Dynamics of suspended sediment transport and yield in a large agricultural catchment, southwest France

The dynamics of suspended sediment transport were monitored continuously in a large agricultural catchment in southwest France from January 2007 to March 2009. The objective of this paper is to analyse the temporal variability in suspended sediment transport and yield in that catchment. Analyses were also undertaken to assess the relationships between precipitation, discharge and suspended sediment transport, and to interpret sediment delivery processes using suspended sediment-discharge hysteresis patterns. During the study period, we analysed 17 fl ood events, with high resolution suspended sediment data derived from continuous turbidity and automatic sampling. The results revealed strong seasonal, annual and inter-annual variability in suspended sediment transport. Sediment was strongly transported during spring, when frequent fl ood events of high magnitude and intensity occurred. Annual sediment transport in 2007 yielded 16 614 tonnes, representing 15 t km−2 (85% of annual load transport during fl oods for 16% of annual duration), while the 2008 sediment yield was 77 960 tonnes, representing 70 t km−2 (95% of annual load transport during fl oods for 20% of annual duration). Analysis of the relationships between precipitation, discharge and suspended sediment transport showed that there were signifi cant correlations between total precipitation, peak discharge, total water yield, fl ood intensity and sediment variables during the fl ood events, but no relationship with antecedent conditions. Flood events were classifi ed in relation to suspended sediment concentration (SSC)–discharge hysteretic loops, complemented with temporal dynamics of SSC–discharge ranges during rising and falling fl ow. The hysteretic shapes obtained for all flood events refl ected the distribution of probable sediment sources throughout the catchment. Regarding the sediment transport during all fl ood events, clockwise hysteretic loops represented 68% from river deposited sediments and nearby source areas, anticlockwise 29% from distant source areas, and simultaneity of SSC and discharge 3%

Fluvial transport of suspended sediment and organic carbon during flood events in a large agricultural catchment in southwest France.

Water draining from a large agricultural catchment of 1 110 km2 in southwest France was sampled over an 18-month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1·8 t km−2 y−1 and 0·7 t km−2 y−1, respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC-discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment