Sensitivity of hydrological models to uncertainty in rainfall input

Abstract

Different approaches used in hydrological modelling are compared in terms of the way each one takes the rainfall data into account. We examine the errors associated with accounting for rainfall variability, whether in hydrological modelling (distributed vs lumped models) or in computing catchment rainfall, as well as the impact of each approach on the representativeness of the parameters it uses. The database consists of 1859 rainfall events, distributed on 500 basins, located in the southeast of France with areas ranging from 6.2 to 2851 km². The study uses as reference the hydrographs computed by a distributed hydrological model from radar rainfall. This allows us to compare and to test the effects of various simplifications to the process when taking rainfall information (complete rain field vs sampled rainfall) and rainfall–runoff modelling (lumped vs distributed) into account. The results appear to show that, in general, the sampling effect can lead to errors in discharge at the outlet that are as great as, or even greater than, those one would get with a fully lumped approach. We found that small catchments are more sensitive to the uncertainties in catchment rainfall input generated by sampling rainfall data as seen through a raingauge network. Conversely, the larger catchments are more sensitive to uncertainties generated when the spatial variability of rainfall events is not taken into account. These uncertainties can be compensated for relatively easily by recalibrating the parameters of the hydrological model, although such recalibrations cause the parameter in question to completely lose physical meaning.

Citation Arnaud, P., Lavabre, J., Fouchier, C., Diss, S. & Javelle, P. (2011) Sensitivity of hydrological models to uncertainty of rainfall input. Hydrol. Sci. J. 56(3), 397–410.     

Multi-batch, incremental assembly of a dynamic magma chamber: the case of the Peninsula pluton granite (Cape Granite Suite, South Africa)

The S-type Peninsula Pluton (South Africa) exhibits substantial compositional variability and hosts a large variety of mafic and felsic magmatic enclaves with contrasting textures and compositions. Moreover, the pluton is characterized by mechanical concentrations of K-feldspar megacrysts, cordierite and biotite, generating a complex array of magmatic structures including schlieren, pipes, and spectacular sheeted structures. Chemical evidence indicates that the pluton is constructed incrementally by rapid emplacement of numerous magma pulses. Field, and textural data suggest that magmatic structures form by local flow at the emplacement level of highly viscous crystal-rich magmas (i.e. crystallinity up to 50?vol.%) through magma mushes assembled from older batches. At the time of arrival of relatively late magma batches, some areas within the pluton had achieved crystal fractions that allowed the material to act as a solid, whilst maintaining enough melt to prevent formation of sharp intrusional contacts. Magmatic structures represent “snapshots” of processes that operate in multiphase crystal-rich mushes and their genesis is due to mechanical and thermal instabilities in the crystal-rich magma chamber that are triggered by the emplacement of pulses of new magma derived from the melting of a compositionally variable metasedimentary source.