HYSTERESIS ANALYSIS OF TURBIDITY AND NITRATE DYNAMICS IN TWO AGRICULTURAL WATERSHEDS WITHIN THE MISSISSIPPI BASIN OF MIDWESTERN UNITED STATES

Abstract:

To effectively manage watersheds and control water quality impairments - including persistent eutrophication, harmful algae bloom (HABs) and increased turbidity, it is imperative to understand a watershed's hydrological and biogeochemical dynamics and its role in the mobilization and transport of nitrate and sediment to nearby surface waterbodies during storms. This paper utilizes a quantitative hysteresis index from high-resolution (15 min) discharge, turbidity and nitrate data during 43 storm events to investigate likely sediment and nutrient sources location and delivery mechanisms in two intensively farmed watersheds. In addition, logistical regression analysis was employed to assess the key environmental driver that determines the trajectory of hysteresis loop. The calculated hysteresis index (HI) metric was used to investigate controls based on the hysteresis class. Logistic regression of a suite of selected independent variables that are representative of the storm event size and the antecedent conditions were investigated. The selected variables used to identify the single independent variable with the most significant predictive influence on hysteresis class include: initial discharge, discharge range, initial nitrate, nitrate range, soil temperature, antecedent precipitation index (API), antecedent soil moisture, and event duration. We also examined intrastorm hysteresis and the influences of seasonality and storm size on storm nitrate loads. Turbidity hysteresis was generally clockwise for both sites, indicating an exhaustible, proximal source of sediments, but anticlockwise for nitrate indicating more distal and plentiful sources of nitrate in the agricultural sites. This research shows that a critical understanding of the individual roles and interplay between nutrient and sediment source location and availability, hydrological connectivity, seasonality and antecedent conditions, and storm size is highly important in order to control nutrient fluxes and understand sediment transport and delivery mechanisms into surface water bodies within intensively farmed watersheds.