Anthropogenic Influences Increase the Risk of Extreme Precipitation that Leads to Bonnet Carré Spillway Operations and Gulf of Mexico Ecosystem Impacts

Abstract:

The Bonnet Carré Spillway (BCS) is a flood control mechanism that diverts excess water from the Mississippi River to Lake Pontchartrain and into the Gulf of Mexico. BCS operations are related to abundant August-July precipitation in four hydrologic catchments drained by the Mississippi River: the Upper Mississippi, the Arkansas-White-Red, the Ohio, and the Lower Mississippi. For the 10 years that the BCS was open since 1973, precipitation in the four catchments exceeded the top decile for five years and was well above average during the other five.

Historic precipitation in each of the four catchments for a period of August 2018 to July 2019 resulted in record discharge of 38 cubic kilometers over 123 days through the BCS. Precipitation during this 12-month period was the highest since at least 1896 and exceeded the former record by 40%. August-July precipitation has increased over the last 125 years in the four catchments and this increase cannot be reconciled by natural variability alone. Based on transient coupled climate model ensembles spanning the last century, the detected change in observed precipitation is attributable to time varying external radiative forcing. The statistically significant shift to wetter conditions makes dry years less likely and very wet years more likely. Consistently, model ensembles indicate robust increases in the risk of extreme August-July precipitation. For exceptional precipitation like 2018-2019, anthropogenic influences more than double the risk of exceeding such a precipitation threshold.

Flow through the BCS into the Gulf of Mexico in 2019 led to changes in water column salinity, turbidity, chemistry, and nutrient-loading that significantly impacted coastal marine ecosystems and fisheries. An economic analysis by NOAA and state partners estimated 2019 economic losses of approximately $380 million for impacts on shellfish and finfish along with impacts to the processing, marketing, and recreation sectors. Coastal industries, still recovering from recent disasters (Hurricane Katrina, Deepwater Horizon oil spill, and other BCS openings) were impacted by additional habitat degradation, declines in fisheries and protected species, and harmful algal blooms resulting from the 2019 BCS event.