Why is the drought 'exceptional'?

Q: We received 35.81 inches of precipitation for the year [at Raleigh-Durham International Airport]. Average is 43.05. Granted, 17 percent less rain than average could be considered a drought. But an "exceptional" drought? Seems rather mild to maybe moderate. ... Seems to me that there are just too many people using water from Falls Lake, due to over-development and selling water to surrounding communities.

Chris Dettlaff, Apex

A: Droughts are not strictly measured in terms of rainfall, but in the effects of rainfall deficits on such factors as stream flows, reservoir levels, groundwater supplies, soil moisture, agriculture, and fire danger, says Ryan Boyles, North Carolina's state climatologist.

Those factors and others go into a calculation of how conditions compare to historic patterns, Boyles says. To be labeled "exceptional," the worst category, a drought must rank in the 0 to 2 percentile of historical records. In other words, there's only a 2 percent or less chance of conditions being that bad in any year.

And we're in it. Judging by a statewide average, 2007 was the driest year in North Carolina, going back to 1895, Boyles says. And record summer heat accelerated evaporation in reservoirs while increasing the demand for water.

Even with normal rain, it would take months to recover from the drought, Boyles says. Without a tropical storm or two this summer or fall, it could be 2009 before reservoirs are full.

And human consumption, which generally increases with population growth, he admits, figures heavily into the equation.