The importance of stage measurement

Stage = 0.378m, Q = 0.521m3/s

In monitoring river discharge, the importance of stage measurement cannot be over-stressed. Accurate and reliable stage measurement in addition to regular maintenance of a site stage-rating curve will be fundamental in obtaining continuous discharge estimation.

For 16 years I've used the same system to monitor stage (pressure transducer and Kadec data logger) with no reliability issues at all. I've just had to change the batteries once per year, and check the offset on each field visit to make sure the visual stage reading matches the data logger reading.

However, recently some issues with the offset adjustment within the data logger have led me to acquire a second back-up system to monitor stage. I chose the inexpensive and widely used Hobo loggers, and have been comparing the Kadec and Hobo loggers since the summer. The Hobo logger system is unvented and so we need to install two sensor/loggers - one to monitor pressure in the water column and another to monitor atmospheric pressure.

The other major difference between the two systems is that the Kadec logger can be easily downloaded on the river bank, while the Hobo logger must first be retrieved from the river and then re-installed after download, making careful note of stage levels during this process.

The two stage hydrographs above show that overall the agreement is good between the two logger systems. However, there are some significant differences, especially during periods of moderate to low flows, in particular the period in early September. The Kadec logger seems to show a more unsteady stage curve during lower flows, and this needs to be further investigated. Perhaps the atmospheric pressure venting or the temperature compensation is not working as effectively as it should be.