THE SENSITIVITY OF HYDROLOGICAL MODELS TO SPATIAL RAINFALL PATTERNS -AN EVALUATION USING OBSERVED DATA

Spatial variability of rainfall is often considered as a major source of temporal variability in the resulting basin hydrograph. Since direc t experimental evidence is not available, this must be verified throug h a modelling approach, provided adequate data are available. A semi-d istributed version of TOPMODEL has therefore been applied to the Real Collobrier experimental basin (71 km2 in southeast France with 21 reco rding raingauges) using an hourly time step and a series of independen t events. First, a set of reference results has been built under the a ssumption of spatial uniformity for the rainfall. Two different densit ies of network have been tested (including 5 or 21 gauges), showing a significant advantage for the dense network rainfall estimate. Next, t he spatial variability of the rainfall field has been tested and confi rmed, with commonly a factor of 3 between simultaneous average rainfal l over subcatchment areas of 6-8 km2. However, the model response refl ects this spatial variability only in secondary peaks which are usuall y an order of magnitude smaller than the bulk of the hydrograph and no t always present in the observed discharges. An extended discussion co nsiders if these results may be dependent on the model or on the setti ng up of the numerical experiments. In fact, it seems that the spatial variability of rainfall, although important, is not sufficiently orga nized in time and space to overcome the effects of smoothing and dampe ning when running off through this rural medium-sized catchment. Such results may not hold for smaller urbanized areas or larger rural basin s.