Converting water logger pressures to depths to storage or flows

How are water pressure logger measurements converted to diverted flows or reservoir storage?  Why does anyone even have to have an electronic pressure logger?  PMC, Onset Computer, In-Situ, , and other manufacturers sell data loggers and water level loggers, not pressure loggers, so why is this post talking about measuring pressures at all?

Most “loggers” record pressure, because that is the easiest physical attribute to measure.  A data logger in water does not know how deep it is, and it does not know how much flow is going by, or how much water is being stored in a

 

reservoir.  Pressures relate directly to static (standing) water depths, and then equations convert the depths to flows, or to reservoir storage volumes.

 

How is pressure converted to depth?  It’s an easy calculation – water that is one foot deep has a pressure of 0.4335 psi at the bottom.  So, if your logger measures 7.00 psi, then the calculation to get depth is 7.00 psi / 0.4335 psi per foot = 16.15 feet of depth.

Now that you can calculate any depth, how do you convert depths to reservoir storage?  That requires an Area-Capacity curve, also known as an Elevation-Storage curve.  The points can be picked off the curve.  For example, in the curve below, a depth of 8.5 feet would correspond to an elevation of 2,802.5 feet, and a reservoir storage volume of 30 acre-feet.

An owner of a reservoir with a capacity over 10 acre-feet must collect monthly storage values.  That’s easily done by hand.  However, a reservoir with a capacity of 50 AF requires weekly measurement; over 200 AF requires daily measurement; and over 1,000 AF requires hourly measurement.  That is really tedious to do by hand.

This is where an Excel spreadsheet can make the task a whole lot easier!  The spreadsheets shown below are just for this.  The first sheet helps translate a graph into a table of elevations and storage volumes.  The second sheet translates collected pressure values into depth and storage values, for as many data points as needed.

How are pressures converted to flows?  The logger is in a stilling well, usually a pipe connected to the inside or outside wall of the weir, flume, or orifice.  It measures pressure, which is easily converted to depths.

As with reservoirs, Excel spreadsheets make the conversion process a whole lot easier.  The sheets below have the rating curve for a suppressed weir, and the second sheet converts pressure to actual water depths over the weir boards.  Even for thousands of hourly readings, the hourly flow volumes are quickly calculated and are ready to send to the Water Board: 

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