Abstract
Abstract. How long a river remembers its past is still an open question. Perturbations occurring in large catchments may impact the flow regime for several weeks and months, therefore providing a physical explanation for the occasional tendency of floods to occur in clusters. The research question explored in this paper may be stated as follows: can higher than usual river discharges in the low flow season be associated to a higher probability of floods in the subsequent high flow season? The physical explanation for such association may be related to the presence of higher soil moisture storage at the beginning of the high flow season, which may induce lower infiltration rates and therefore higher river runoff. Another possible explanation is persistence of climate, due to presence of long-term properties in atmospheric circulation. We focus on the Po River at Pontelagoscuro, whose catchment area amounts to 71 000 km2. We look at the stochastic connection between average river flows in the pre-flood season and the peak flows in the flood season by using a bivariate probability distribution. We found that the shape of the flood frequency distribution is significantly impacted by the river flow regime in the low flow season. The proposed technique, which can be classified as a data assimilation approach, may allow one to reduce the uncertainty associated to the estimation of the flood probability.
Highlights
Perturbations occurring in large catchments may impact the flow regime for several weeks and months, providing a physical explanation for the occasional tendency of floods to occur in clusters (Montanari, 2012)
With the idea that extreme floods may be induced by long term stress, rather than a short sequence of extreme rainfall, this paper explores the following research question: can higher than usual river discharges in the low flow season be associated to a higher probability of floods in the subsequent high flow season? An application in the Po River is carried out in order to set up a methodology to update the uncertainty associated to the estimation of flood occurrence probability
In order to infer the actual impact of the dependence between peak flows and average flow in the low flow season, the unconditioned flood frequency distribution and the updated distributions inferred for several levels higher-than-average values of mean flow (e.g. 70, 80, and 95 % quantiles) in the pre-flood season were compared
Summary
Perturbations occurring in large catchments may impact the flow regime for several weeks and months, providing a physical explanation for the occasional tendency of floods to occur in clusters (Montanari, 2012). The physical explanation for such association may be related to the presence of higher than usual soil moisture storage, which may induce lower infiltration rates and higher river runoff. Another possible explanation is persistence of climate, due to presence of long-term properties in atmospheric circulation. It is well known that river flows are affected by forms of persistence that are not fully understood yet (O’Connell et al, 2015). These are referred to as the “Hurst Phenomenon”, or the “Hurst Effect”.
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