Serving Boone, Blowing Rock, Banner Elk, and other towns of the North Carolina High Country
Founded 05-05-05
January 31, 2008 issue
Story by Kathleen McFadden
The scattered piles of snow in the High Country attest to our recent—and welcome— precipitation, but did that snowfall have any appreciable effect on the local drought conditions?
Not really. We need rain and lots of it.
One Inch of Rain
Dr. William Anderson, a hydrologist and assistant professor at Appalachian, quantified the effect of an inch of rain in Watauga County: “Given a county area of 313 square miles, one inch of rain produces about 5,440 Mgal of water.”
An Mgal is one million gallons, so 5,440 Mgal is a considerable amount of water. To visualize how much one million gallons is, consider that a good-sized bathtub holds 50 gallons, so a million gallons would be 20,000 baths.
Here’s another way to picture it: a swimming pool large enough to hold one million gallons would be approximately 267 feet long (almost as long as a football field), 50 feet wide and 10 feet deep.
But not all the water from a one-inch rainfall would soak into the ground where we need it to go. Rainfall produces runoff, but how much runoff is associated with a one-inch rainfall depends on a host of variables.
Anderson explained, “It is impossible to say how much of [a one-inch rainfall] will become runoff because of the many factors that determine what happens to the water. For example, some will evaporate, but this amount depends on seasonal patterns and daily weather conditions. Some will also be used by plants, but again this depends on seasonal variations. The amount of runoff may even depend on what time of day the one inch of rain happens to fall. Runoff also varies spatially depending upon sediment (soil) type and thickness, slope, land use, and many other factors.”
One Inch of Snow
Taking the rainfall calculation down to the acre level, one inch of rain falling on one acre of land is equal to about 27,154 gallons of water. Snow does not have anywhere near the same impact.
One inch of snow falling evenly on one acre of ground is equivalent to about 2,715 gallons of water—one-tenth the impact of rain. This figure is based on the rule-of-thumb that 10 inches of snow is equal to 1 inch of water and can vary considerably, depending on whether the snow is heavy and wet or powdery and dry.
Heavy, wet snow has a high water content, and 4 or 5 inches of this kind of snow contains about 1 inch of water. So an inch of very wet snow over an acre might amount to more than 5,400 gallons of water, while an inch of powdery snow might yield only about 1,300 gallons.
Snow produces runoff just as rain does, but the volume is equally impossible to calculate. Anderson said, “It depends on the characteristics of the snow. If it is a ‘wet’ snow, I would expect more runoff than a ‘dry’ snow; however, the exact amount will vary.”
Do Septic Systems Recharge Groundwater?
Because homes with septic system put their water back into the ground, does that cancel the amount of water those homes draw from their wells?
Dr. William Anderson, a hydrologist and assistant professor at Appalachian, said, “No, it does not. Most residences that use well water draw their water from fairly deep fracture zones (300 to 500 feet in depth). The groundwater moving through those deep fractures is on a temporally long travel path and will not discharge to adjacent streams for many years (decades to centuries, perhaps a bit more or less). So, removing water from those deep regions lowers the amount of energy (water levels) in those fractures.
Septic systems take this deep water and place it near the surface into a much shorter travel path. Thus, it is not that septic cancels out pumping. Rather, the transfer of water from deep fractures to septic short circuits the natural hydrogeologic system and shortens the residence time of the water in the system.”
Watauga County Well Monitoring
Dr. William Anderson, a hydrologist and assistant professor at Appalachian, is studying recharge rates at wells in Watauga County, along with other local water parameters.
Anderson said that the water levels in his three monitoring wells around the county dropped an average of 2 meters (about 6 feet) during the summer months, but have since nearly returned to last winter’s levels. The maximum decline in the three wells was about 3 meters (about 9 feet).