New Nutrient Flux Estimates for 2004

The nutrient flux estimates for July 2003 through June 2004 have been calculated. The 2004 flux of total nitrogen to the Gulf of Mexico was 1.24 million metric tons, the seventh smallest total nitrogen flux in the past 26 years. The 2004 flux of total phosphorous to the Gulf of Mexico was 172,800 metric tons, the fifth largest total phosphorous flux in the past 26 years. The 2004 flux of nitrate plus nitrite as nitrogen to the Gulf of Mexico was 800,500 metric tons, the eighth smallest nitrate plus nitrite flux in the past 24 years. Statistical models have demonstrated that the size of the hypoxic zone is more closely related to the flux of nitrogen during spring months, than during the rest of the year. The monthly flux of total nitrogen for April, May, and June of 2004 was 139,241, 152,800 and 192,925 metric tons, respectively. The monthly flux of nitrate plus nitrite as nitrogen for April, May, and June of 2003 was 94,593, 101,389, and 137,738 metric tons, respectively. A minor error in the discharge data from 2003 was detected in 2004. The correction of this error resulted in minor changes to 2003 estimates of nutrient flux.

Several changes have been made to the models that were used in the CENR report to calculate estimates of nutrient flux and yield. The resulting nutrient flux and yield estimates differ slightly from those published in the CENR report. These new estimates are calculated using the same method used in the CENR report (ESTIMATOR; Gilroy et al., 1990; Cohn et al., 1992), but the model structure and calibration periods have been changed. The new models do not include a time-squared term. Different upstream flow terms are used in the Low Mississippi River model than were used in the CENR report. The model calibration period differs from that in the CENR report.

The new flux estimates were calculated using the ESTIMATOR model with terms for flow and flow-squared, seasonal sine and cosine functions, and a linear time trend. Flow terms from upstream stations (Mississippi River at Thebes, IL and Ohio River at Grand Chain, IL) were used in addition to at site flows for the lower Mississippi River model. Upstream flows were lagged 10 days to account for travel time between the upstream and downstream sites.

The data set used to construct flux estimates consists of instantaneous sample data only. Results from composite samples, collected prior to October 1967 are not used. The first 10 years of flux estimates are calculated using the first 10 years (July through June) of data. Each subsequent year of load estimates are calculated using samples from the current year and the previous 9 years. This "moving window" approach allows a sufficient number of samples in each model run to represent the full range of flow and nutrient concentration conditions.

Low_Miss_MO_flux.xls ­ monthly nutrient flux estimates, 1968 through 2004 for the Mississippi River at St Francisville, LA
Low_Miss_AN_flux.xls ­ annual (July-June) nutrient flux estimates, 1968 through 2004 for the Mississippi River at St Francisville, LA
Atchaf_MO_flux.xls ­ monthly nutrient flux estimates, 1979 through 2004 for the Atchafalaya River at Melville, LA
Atchaf_AN_flux.xls ­ annual nutrient flux estimates, 1979 through 2004 for the Atchafalaya River at Melville, LA
Ohio_MO_flux-yield.xls ­ monthly nutrient flux and yield estimates, 1995 through 2000 for the Ohio River at Grand Chain, IL
Up_Miss_MO_flux-yield.xls ­ monthly nutrient flux and yield estimates, 1995 through 2000 for the Mississippi River and Thebes, IL
Gulf_MO_flux-yield.xls ­ monthly nutrient flux and yield estimates, 1979 through 2004 to the Gulf of Mexico
Gulf_AN_flux-yield.xls ­ annual nutrient flux and yield estimates, 1979 through 2004 to the Gulf of Mexico

All flux estimates are in metric tons as N, P, or SiO2 and all yield estimates are in kilograms per square kilometer per month or year. Discharge estimates are given in cubic feet per second.

The nutrient flux and yield estimates will be updated every year in early July so that nutrient flux estimates for the previous 12 months (July through June) can be provided in a timely manner and coordinated with surveys of the Gulf of Mexico hypoxic zone, which occur in mid-July.

Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and Sommers, R.M., 1992. The validity of a simple statistical model for estimating fluvial constituent loads: An empirical study involving nutrient loads entering Chesapeake Bay. Water Resources Research 28:2352-2363.

Gilroy, E.J., Hirsh, R.M., and Cohn, T.A., 1990. Mean square error of regression-based constituent transport estimates. Water Resources Research 26: 2069-2077.