Temporal variability of soil organic carbon transport in the Enxoé agricultural watershed.

The temporal variability of particulate (POC)and dissolved (DOC) organic carbon concentrations wasanalyzed in the Enxoe ́temporary river, southern Portugal,between September, 2010 and August, 2013. The overallaim was to study the variability of those elements duringstorm events, and determine their origin and the maintransfer mechanisms to the river. Twenty-one flood eventswere observed. An empirical model was used to describechanges in solute concentrations, and the magnitude androtational patterns of the hysteretic loops during floodevents. POC and DOC concentrations varied between0.49–88.93 and 0.25–25.75 mg L-1, respectively. POC andDOC annual yields varied between 0.06–2.15 and0.03–1.47 t km-2, respectively. Flood events had greatereffect in POC than in DOC variability. POC had mostly aterrestrial origin, with exports being related to soil erosionand runoff. POC revealed a flushing behavior during theentire monitored period, and clockwise or anticlockwisetrajectory loops whenever the predominant origin of theexports was in river bed deposits or arable lands, respec-tively. DOC had also a terrestrial origin, but it revealed acontrasting dilution behavior and, in general, anticlockwisehysteresis loops. DOC showed a delay in the arrival ofsolutes to the river, consistent with mass flow throughsubsurface flow. DOC exports were thus associated withsoil weathering and crop mineralization. This workhighlights the main processes involved in POC and DOCloads in a temporary river during flood events, with aprecise quantification of those elements.