Extreme drought in many regions of the world is creating an opportunity for many stream hydrographers to record a historic event. A report from Poland shows ‘no end of secrets’ being revealed by rivers as they dry up.
July 2015 was the hottest month in recorded history. The Clausius Clapeyron relation tells us that the moisture holding capacity of the atmosphere is non-linear with temperature. Over global land surfaces, the July average temperature was almost a degree higher than the 20th century average. This translates into an additional capacity of about a gram of water per kilogram of air. That means that the atmosphere has had the capacity to suck up about 10% more water than ever before. The net result is that many regions are dryer than they have ever been.
Low-flow measurements being made during these extreme conditions will inform hydrological science and watershed management decades into the future. In distant times, end-users of the data may have questions about the reliability and accuracy of the data. The time to answer those questions is now, while we’re in the field.
Extreme low-flow is a good time to be extremely careful in field procedures and documentation.
A lot can go wrong with the measurement of small flow. There may not be sufficient channel width to sample channel depth with sufficient resolution. There may not be sufficient depth to sample the velocity profile with sufficient resolution. There might not be sufficient velocity for an accurate velocity measurement. There may be current angles, or even reverse flow, but the force of the current may be insufficient to accurately measure direction of flow. There might not be enough cross-sectional area to get a representative velocity field for ADCP measurements. The control feature may not be sensitive to very small changes in stage. Small rearrangements of the stream bed may result in large effects on the stage-discharge relation. To make matters more complicated, high stream temperature and slow velocity are favorable to the growth of aquatic vegetation on the control.
Low flow measurement provides an excellent opportunity to be especially thorough in procedures and documentation. There is no better time to:
- survey the cease-to-flow elevation of the control section
- survey a cross-section of the control reach
- photograph the control and measurement reaches
- sample the bed material size distribution
- walk the stream upstream and downstream of the gauge
- inspect and improve water level orifice/intake/sensor placement
This is also an excellent time to conduct replicate gaugings. Changing equipment and/or initial point for soundings provides a wealth of information that is helpful for accurate calibration of the low end of the rating curve. In the time it would take to do one high flow gauging it may be possible to do several low flow gaugings.
Unfortunately, I don’t get out in the field much anymore. I’ll be very interested to hear first-hand experiences from hydrographers who are recording some of the lowest flows in the past century. What are you learning about your streams, your gauges, and your methods and procedures as you monitor change through extreme low flow? What are your ‘best practices’ for monitoring low-flow?
Photo Credit: https://www.ncdc.noaa.gov/sotc/global/201507 accessed on August 28, 2015