SPATIAL EXTENSION OF EXTREME RAINFALL EVENTS - RETURN PERIOD OF ISOHYETS AREA AND INFLUENCE OF RAIN GAUGES NETWORK EVOLUTION

Those last 10 yrs, many cities in southern Europe have been affected b y heavy rainfall events leading to severe runoffs. Thus, the assessmen t of rainfall risk needs a better knowledge of the climate hazards and particularly the rainfall hazards. A spatial stochastic analysis is p roposed here, based on rainfall events selected during the 1958-1993 p eriod in the South of France, using the French weather service daily r ainfall database. The isohyet areas defined at various rainfall thresh olds tau are estimated. Because of the sampling constraints, two diffe rent time steps have been considered: 24 and 48 h. A two parameters ga mma distribution law has been fitted to each sample of isohyet areas, and the two parameters are related to the threshold tau. For those 36 yrs, the evolution of the network of rain gauges in space and time has been described. The empirical expression of the probability to observ e a given rainy area with a given network configuration, using simulat ion, has been proposed. The bias introduced in sampling events has bee n estimated and it is shown that its influence on isohyet areas quanti les could be negligible, for studies based on daily rainfall gauges. ( C) 1997 Elsevier Science B.V.