Published: 2011-05-09 08:50:04
Updated: 2011-05-09 08:50:04
Posted May 9, 2011
By Mike Moss
A lot of our attention in the past few weeks has been on severe weather, but I thought today I'd take a quick look at water resources and rain, and how we stand in terms of drought in some areas. A lot of us wouldn't really notice much problem with moisture at the moment, as we've had a pretty substantial stretch with fairly frequent rain-producing systems crossing the region. This has ameliorated the previous drought conditions to a reasonable degree, and last week's drought monitor update (first image above) shows that a good portion of our viewing area remains in the "abnormally dry" category rather than a drought status. However, just south and southwest of the Triangle, we still have moderate drought in place, and the portion of the state affected by moderate drought grew a bit in the past week or two, now extending across parts of the southern coastal plain and along the central and southern coast.
The frequency of light to moderate rainfall events has kept short-term, shallow moisture (like topsoil and crop moisture) in pretty good shape, and we're near normal on those measures over much of the state. This also reflects in generally lush vegetation when it comes to trees, bushes, grass and weeds.
A bit longer-term, though, we're still running somewhat short on rain, which is evident in maps of precipitation (second image) and departure from normal (third), both taken from the National Weather Service Precipitation Analysis site. These maps total up radar and gauge-estimated values from the past 90 days, and show an area just southwest of the Triangle that is about 4-6 inches below normal over that 3-month span, while a few pockets toward the southeastern coast are running 6-8 inches below normal, leading to that extension of the moderate drought area.
Another even longer-term indicator is groundwater, and we still have some improvement to do there, even outside the designated drought regions. As an example, take a look at the last two images, which are graphs of groundwater levels at the USGS site in Chapel Hill. The first shows the measured water-table depth (thick red line) most recently at about 45 feet below the surface. For May, this is near the bottom of the dark red box indicating the lowest 10% of historical readings. The graph does show that the level has bottomed out and recovered a bit recently, but when you compare that upward trend to the long-term median trend (black triangles) , you can see that since about Dec/Jan, the trend has been downward through a time of year when historically it would be on the rise.
A second graph from this site goes farther back, showing a fairly dry 2009 (compared to the long-term median, green line) and then a recovery to a largely normal 2010, before drying set in late last year. Note also on this image that the light blue shading in the background shows the daily range of highest and lowest past readings, and the most recent readings (in red) have been right at the bottom of that range.
It does appear we may be in for a fairly active stretch later this week into next, with several rounds of showers and storms possible, and projections from the Climate Prediction Center place us in a good chance for above normal precipitation in the 6-14 day time frame. Hopefully that will provide for some additional "catching up" on long-term moisture...