Flood triggering in Switzerland: the role of daily to monthly preceding precipitation

Abstract

Abstract. Determining the role of different precipitation periods for peak discharge generation is crucial for both projecting future changes in flood probability and for short-and medium-range flood forecasting. In this study, catchmentaveraged daily precipitation time series are analyzed prior to annual peak discharge events (floods) in Switzerland. The high number of floods considered -more than 4000 events from 101 catchments have been analyzed -allows to derive significant information about the role of antecedent precipitation for peak discharge generation. Based on the analysis of precipitation times series, a new separation of flood-related precipitation periods is proposed: (i) the period 0 to 1 day before flood days, when the maximum flood-triggering precipitation rates are generally observed, (ii) the period 2 to 3 days before flood days, when longer-lasting synoptic situations generate "significantly higher than normal" precipitation amounts, and (iii) the period from 4 days to 1 month before flood days when previous wet episodes may have already preconditioned the catchment. The novelty of this study lies in the separation of antecedent precipitation into the precursor antecedent precipitation (4 days before floods or earlier, called PRE-AP) and the short range precipitation (0 to 3 days before floods, a period when precipitation is often driven by one persistent weather situation like e.g., a stationary low-pressure system). A precise separation of "antecedent" and "peak-triggering" precipitation is not attempted. Instead, the strict definition of antecedent precipitation periods permits a direct comparison of all catchments.The precipitation accumulating 0 to 3 days before an event is the most relevant for floods in Switzerland. PRE-AP precipitation has only a weak and region-specific influence on flood probability. Floods were significantly more frequent after wet PRE-AP periods only in the Jura Mountains, in the western and eastern Swiss plateau, and at the outlet of large lakes. As a general rule, wet PRE-AP periods enhance the flood probability in catchments with gentle topography, high infiltration rates, and large storage capacity (karstic cavities, deep soils, large reservoirs). In contrast, floods were significantly less frequent after wet PRE-AP periods in glacial catchments because of reduced melt.For the majority of catchments however, no significant correlation between precipitation amounts and flood occurrences is found when the last 3 days before floods are omitted in the precipitation amounts. Moreover, the PRE-AP was not higher for extreme floods than for annual floods with a high frequency and was very close to climatology for all floods. The fact that floods are not significantly more frequent nor more intense after wet PRE-AP is a clear indicator of a short discharge memory of Pre-Alpine, Alpine and South Alpine Swiss catchments. Our study poses the question whether the impact of long-term precursory precipitation for floods in such catchments is not overestimated in the general ...

The influence of antecedent conditions on flood risk in sub-Saharan Africa

“…The role of weather-scale meteorological conditions (particularly rainfall) in flood generation is generally accepted (Webster et al, 2011;Jongman et al, 2014;Froidevaux et al, 2015). Our results showed that the flood events were preceded by 7-day precipitation (PRE7) of similar magnitude compared to the maximum observed 7-day precipitation during the same month in the no-flood years.…”

“…The length of the precipitation period leading to a flood depends highly on the local characteristics. For example, a 2-day precipitation sum is best correlated with flood frequency and magnitude in the high ranges of the Swiss Alps, but longer-duration precipitation affects flood occurrence more in the western and eastern Swiss Plateau (Froidevaux et al, 2015). Hence, we expect to be on the safe side by using a relatively long synoptic time window (7 days), similarly to Webster et al (2011).…”

“…For instance, evapotranspiration and soil saturation are considered important in flood forecasting (Sivapalan et al, 2005;Merz et al, 2006;Parajka et al, 2010;Fundel and Zappa, 2011). Reager et al (2014) demonstrated that basin-scale estimates of total water storage, including soil moisture, could be used to characterize regional flood potential for the Missouri 2011 floods several months in advance.…”

Stochastic generation of multi-site daily precipitation focusing on extreme events

“…For hydrological applications, it can be assumed that a precipitation model preserving these properties has a better chance of adequately reproducing flood properties for small sub-basins as well as for large basins. This statement is supported by empirical evidence provided by Froidevaux (2014) and Froidevaux et al (2015) for our study area (i.e., Switzerland). Using 60 years of gridded precipitation data, Froidevaux et al (2015) show that, in Switzerland, high discharge events are usually triggered by meteorological events with a duration of several days, in late summer and autumn.…”

“…For Switzerland, Froidevaux et al (2015) show that the generation of floods is mainly influenced by areal precipitation amounts accumulated over short periods (e.g., 1 to 3 days). These results are obtained by analyzing a wide variety of basins, their areas ranging from 10 to 12 000 km 2 .…”