High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

Abstract

Storm events can drive highly variable behavior in catchment nutrient and water fluxes, yet short‐term event dynamics are frequently missed by low‐resolution sampling regimes. In addition, nutrient source zone contributions can vary significantly within and between storm events. Our inability to identify and characterize time‐dynamic source zone contributions severely hampers the adequate design of land use management practices in order to control nutrient exports from agricultural landscapes. Here we utilize an 8 month high‐frequency (hourly) time series of streamflow, nitrate (NO3‐N), dissolved organic carbon (DOC), and hydroclimatic variables for a headwater agricultural catchment. We identified 29 distinct storm events across the monitoring period. These events represented 31% of the time series and contributed disproportionately to nutrient loads (42% of NO3‐N and 43% of DOC) relative to their duration. Regression analysis identified a small subset of hydroclimatological variables (notably precipitation intensity and antecedent conditions) as key drivers of nutrient dynamics during storm events. Hysteresis analysis of nutrient concentration‐discharge relationships highlighted the dynamic activation of discrete NO3‐N and DOC source zones, which varied on an event‐specific basis. Our results highlight the benefits of high‐frequency in situ monitoring for characterizing short‐term nutrient fluxes and unraveling connections between hydroclimatological variability and river nutrient export and source zone activation under extreme flow conditions. These new process‐based insights, which we summarize in a conceptual model, are fundamental to underpinning targeted management measures to reduce nutrient loading of surface waters.

Multitracer Field Fluorometry: Accounting for Temperature and Turbidity Variability During Stream Tracer Tests

“…Discrete water samples were also collected for laboratory analysis of tracer concentrations. For full details of the study site and field experiments see Blaen et al () and supporting information Text S2 and Table S1.…”

Monitoring the riverine pulse: Applying high‐frequency nitrate data to advance integrative understanding of biogeochemical and hydrological processes

“…For example, NO 3 − and TP typically show differing responses during storms (Bowes et al, ; Lloyd et al, ), although similar behavior has been noted as well (Thomas et al, ). Clockwise hysteresis is common for TP as concentrations increase on the rising hydrograph limb, reflecting contributions from in‐ or near‐channel sediment sources, whereas NO 3 − often shows dilution on the rising limb, sometimes preceded by a short‐lived spike, which can manifest as either clockwise or anticlockwise hysteresis (Blaen et al, ). Similarly, NO 3 − and DOC often show opposing hysteresis patterns interpreted to reflect differing hydrologic source areas (Blaen et al, ).…”

Carbon and nutrient export regimes from headwater catchments to downstream reaches

“…Finally, this study focused mainly on manual grab sampling data, with a limited number of high-frequency sampling of storm events; however, the storm events monitored exhibited different export regimes from seasonal variations, with for example NO − 3 and SRP exhibiting alternatively synchronous and opposite dynamics, whereas NO − 3 and SRP consistently had opposite dynamics on a seasonal basis. These short terms variations have implications for N : P load ratios and further research involving high-frequency sensors will allow their quantitative evaluation Blaen et al, 2017), including seasonal variability of storm event patterns (Dupas et al, 2015b;Minaudo et al, 2017).…”

“…Storm event patterns (accretion or dilution) provide insight into the NO − 3 , DOC and SRP concentrations in the flow pathways activated during storms, i.e., overland flow and shallow sub-surface flow, relative to the baseflow concentration (Dupas et al, 2016;Buda and DeWalle, 2009;Jiang et al, 2014a;Blaen et al, 2017). In both US-Agr and LS-Agr, DOC and SRP storm event dynamics exhibited a majority of accretion patterns, suggesting high C and P source in the uppermost soil layers compared to deeper soil layers (Dupas et al, 2015b;Outram et al, 2014;Bieroza and Heathwaite, 2015).…”