framework. This shows the importance of the numerical effect in the dependence of the classical topographic index to DEM resolution, and reduces the need to recalibrate TOPMODEL when changing DEM resolution. 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