As the concentrations of CO2 in the atmosphere increase, the warming produced by the greenhouse gas effect is strengthened. Computer modeling of the climate predicts that there will be feedbacks that significantly increase the impact from the increasing CO2. Even if the concentration of CO2 in the earth’s atmosphere were to stabilize at this level, the changes in the climate of the earth in response to the atmospheric CO2 levels would continue for hundreds of years. In reality, the global emissions of CO2 will not stabilize or decrease any time soon and will continue to rise for at least a generation. What is going to happen will happen, so we need to plan for change and make decisions for the next 30-50 years based on likely outcomes.
It is my plan to live for another 40 years. My relatives do pretty well and I am an optimist and a “real food” and exercise devotee. So when it came time to select a place to live in retirement, climate change was one of the factors taken into consideration- water availability, distance from the coast, elevation, along with proximity to family, medical service and an airport and several other factors. Now, I find myself in northwest Prince William County a place that the Washington Post recently described as becoming “a regional retirement mecca, a small-scale version of Florida on the outskirts of Washington.” I made my choices based to a large extent on general projections of climate released by various groups, not having the tools or resources to do much more.
Now, however, the Interstate Commission on the Potomac River Basin (ICPRB) has completed a study in water supply availability and the health of the Potomac Watershed for various climate scenarios. The focus of their study was the Potomac River, which supplies water to the Washington Aqueduct, Washington Suburban Sanitary Commission (WSSC), and Fairfax Water who all funded the study. The Potomac River supplies 78% of the regions drinking water and the water utilities of the region must plan for the future. In addition, there must be adequate flow of the Potomac below Little Falls to ensure that the balance of saline and fresh water for the health of the Chesapeake Bay estuary. So, on the water rate payer’s nickel I get to see what the future might look like here in in Prince William County.
The National Research Program of the U.S. Geological Survey (USGS) actually performed the study using six of the global climate models and three atmospheric CO2 scenarios to create 18 separate possible scenarios. The USGS then “downscaled” the 18 global climate predictions to the Potomac River basin and to other areas as part of a separate project on climate change being conducted by the Chesapeake Bay Program Office and the USGS’s Virginia Water Science Center (your tax dollars at work). In addition, the Chesapeake Bay Program’s Phase 5 Watershed Model was used to estimate the impact of changing temperatures and precipitation on Potomac basin stream flows.
The most advanced types of models currently being used to project future global climate are general circulation models (GCMs). A GCM is a numerical model which represents the important physical, chemical, and biological processes on the Earth’s surface, in the atmosphere, and/or in oceanic systems that affect climate. The USGS used models from the National Center for Atmospheric Research (USA), Norway, Australia, Russia and Japan as listed in the chart below.
|From ICPRB publication|
|IPCC Climate Change 2007|
Though it is predicted by the climate models that precipitation will increase on a global scale, when dealing with only the Potomac River basin, the models differed on whether precipitation will increase or decrease. The models project that the total annual precipitation varies from plus 9% to minus 9% or that the rainfall/ snowmelt that averaged 42.2 inches during the reference period (1988-1999) may stay within 4 inches of that average. Though, it is to be noted, that year to year weather variations in rainfall in the region are large, precipitation has varied from over 80 inches to below 20 inches in the past. Also, in the 18 climate scenarios, the increase in annual average temperature by 2040 increases for the area from 1.3 to 4.1 degrees Fahrenheit when compared with the reference period of 1988 to 1999. The average increase, over all scenarios is 2.7 degrees Fahrenheit. (These temperature predictions were the basis of the energy savings and water savings projects for my home that were geared for a slightly warmer, drier climate, though I am still hoping for wetter.)
Though annual rainfall increases in half of the climate change scenarios, flow in the Potomac River falls in most scenarios. Changes in both temperature and precipitation affect stream flows. Changes in rain or snow affect the amount of water that runs off the land surface and enters streams during rainfall. Precipitation also affects the amount of water recharging groundwater aquifers, which are the primary source of stream flow during dry weather periods. Increasing temperatures will cause more rain to be lost to evaporation from the soil, streams, and will increase transpiration, the water released to the atmosphere by plants. These increases in evaporation and transpiration will tend to reduce flow in streams which in turn reduces flow in the Potomac. With rising population, this could require changes in water use and supply for the area and reduce groundwater availability for private well owners like me.
Average annual basin-wide evaporation and transpiration is predicted to increase by 6-8%. Groundwater recharge decreases under all but three of the climate scenarios, and as I watch the statistically low water level in the monitoring well up the road, I worry about my well water supply. According to study results, the seasonal pattern of groundwater recharge does not change significantly under climate change, with January, February, and March remaining the months of greatest recharge. However, the average annual amount of groundwater that provides base flow to streams and the water in my well, is predicted to decrease in 16 out of the 18 scenarios by as much as 34% in one case.
Results for the 18 climate scenarios fell into three categories: minor impact, moderate impact, and major impact. The biggest impact is the ability of the regional water utilities to continue to supply water on demand during droughts as the climate changes. Six of the scenarios are predicted to have little impact on the system during a moderate drought and the projected population of the region can be supplied with drinking water from the Potomac River and current systems and operations. Six of the climate change scenarios fall into the “moderate impact” category. Under these scenarios the region is predicted to experience more frequent and stricter water use restrictions, but no water supply shortages during a moderate drought. Reservoir levels are predicted to fall to significantly lower levels during a drought than would occur in the absence of climate change with the projected and assumed increase in population.
However, the remaining six climate change scenarios are scary. Under these dreadful six scenarios, unless we make changes in the water supply systems we run out of water. These scenarios predict that both mandatory and emergency water use restrictions would be imposed and most system reservoirs would become empty or close to empty during a moderate drought. In addition, these six scenarios predict on some days of the drought the Potomac River would fail to provide sufficient water to meet demand and environmental needs. Our regional water utilities: The Washington Aqueduct, Washington Suburban Sanitary Commission (WSSC), and Fairfax Water working together with the ICPRB can make changes to the structure and operation of the water supply system to make it more robust. The Potomac River will continue to supply water to the region, we will have to use it more wisely to ensure adequate water supply in the future, but there will be water. Clearly though, development in the groundwater recharge zones (mostly the Rural Crescent in Prince William County) and open areas needs to be limited to protect the groundwater and stream base flow that supplies our water.