es504029c_si_001.pdf (284.04 kB)
Mississippi River Nitrate Loads from High Frequency Sensor Measurements and Regression-Based Load Estimation
journal contribution
posted on 2015-12-17, 05:31 authored by Brian A. Pellerin, Brian
A. Bergamaschi, Robert
J. Gilliom, Charles G. Crawford, JohnFranco Saraceno, C. Paul Frederick, Bryan D. Downing, Jennifer C. MurphyAccurately quantifying nitrate (NO3–) loading from the Mississippi River is
important for predicting
summer hypoxia in the Gulf of Mexico and targeting nutrient reduction
within the basin. Loads have historically been modeled with regression-based
techniques, but recent advances with high frequency NO3– sensors allowed us to evaluate model performance
relative to measured loads in the lower Mississippi River. Patterns
in NO3– concentrations and loads were
observed at daily to annual time steps, with considerable variability
in concentration-discharge relationships over the two year study.
Differences were particularly accentuated during the 2012 drought
and 2013 flood, which resulted in anomalously high NO3– concentrations consistent with a large flush of stored
NO3– from soil. The comparison between
measured loads and modeled loads (LOADEST, Composite Method, WRTDS)
showed underestimates of only 3.5% across the entire study period,
but much larger differences at shorter time steps. Absolute differences
in loads were typically greatest in the spring and early summer critical
to Gulf hypoxia formation, with the largest differences (underestimates)
for all models during the flood period of 2013. In additional to improving
the accuracy and precision of monthly loads, high frequency NO3– measurements offer additional benefits
not available with regression-based or other load estimation techniques.