Catchment size and contribution of the largest daily events to suspended sediment load on a continental scale

Abstract

Abstract The classic approach defines an extreme event as a rare event identified by magnitude–frequency analysis and quantified by its deviation from a central value. They are key to understand geomorphological dynamics, since they are responsible for a considerable amount of work and “time compression”. Time compression means that most of the geomorphic work (particularly sediment transport) is produced in very short temporal intervals (i.e. in few events). Moreover, it is well known from magnitude–frequency analyses that events not necessarily extreme by magnitude could be responsible for a large amount of geomorphic work. To analyse the time compression of geomorphological processes, a new approach has recently been suggested, based on the effects of “largest events”, defined by rank, whatever the magnitude. In this paper, we analyse the relation between the relative contribution of the largest daily event to the total suspended sediment load in relation to catchment size at the continental scale. For this purpose, we used two massive datasets of daily suspended sediment load facilitated by the United States Geological Survey (i.e. USGS Ancillary database), and the Hydrological Service of Canada (i.e. HYDAT dataset). The whole study area covers most of the temperate and sub-polar latitudinal climate conditions. Original data include information from more than 1800 catchments comprising appr. 3 × 10 6 daily events. The data show that the contribution of few events represents a high proportion of the total basin suspended sediment load. For instance, the mean contribution of the 25-largest daily events varies between 63% and 46% of the total load depending on the basin area, while the mean contribution of just the 5-largest events varies between 39% and 23%. The analysis suggests that the relative contribution of different n-largest aggregated daily events on total suspended sediment load decreases as catchment size increases. This is a general pattern and means that, for instance, time compression of suspended load is exacerbated in small catchments; hence, relatively few events are the key to explain the catchment sediment export. In addition, the relative contribution of the n-largest daily event to the suspended sediment load seems to be independent of climate conditions in small catchments, particularly in those 2 .