SIGNIFICATIVITE DES TENDANCES PLUVIOMÉTRIQUES PLURIANNUELLES

The significance of multiannual pluviometrie trends . The interpretation of the climatic trends quoted in the literature is often influenced by the method used, by the length of the series and by the way h ow the components of the temporal series have been selecte d. This is why this paper examines the annual preci pitation sums from 20 European stations from the point of vi ew of several stochastic hypotheses. The global tre nd, determined by polynomial adjustment over the whole time series, does not always coincide for the last subperiod (10-20 years) with the direction of the tren d of the sub-period in question. The differentiatio n method has technical advantages (low error risk due to the technique used), but it does not allow the visuali sation and interpretation of the dynamics of the mean level of the series globally or by sub-periods. This paper demonstrates that the series of annual precipitation can be considered as a stochastic non-ergodic process c omposed of an ergodic stationary series and an aleatory var iable determined by the existence of trends common to sequential series. Only sequential trends are free of the subjectivism resulting from the increase or decrease of the length of temporal series. By the elimination o f the component of sequential trends, in order to s tationarise the series, the global trend is eliminated automati cally. This approach allows stochastic modelling wi th a lower risk of producing errors due to work methods; moreo ver, it has the advantage to allow the visualisatio n of the dynamics of the mean level of the series.