Scott River flow correction at USGS gage: a win for data accuracy
Flows in the Scott River at the primary monitoring gage appeared to have doubled overnight last week. What happened? It certainly didn’t rain. The answer? The U.S. Geological Survey (USGS) finally recalibrated the flow gage. The gage was corrected on August 24th, after two months of no on-site field-checks by USGS. This gage, located below Scott Valley at river mile 21, is the flow measurement site that is used by the State Water Resources Control Board (State Water Board) to determine whether its “Drought Emergency Minimum Flows” are being met on the Scott.
AgWA had been encouraging the State Water Board and California Department of Fish and Wildlife to coordinate with USGS for several months in order to get the gage calibrated.
“Getting accuracy is very important when the State Water Board is using the USGS flow gage as a regulatory tool,” says Sari Sommarstrom, a retired watershed consultant who has observed Scott River flows for 30 years.
“My experience is that the USGS flow gage needs to be field-checked at regular intervals, particularly during low flows, to ensure its accuracy and readings and be adjusted as needed,” Sommarstrom says. “When it hadn’t been checked since June 21st, I was concerned that the real-time readings online might not be accurate by early July.”
The State Water Board imposed curtailments in Scott Valley in July: surface water diversions were curtailed on July 2nd, and groundwater was curtailed on July 14th.
“The State’s Drought Regulation assumes the worst about our water situation, so without accurate, shared monitoring data, Scott Valley water users can suffer,” says Sommarstrom.
Calibration of the gage entails measuring the cross-section of the river channel at the gage and getting actual flow (“discharge”) readings across multiple points on this line. Stream channel bottoms can change shape over time, especially in a sandy streambed like the Scott River, causing the flow calculation to change.
Clearly, an uncalibrated gage can cause significant errors in the daily flow readings. In this case, the corrected flow reading more than doubled, jumping from (the uncorrected) 3.37 cubic feet per second (cfs) on 8/19 to a corrected real-time reading of 8.94 cfs on 8/24. This was after the USGS performed a site visit on 8/23. USGS has since corrected their “preliminary data” backwards to the previous site visit, so the accurate readings are now showing on the USGS website. This correction practice is a standard method with stream gage measurements. USGS told Sommarstrom that they’d been delayed due to staff shortage, vacations, and the McKinney Fire risk.
AgWA has been encouraging accurate data collection, as well as sharing of monitoring data between the state water and wildlife agencies and entities such as AgWA and the tribes.
AgWA argues that accurate facts about the Scott River and its fish populations need to be brought to light. For example, how much flow does it take for a Chinook to get upstream to spawn? And is the USGS flow gage reading that level accurately? These are crucial questions for our farmers and ranchers, who are sacrificing the water they need to stay in business in the name of saving fish.
Another question that is undoubtedly influencing the State Water Board’s actions: Is the current drought unprecedented? This question can be answered with accurate data.
“We had voices saying the flows this month were at ‘record lows’,” Sommarstrom says. “But it turns out they’re not at record lows. In fact, since USGS began taking readings in 1941, the record-low for today (August 30th) was in 1994, at 4.5 cfs. Today, it’s at 12.8 cfs. While this drought is certainly real and something we need to deal with, the hyperbole needs to stop. We need accurate information—and that starts with having reliable data.”
For those interested in viewing daily flow readings at the USGS gage (river mile 21) visit: USGS Current Conditions for USGS 11519500 SCOTT R NR FORT JONES CA. You can receive email updates by subscribing to WaterAlert.
