Southern Research Station Headquarters - Asheville, NC
![[Images] Five photos of different landscape](/images/imstr1.jpg "[Images] Five photos of different landscape")
Issue 10
We’re All Downstream
by Zoë Hoyle
Global warming and climate change are in the news every day now. It may seem natural to tie this summer’s drought to Earth’s warming, but it’s been hot and dry before: Just think of the Dust Bowl years of the 1930s. So is this summer a one-off, or a bitter taste of things to come? Either way it brings up questions. Will there be enough water in the South for our future needs under present climate change scenarios?
Summer in the South, 2007. Temperatures reach the 90s and 100s for weeks on end. There’s literally no rain for months. In Georgia, they talk about the kind of drought that only comes once a century as the waters in Lake Lanier drop perilously low. Water restrictions are imposed on the Research Triangle area of North Carolina in September; as the month passes with no rain, restrictions are put in across the State. Crops fail, livestock suffers, river adventures close down; lawns turn brown, and farmers despair.
Researchers with the SRS Southern Global Change Team in Raleigh, NC, have focused on this very question since well before climate change became a daily preoccupation. Along with collaborators, team leader Steve McNulty, research hydrologist Ge Sun, and resource information specialists Jennifer Moore Myers and Erika Cohen have developed a supply-and-demand modeling framework called the Water Supply Stress Index (WaSSI) that will allow local planners and policymakers to run water supply scenarios at local, regional, and national levels.
Population Pressure
Population is growing faster in the South than anywhere else in the United States, a trend that’s expected to continue well into the next century. By 2045, the overall population of the Southern States is expected to almost double that recorded in 1990. The real story is at the individual watershed level, where population change is expected to vary from a 20-percent decrease to a 500-percent increase.
The Southern Forest Resource Assessment and other sources project 12 million acres of forested land lost to other land uses by 2020. Almost all global climate change models predict higher temperatures, variable rainfall, and more frequent and intense storms. The combination may bode ill for future water supplies in heavily populated areas.
“As the population in the South continues to grow during the next 20 plus years, surface and groundwater reserves will be reduced in many areas,” says McNulty. “Water limitations could occur in new places across this traditionally water rich region.”
Tools for modeling water availability at the regional level are not that easy to find, and models developed for both smaller and larger spatial scales have not been designed to work together. To construct WaSSI, SRS researchers combined a hydrologic model with projections from the two major climate change models and with land use and population change projections. They then applied the model to look at eight different scenarios—combinations of climate, land use change, and population—that might take place in the South over the next two decades.
Water yield, the total water that flows out of a particular basin in a specified time, is mainly controlled by climate, land use, and consumption by humans and ecosystems.
“We found that the Southern United States has approximately six times more water coming in, on the average, than is being used by people,” says McNulty. “You could think of all those rivers flowing into the ocean as excess water.”
That’s good news overall, but you can have plenty of water flowing through a region and still not have enough in a particular river or watershed to support an intensely populated area like the Research Triangle or Atlanta.
“Here in Raleigh we’ve been having a drought every 2 or 3 years, and we keep adding people. The population for Wake County is supposed to rise by 50 percent between now and 2025,” says McNulty. “When we get a dip in our water supply due to drought, it has an enormous impact. All of our scenarios suggest that water yield will increase overall, but in some areas of high population density water demand is already greater than local supply. These shortages will increase and expand in area during the next 20 years.”
You Don’t Miss Your Water
The greatest area of uncertainty in running water supply scenarios has to do with ground water, more specifically, with knowing how much water is stored in underground aquifers in the Southeast. Aquifers can take centuries, even millennia, to recharge; even years with high rainfall may not be enough to move areas with depleting aquifers out of chronic water stress conditions. And nobody really knows how much water is down there right now.
“There are parts of Texas, for instance, where they receive very little rainfall,” says McNulty. “Those are areas that are already heavily dependent on ground water supplies. We know where the Nation’s major aquifers are, but we don’t know how much water is in them. That puts some areas at severe risk in the future, because it may become very costly to get that water out.”
That’s why it’s important to have locally explicit models. In Texas, ground water is the critical factor. Fluctuations in rainfall from climate change will have a much larger effect in Texas than in the parts of western North Carolina and eastern Tennessee that sometimes get 90 inches of rain a year. Recent findings by the Southern Global Change Team show that by the 2040s, areas with limited aquifer reserves and heavy ground-water use will probably begin to run out of ground-water.
“The point of creating scenarios is to give municipal planners and policymakers time to try to prevent severe water shortages in the future,” says McNulty. “Now is the time to start measures—whether it’s developing networks to move water from other areas, building reservoir systems, or starting water conservation measures.”
