It may not be obvious to the casual observer, but streamflow in North Carolina (and lots of other places) is composed of two elements, runoff (which for us includes overland flow into and rainfall directly on the stream), and baseflow. DWR is very interested in quantifying baseflow because it represents the amount of water discharging from the subsurface into the stream. Sometimes when rain events are very widely separated a small stream can go completely dry. But normally, there is some flow in the stream even between rain events.

What keeps that stream flowing between rainfall events? Well, a large amount of water is stored in the subsurface. That stored water is groundwater which slowly moves toward a local stream and discharges forming part of the streamflow measured in gages. That discharge is not constant, but changes as the amount of water stored in the subsurface changes over time. Measuring that changing amount of groundwater discharge or baseflow gives DWR a sense of drought conditions.

As you may have seen on the Drought Indicator Wells page, baseflow measurements can be directly correlated to groundwater levels by comparing percentiles or a ranking of the current baseflow rate or groundwater level to historical rates or water levels, respectively.

The graph to the right illustrates the daily streamflow from a gage on Potecasi Creek in Hertford County (in blue). Baseflow is separated out and plotted in red using the Lyne and Hollick algorithm. As you can see, baseflow never exceeds streamflow, but during large portions of this plot baseflow makes up a significant amount of streamflow. In fact, baseflow during this time period totals to about 72% of streamflow. Runoff accounts for the peak areas between the streamflow and baseflow curves.

Additional information is available about the DWR Drought Image gages on our map interface.