Intra-Annual Variability of Riverine Nutrient and Sediment Loadings Using Weighted Circular Statistics

Abstract

This study analyzed the intra-annual patterns of nutrient and sediment exports from eight watersheds in the Lake Erie and Ohio River basins over long-term monitoring durations (8–32 years). These primarily agricultural watersheds are the major source of pollutants delivered to Lake Erie, and through the Mississippi River system to the Gulf of Mexico. Suspended sediment (SS), total phosphorus (TP), and total oxidized nitrogen (TON) (NO3−N+NO2−N) levels were analyzed using weighted circular statistics in two phases based on (1) the entire observed daily data sets; and (2) the largest events. The data analysis evaluated the timing of river flows, pollutant concentrations, and corresponding loads in 1 year for each watershed. The results showed that pollutant concentrations could occur in any season, and their corresponding loads were concentrated during high-flow periods in winter and early spring, even though the largest rain events occurred in late spring and early summer. Circular statistical analyses of data and large-event examinations indicated that SS loads were more evenly distributed over the year compared with the TP and TON loads. The results indicated that a temporal difference exists between the peak riverine pollutant export and the poorest water quality conditions in the receiving water bodies. This difference underscores the need for better management through coordinated monitoring and regulatory efforts during periods of high load export. The weighted circular analysis method provided a clearer visualization and analysis tool through finer time scales than the traditional monthly analysis of nutrient and sediment data.