How can the Drought Management Advisory Council say that the triangle is "abnormally dry" when the rainfall since January 2007 (more than a year and a half) is above normal?
Posted September 16, 2008 12:34 p.m. EDT
MIKE MOSS SAYS: Gary, Rainfall has certainly been more copious in the past few months and has driven our rainfall levels well above normal in the short ot medium term (a few months) and close to normal (about -.10" as I write this) since the beginning of 2007.
The result has been very well-stocked surface water impoundments and reservoirs and moist topsoil (with the exception of the mountains, where very dry conditions continue and severe to extreme drought designations remain in place). On the other hand, groundwater levels recover more slowly, and respond better to fairly frequent moderate rains as opposed to less frequent very heavy rains (as with the remnants of Fay and the passage of Hanna) in which the high rain rates can result in a proportianally large amount of runoff compared to infiltration. The runoff is great for quickly filling reservoirs and also causes big short-term spikes in streamflow. However, the streamflows after such events tend to rapidly decline again, and do not become as persistent and steady as would be ideal until groundwater levels are well recharged.
That last point is probably what the Drought Management folks and the U.S. Drought Monitor are being cautious about with the designations for central NC, due to streamflows that continue to show fairly rapid (but not extreme) declines after rainy episodes, and also to the fact that several of the groundwater wells in the area continue to show levels that are still within or near the lowest 25% of historical readings and, to use the site in Chapel Hill as an example, remains about 2 feet below the median for this time of year. That is a big improvement over several months back, when it was 4 feet or more below normal, but probably plays into the decision last week to call our area "abnormally dry" despite the full reservoirs.