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We determine the volume of water stored in each of our dams by combining a measurement of the water level in the dam with a model of the ground profile below the water surface. The level measurement is currently performed with either a float-well device or a submerged pressure sensor. In both cases the instrument is calibrated using gauge boards installed at the dams.

As with all physical measurements there is a degree of uncertainty when measuring our water levels. This uncertainty, which is typically 1-2cm, is partly due to the calibration process and partly due to the finite resolution of the measurement instrument. This small uncertainty in the measured water level can translate into a large uncertainty in dam volumes because some of our dams have a very large surface area.

This effect has been quantified and used to determine the overall uncertainty in the volume of water in the individual hills dams and also in the total volume in all of the dams. The uncertainty in the total volume resulting from the measurement of water levels is ~0.4 - 0.5 gigalitres.

Our knowledge of the ground profile below the water surface is not perfect and as a result there is some additional uncertainty in the stored volume in each dam. Unfortunately it is not possible to quantify this effect from existing survey data.

If the uncertainty in total dam volumes is greater than ~0.4 - 0.5 gigalitres we will present the total dam volumes to a single decimal place going forward.