Temporal variability of nitrate transport through hydrological response during flood events within a large agricultural catchment in south-west France.

The temporal variability of nitrate transport was monitored continuously in a large agricultural catchment, the 1110km(2) Save catchment in south-west France, from January 2007 to June 2009. The overall aim was to analyse the temporal transport of nitrate through hydrological response during flood events in the catchment. Nitrate loads and hysteresis were also analysed and the relationships between nitrate and hydro-climatological variables within flood events were determined. During the study period, 19 flood events were analysed using extensive datasets obtained by manual and automatic sampling. The maximum NO(3)(-) concentration during flood varied from 8.2mgl(-1) to 41.1mgl(-1) with flood discharge from 6.75m(3)s(-1) to 112.60m(3)s(-1). The annual NO(3)(-) loads in 2007 and 2008 amounted to 2514t and 3047t, respectively, with average specific yield of 2.5tkm(-12)yr(-1). The temporal transport of nitrate loads during different seasonal flood events varied from 12t to 909t. Nitrate transport during flood events amounted to 1600t (64% of annual load; 16% of annual duration) in 2007 and 1872t (62% of annual load; 20% of annual duration) in 2008. The level of peak discharge during flood events did not control peak nitrate concentrations, since similar nitrate peaks were produced by different peak discharges. Statistically strong correlations were found between nitrate transport and total precipitation, flood duration, peak discharge and total water yield. These four variables may be the main factors controlling nitrate export from the Save catchment. The relationship between nitrate and discharge (hysteresis patterns) investigated through flood events in this study was mainly dominated by anticlockwise behaviour.