Image cross‐correlation techniques, such as particle image velocimetry (PIV), can estimate water surface velocity (v _surf) of streams. However, discharge estimation requires water depth and the depth‐averaged vertical velocity (U _m ). The variability of the ratio U _m /v _surf introduces large errors in discharge estimates. We demonstrate a method to estimate v _surf from Unmanned Aerial Systems (UASs) with PIV technique. This method does not require any ground control point (GCP): the conversion of velocities from pixels per frame into length per time is performed by informing a camera pinhole model; the range from the pinhole to the water surface is measured by the drone‐borne radar. For approximately uniform flow, U _m is a function of the Gauckler‐Manning‐Strickler coefficient (K _s ) and v _surf. We implement an approach that can be used to jointly estimate K _s and discharge by informing a system of two unknowns (K _s and discharge) and two nonlinear equations: i) Manning's equation and ii) mean‐section method for computing discharge from U _m . This approach relies on bathymetry, acquired in situ a priori, and on UAS‐borne v _surf and water surface slope measurements. Our joint (discharge and K _s ) estimation approach is an alternative to the widely used approach that relies on estimating U _m as 0.85·v _surf. It was extensively investigated in 27 case studies, in different streams with different hydraulic conditions. Discharge estimated with the joint estimation approach showed a mean absolute error of 19.1% compared to in situ discharge measurements. K _s estimates showed a mean absolute error of 3 m^1/3/s compared to in situ measurements.

Holistic water quality models to support decision-making in lowland catchments with competing stakeholder perspectives are still limited. To address this gap, an integrated system dynamics model for water quantity and quality (including stream temperature, dissolved oxygen, and macronutrients) was developed. Adaptable plug-n-play modules handle the complexity (sources, pathways) related to both urban and agricultural/natural land-use features. The model was applied in a data-rich catchment to uncover key insights into the dynamics governing water quality in a peri-urban stream. Performance indicators demonstrate the model successfully captured key water quantity/quality variations and interactions (with, e.g., Nash-Sutcliff Efficiency ranging from very good to satisfactory). Model simulation and sensitivity results could then highlight the influence of stream temperature variations and enhanced heterotrophic respiration in summer, causing low dissolved oxygen levels and potentially affecting ecological quality. Probabilistic uncertainty results combined with a rich dataset show high potential for ammonium uptake in the macrophyte-dominated reach. The results further suggest phosphorus remobilization from streambed sediment could become an important diffuse nutrient source should other sources (e.g., urban effluents) be mitigated. These findings are especially important for the design of green transition solutions, where single-objective management strategies may negatively impact aquatic ecosystems.

Sulfonamides are widely used antiobiotics and a threat to water resources and related ecosystems. While the direct discharge from untreated sewer or wastewater to surface water is a well-known pathway to the aquatic environment, only a limited number of studies have looked at the discharge and fate of sulfonamides from a contaminant plume to surface water so far.In this study, we investigated a sulfonamide contaminant plume discharging to a stream in Denmark. The sulfonamides (originating from a former production facility), are transported through a multilayered sandy aquifer and discharge to a groundwater-fed stream located down gradient. Our objectives were to evaluate if a screening using fluorescence properties could be used to delineate the sulfonamide contaminant plume and support the contaminant mass discharge estimation (both by transect method and in-stream measurements).Direct push technics in combination with a fluorescence screening allowed a relatively unexpensive coarse delineation of high concentrations areas (as opposed to laboratory analysis) down to 15 m.b.g.s. and ergo optimization of monitoring wells / screen locations in the transect. Chemical analyses were combined with slug test and hydraulic gradient estimates via continuous monitoring to quantify the sulfonamide flux and its temporal variations in a 2 km-long transect along the stream (24 monitoring wells with 3 - 6 screens).The estimated sulfonamide mass discharge (transect based) is in good agreement with the mass discharge calculated from in-stream measurements, highlighting the relevance of the screening approach to select appropriate measurement point locations. Furthermore, the comparison between the flux in both stream and groundwater compartments shows that the degradation of sulfonamides seems relatively limited in the near-stream and hyporheic zone, with the exception of the sulfanilic acid. The results of this study will be used for the prioritization of remedial actions along the main discharge zones.