Abstract:Variability of suspended sediment concentration (SSC) versus discharge relationships in streams is often high and illustrates variable particle origins or availability. Particle availability depends on both new sediment supply and deposited sediment stock. The aim of this study is to improve SSC-discharge relationship interpretation, in order to determine the origins of particles and to understand the temporal dynamics of particles for two small streams in agricultural catchments from northwestern France. SSC and discharge were continuously recorded at the outlets and data were examined at different time-scales: yearly, monthly, with distinction between flood periods and non-flooding periods, and individual flood events. Floods are classified in relation to SSC-discharge hysteresis, and this typology is completed by the analysis of SSC-discharge ranges during rising and falling flow. We show that particles are mainly coming from channel, banks, either by hydraulic erosion or by cattle trampling. Particle availability presents a seasonal dynamics with a maximum at the beginning of autumn when discharge is low, decreasing progressively during autumn to become a minimum in winter when discharge is the highest, and increasing again in spring. Bank degradation by cattle is the determining factor in the suspended sediment dynamics. Cattle bank-trampling produces sediment, mostly from spring to autumn, that supplies the deposited sediment stock even outside floods. This hydrologically independent process hides SSC-discharge correlation classically linked to hydraulic erosion and transport. Differences in SSC-discharge relationships and suspended sediment budgets between streams are related to differences in transport capacity and bank degradation by cattle trampling and channelization.

International audienceThis paper investigates particulate phosphorus (PP) and soluble reactive phosphorus (SRP) concentrations at the outlet of a small(5 km²) intensively farmed catchment to identify seasonal variability of sources and transport pathways for these two phosphorusforms. The shape and direction of discharge-concentration hystereses during floods were related to the hydrological conditions inthe catchment during four hydrological periods. Both during flood events and on an annual basis, contrasting export dynamicshighlighted a strong decoupling between SRP and PP export. During most flood events, discharge-concentration hystereses forPP were clockwise, indicating mobilization of a source located within or near the stream channel. Seasonal variability of PPexport was linked to the availability of stream sediments and the export capacity of the stream. In contrast, hysteresis shapes forSRP were anticlockwise, which suggests that SRP was transferred to the stream via subsurface flow. Groundwater rise in wetlandsoils was likely the cause of this transfer, through the hydrological connectivity it created between the stream and P-rich soilhorizons. SRP concentrations were the highest when the relative contribution of deep groundwater from the upland domain waslow compared with wetland groundwater. Hence, soils from non-fertilized riparian wetlands seemed to be the main source ofSRP in the catchment. This conceptual model of P transfer with distinct hydrological controls for PP and SRP was validthroughout the year, except during spring storm events, during which PP and SRP exports were synchronized as a consequenceof overland flow and erosion on hillslopes

of stream water quality response to storm events captured using high-frequency and multi-parameter data,

International audiencePhosphorus (P) transfer during storm events represents a significant part of annual P loads in streams and contributes to eutrophication in downstream water bodies. To improve understanding of P storm dynamics, automated or semiautomated methods are needed to extract meaningful information from ever-growing water quality measurement data sets. In this paper, seasonal patterns of P storm dynamics are identified in two contrasting watersheds (arable and grassland) through Dynamic Time Warping (DTW) combined with k-means clustering. DTW was used to align discharge time series of different lengths and with differences in phase, which allowed robust application of a k-means clustering algorithm on rescaled P time series. In the arable watershed, the main storm pattern identified from autumn to winter displayed distinct export dynamics for particulate and dissolved P, which suggests independent transport mechanisms for both P forms. Conversely, the main storm pattern identified in spring displayed synchronized export of particulate and dissolved P. In the grassland watershed, the occurrence of synchronized export of dissolved and particulate P forms was not related to the season, but rather to the amplitude of storm events. Differences between the seasonal distributions of the patterns identified for the two watersheds were interpreted in terms of P sources and transport pathways. The DTW-based clustering algorithm used in this study proved useful for identifying common patterns in water quality time series and for isolating unusual events. It will open new possibilities for interpreting the high-frequency and multiparameter water quality time series that are currently acquired worldwide