Rain and runoff results
Posted October 4, 2010
The two rounds of very heavy rainfall across central and eastern NC that occurred last week had immediate and more gradual impacts on the state. Unfortunately, some of those impacts included serious flooding in parts of the coastal plain and near the coast. On the other hand, a large portion of the state was very parched, with a combination of dry weather and heat leading to many areas being designated in moderate to severe drought leading into the last week of September. The scale of the rains last week made for some big changes, though, with most of the state now listed as abnormally dry, parts of the southern coast out of any drought-related designation, and a few areas still in moderate drought.
The rains certainly moistened the ground, got rivers and streams flowing again, and significantly recharged some water storage and supply facilities. To highlight a few quick pieces of data, I've included a NOAA/NWS map here showing rainfall totals across NC for the two weeks ending October 4, estimated from a combination of gauge readings and radar. You can see that much of central NC received 4-8 inches during that period, with much of the coastal plain in double-digits and some coastal areas reaching the 20-23 inch range. At the Raleigh-Durham airport, the spike of over six inches of rain in the past week or so is evident at the end of the graph of one-year rain versus normal, and that amount of rain, after trailing below normal for some time since summer, puts us back to almost 2 inches above normal for that overall time frame.
Many streams across the region, even 3-4 days after the rain ceased, are running in the normal to well above normal discharge range, an example being the graph of Contentnea Creek near Lucama. The triangles on the graph indicate long-term median flow values. The large streamflows have also fed into some of our reservoirs in a big way, with Falls Lake still a little below normal but up more than a foot since last week, and Jordan, with a much larger watershed, having shot up almost 2 feet to just about match its normal level, or "guide curve." It takes a little longer to judge the more gradual response of groundwater, but we should see an uptick there as well. I looked up some graphs for monitor wells in Chapel Hill and Grantham, but for now the data in the USGS graphs ends on September 19th and does not reflect the recent rains. I'll revisit that once updated info becomes available.