Monsoonal storm events in semi-arid montane watersheds generate short-lived but disproportionately large nutrient and suspended sediment load pulses with important downstream ecological consequences. Rapid increases in discharge during storm events mobilize and transport nutrients and sediment from hillslopes, soils, and floodplains into stream networks, often accounting for a substantial fraction of annual material export. These flood-driven expansion and contraction dynamics create critical hydrologic connections that regulate when, where, and how nutrients and sediment enter the aquatic system and are delivered to downstream waters. Understanding the controls on nutrient and sediment loading during storm events is therefore essential for predicting watershed-scale nutrient export under a changing climate.
We employed a nested watershed approach to quantify storm-driven nutrient dynamics across spatial scales. In situ sensors were deployed throughout the stream network to continuously measure flow, nitrate, dissolved organic carbon, and total suspended solids at 15-minute intervals, enabling capture of rapid changes in concentration and discharge during monsoonal storm events. Storm events were defined as periods when discharge exceeded two times base flow and were detected at a minimum of two locations within the watershed. Identified storms were then used to calculate nutrient loads by integrating solute concentrations over the duration of each event, quantified as the area under the concentration–time curve.
We hypothesized that nutrient loads would scale linearly with contributing drainage area. We found this to be true of dissolved organic carbon and total suspended solids, where there is a linear increase in load with contributing area. However, nitrate loads followed a unimodal pattern, suggesting non-conservative behavior during storm events. This talk explores the mechanisms underlying this deviation by integrating storm-based metrics, including hysteresis indices and flushing indices, to evaluate how event-scale transport, source activation, and in-stream processing shape the predictability and scaling of nitrate loads across stream networks.