The sensitivity of hydrological models to spatial rainfall patterns: an evaluation using observed data

Abstract

Spatial variability of rainfall is often considered as a major source of temporal variability in the resulting basin hydrograph. Since direct experimental evidence is not available, this must be verified through a modelling approach, provided adequate data are available. A semidistributed version of TOPMODEL has therefore been applied to the Réal Collobrier experimental basin (71 km 2 in southeast France with 21 recording raingauges) using an hourly time step and a series of independent events. First, a set of reference results has been built under the assumption of spatial uniformity for the rainfall. Two different densities of network have been tested (including 5 or 21 gauges), showing a significant advantage for the dense network rainfall estimate. Next, the spatial variability of the rainfall field has been tested and confirmed, with commonly a factor of 3 between simultaneous average rainfall over subcatchment areas of 6–8 km 2. However, the model response reflects this spatial variability only in secondary peaks which are usually an order of magnitude smaller than the bulk of the hydrograph and not always present in the observed discharges. An extended discussion considers if these results may be dependent on the model or on the setting up of the numerical experiments. In fact, it seems that the spatial variability of rainfall, although important, is not sufficiently organized in time and space to overcome the effects of smoothing and dampening when running off through this rural medium-sized catchment. Such results may not hold for smaller urbanized areas or larger rural basins.