In NC, Low-Flows Increasing

Detailed in a July 2017 news story, the occurrence of extreme low-flow conditions in NC piedmont streams is increasing.  The story is based on USGS research,  that looked at the 7-day, 10-year (7Q10, W7Q10) low-flow discharges.  The 7Q10 is the lowest average discharge over a period of one week with a recurrence interval of 10 years. The measure identifies rare and extreme events of low water flow in streams.

The USGS report summary findings for 7Q10 trends at 63 gages with 30 or more years of climate date include the following:

• The 7Q10 discharges did not change for 3 sites,
• 5 sites had increased discharges with a median of +13.2%;
• 55 sites had decreased discharges with a median of -22%;
• The median percentage change for all 63 stream gages was –18.4 percent.

Factors such as increased development, increased water withdrawals from agricultural, industrial, and municipal users, and erratic rainfall patterns contribute to the reduced flow critical for drinking water and dilution of pollutants.

Indeed, findings from UNC researcher Nathan Hall presented at this year’s WRRI conference pointed out that recent algal blooms in the middle Cape Fear River are directly tied to low-flow events.  Hall summarized the following:

1. Recent, extreme, summertime low‐flow events provide sufficient light and travel times for algal bloom development.
2. Stratification likely plays an important role in determining overall phytoplankton biomass and competitive dominance by positively buoyant cyanobacteria.
3. Recent increase in blooms likely related to flow rather than increasing nutrient concentrations because nutrients are, and have been, replete for a long time.

What’s this all mean? Most importantly, our water is a finite resource that needs to be protected to insure that both human and natural uses are sustainable.  Water wars can be found throughout the US and given the trends found in this USGS research, we need mindful policies to insure they don’t happen here.