Using State-of-the-Art Simulations to Determine Climate Effects on the Mississippi River

MES/CEE Assistant Professor Samuel Muñoz, in collaboration with Sylvia Dee and James Doss-Gollin from Rice University, was awarded a $700K NSF grant for “Evaluating the Past and Future of Mississippi River Hydroclimatology to Constrain Risk via Integrated Climate Modeling, Observations, and Reconstructions.” The Mississippi River is a critical artery for industry, agriculture, and commerce, but perennial flooding and droughts cause major economic disruptions. Climate change is altering water availability globally, but the effects of climate change on the flow of the Mississippi River are not well understood. In this project, we combine state-of-the-art model simulations with data describing changes in the hydrology of the Mississippi River to better understand its sensitivity to changes in climate.

The Mississippi River, the longest of its kind in North America and one of the longest in the world, is fed by a variety of sources from across the continent. From snowpack melt to rains and lakes, water emptying from the Mississippi Delta into the Gulf of Mexico travels from locations as disparate as the Great Lakes and western Montana.

Climate change has exacerbated extreme weather and shifted the rain and snowfall patterns which feed the Mississippi. Many Americans make their homes along the river, and in recent decades housing development has continued to expand into areas once thought to be less risky flood zones. Scientists must grapple with the effects climate change will have on the river’s massive drainage basin to assess the threat to local communities.

Through the grant, Munoz is hoping to puzzle out the effects of competing climate-charged forces on the mighty river’s water levels. As the Earth warms, less snowpack will develop in the winter months, decreasing spring melt and drainage into the river. The hot weather will also increase evaporation. However, warmer air carries more moisture, thus increasing the chance of rainfall and severe storms.

Predicting the future is difficult business, but Munoz has a unique view of the problem: the past. As a paleoclimatologists, Munoz studies the fossil record, sediment cores, and other tools of peering into the past to understand how millennia’s worth of oscillating climates have affected the water levels of the Mississippi River. Already, Munoz has used this method to compare water discharge during the warmer climate of the Medieval Climate Anomaly and the colder of the Little Ice Age. Armed with an understanding of how temperature changes have affected water discharge in the past, he can plug this information into climate models and more accurately predict what lies in store. The results will help communities and water resource managers along the Mississippi River prepare for the emerging flood risks of the coming decades.

Abstract Source: NSF

The Mississippi River drains a continent, gathering water from as far as Pennsylvania and Montana and discharging it into the Gulf of Mexico. The channeling of so much water into a single river naturally raises the stakes for flooding, and the river has one of the most extensive flood mitigation systems in the world. But there are limits to the protection afforded by the current system and we do not know if it will prove adequate as climate warms. The effect of warming on flood risk is determined by a balance between competing effects: warming by itself would likely reduce discharge by increasing the amount of moisture that evaporates from the land rather than running off into streams. Warming also reduces snowpack, leading to a reduction in the springtime streamflow resulting from snow melt. On the other hand warmer air generally holds more moisture, which leads to increases in both mean and extreme precipitation, thus increasing flood risk.

This projects seeks to determine the net effect of these competing influences on Mississippi River discharge, which serves as a broad-brush proxy for flood risk. Much of the work involves assessments of Mississippi discharge in climate models, particularly the Community Earth System Model (CESM), which uses a River Transport Model (RTM) for the Mississippi. Preliminary results show river discharge increasing dramatically (by perhaps 130%) over the 21st century under a high greenhouse gas emissions scenario (RCP8.5) but remaining within historical norms under lower emissions (RCP4.5). This contrast suggests that the balance between streamflow increases from precipitation and decreases from evaporation and snow melt could be temperature dependent, with precipitation winning out at higher levels of warming.

The project also considers the factors driving Mississippi discharge over the last millennium using a combination of weather and streamflow observing networks, paleoclimate reconstructions, and ensembles of present-day and last millennium climate model simulations. Recent work by the Principal Investigators (PIs) suggests that the warm period of the Medieval Climate Anomaly featured lower discharge than the Little Ice Age, a result that is generally captured by CESM simulations of the last millennium. The result is also in keeping with the moderate warming scenario for the 21st century.

The work is of societal interest due to the focus on flood risk, and the PIs are working with stakeholders including the US Army Corps of Engineers and the Mississippi River Cities and Towns Initiative to understand the practical significance of their results and provide decision support. The project also involves education and outreach, supporting summer research experiences for high school students and working with the Girl Scouts of America to encourage girls and young women to consider careers in science. The project provides support and training to a postdoctoral researcher and two graduate students.

This award reflects NSF’s statutory mission and has been deemed worthy of support through evaluation using the Foundation’s intellectual merit and broader impacts review criteria.

Related Faculty: Samuel Munoz

Related Departments:Civil & Environmental Engineering