Sediment and nutrient dynamics during storm events in the Enxoé temporary river, southern Portugal

Abstract

In temporary (or intermittent) rivers the first storm event after a dry period is responsible for transferring large amounts of sediment and nutrients into water reservoirs, thereby justifying close monitoring. The objective of this study was to analyse the contribution of storm events to sediment and nutrient transport in the Enxoé temporary river (southern Portugal) using detailed monitoring collected during three hydrological years (September, 2010 to August, 2013), and identify possible sediment and nutrient source areas based on the interpretation of hysteresis in the concentration–discharge relationship. The Enxoé River was monitored for suspended sediment concentration (SSC), total phosphorus (TP), particulate phosphorus (PP), soluble reactive phosphorus (SRP), and nitrate (NO3−). An empirical model was used to describe changes in solute concentrations, and the magnitude and rotational patterns of the hysteretic loops. Twenty-one storm events were registered. SSC, TP, PP, SRP, and NO3− concentrations varied between 1.6 and 3790.1, 0.05–11.4, 0–7.6, 0–0.67, and 0–27.84mgl−1, respectively. The highest SSC, TP, and PP concentrations were registered during the first storm event after an extended drought period. Annual sediment yields (13–480kgha−1y−1) and nitrate (4.4–45.5kgha−1y−1) were relatively low, while phosphorus losses (0.04–0.96kgha−1y−1) reached relatively high values during humid years. Sediment and phosphorus transport was influenced by the stream transport capacity and particle availability, whereas nitrate loads were influenced by rainfall, soil hydraulic characteristics, and land management. This work highlights the main processes involved in sediment and nutrients loads in a temporary river during storm events, with a quantification of the relevant elements.