Converting Logger Pressure to Depth & Storage/Flows

If you have a pipeline as part of your diversion, then an in-line meter with an integrated data collector can be installed.  The data files from these units are

McCrometer McPropeller inline meter with data collector

easily readable in Excel, and the files can be sent directly to the Water Board to meet the requirements of SB 88.

What if you don’t have a pipeline?  Then your flow needs to be measured in the open ditch with a weir, flume, or orifice.  These devices measure the flow but they don’t record the data.  To continuously record data, a submersible logging instrument must be used to measure the water pressure at the bottom of the box.  These logging instruments are commonly put into stilling wells that are inside or outside the measurement device.

Flume with attached stilling well for water level logger

How are water pressure logger measurements converted to diverted flows or reservoir storage?  Why does anyone even have to have an electronic pressure logger?  Onset ComputerPMCIn-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?

Bluetooth Hobo Logger, cabled to recorder and barometric compensator unit – least expensive option for a single location

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 1.60 psi, then the calculation to get depth is 1.60 psi / 0.4335 psi per foot = 3.69 feet of depth.

Note that water level loggers can be of two types.  The least expensive are completely submersible, and do not compensate for barometric pressure.  For an idea of  the readings of barometric pressure in a measurement device, a 2 foot deep logger records a pressure of 0.8670 psi.  Atmospheric pressure at sea level is about 14.7 psi, and high in the mountains may be 12.0 psi.  Air pressure is much greater than those measured in ditches.  Usually two of these loggers are used at once, one in the water, and one out of the water measuring only air pressure.  This also eliminates the variability in pressure due to weather changes.

Stilling well in a weir

The second type of data logger compensates for barometric pressure at the same time water pressure is being recorded.  That way, the water and air pressure data sets do not have to be combined before conversion to depths.  These loggers were always more expensive until the Bluetooth Hobo water level logger came along; as of February 2019 I found that it is the least expensive option for a single location.

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.

For diversion ditches from a stream, 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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AllWaterRights - Rights To Water Engineering, Inc.

Solving Diverters' Headaches To Provide Peace Of Mind And Help Stay Out Of Trouble. Helping California residents understand, define, and protect their water rights since 2005.

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