Hello.
I'm using a water level logger named 'HOBO U20'.
According to its manual, the Logger records absolute pressure, which
is later converted to water level readings by the software. Normally, atmospheric pressure is 100 kPa at sea level, but changes with weather and altitude. Because my study site is in the middle of the mountain area, HOBO U20 Water Level Logger can be used as a barometric reference to compensate for barometric pressure changes.
I use two loggers and one is set in the stream to collect water level data and the other is set on the ground as a barometric reference.
I would like to know the principle to deduce the water level from both pressure data. Is there the relationship like some mathematical formula ?
The water level (h) is given by h=(P-Pr)/(rho * g) where P is the pressure measured by the sensor in the stream, Pr is the pressure measured by the reference sensor (atmospheric pressure), rho is the density of the water in the stream, and g is the acceleration due to gravity.
You may need to take care regarding the density of water. This is temperature dependent, but also depends on the water quality (e.g. dissolved salts and suspended sediment).
Dear Barry Croke. Thank you so much! :D
Sorry, would you recommend some papers related to this concept, please ? :)
I agree with Barry Croke fully. For variation of density of water with temperature, please refer to any fluid mechanics text.
Dear Beomjeong,
you could review this paper;
http://www.filecciageologia.it/Download/Articoli_Fileccia_pdf/Correcting_GW_levels_using_BE_.pdf
I found the paper that Dr. Abushandi most interesting for well water level adjustments, and certainly makes sense. I learned a few things. However, water level in surface streams would have an immediate or nearly so response to barometric pressure change, since they are open to the air pressure. From the article, the subject is more complex that I imagined, and questions might emerge on stream levels affected by groundwater inputs (gaining streams) or the effect of pressure changes to losses to groundwater (loosing streams). However, streams typically use a stage discharge relationship with flow measurements correlated to stream stage for each measure. For stream gauging, a vented transducer give you a barometric adjusted value of water level, while the unvented are more convenient, they need adjustment. In all my stream gauging, we often do take air and water temperatures, but I do not remember adjusting water levels or flow quantities for temperature readings due to differences in water density. Since the goal of stream measurements with meters is typically to be within 10%, perhaps without a weir or flume of high quality and connectivity of drainage (watershed outlet) boundary to impermeable substrate, the measurement of surface water just is expected to have some precision errors.
I have use transducers from another company (In situ) and they had an excel type program that made the adjustments to the water quality sonde (self contained unit included transducer) and nearby barometer transducer. I dont remember if it used temperature in addition to pressure to make adjustments in water level. I have a file in another computer, so I will let you know if I find anything additional.
In case you did not know, the HOBOware software has a built-in barometric compensation tool. You just load the data from the baro-sensor and the data from the actual water level logger and the software does all the magic.
Dear Oliver Koit.
Yes. you're right, but it didn't work to use the barometric data as a reference. I followed a tutorial video however error message popped up, so I'm trying to do with manual way.
so pity the Q is not simple as it seems at first glance especially for confined aquifer.
Hi.
Maybe it can be helpful.
there is note to show how to read the data.
http://www.solinst.com/products/dataloggers-and-telemetry/3001-levelogger-series/operating-instructions/quick-start-guide/3001-quick-start-guide.pdf
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