Calculation of Mean Particle Size vs. Median

[Hodge.WaterResources 0]

The model inputs for a sediment source in PTM require a standard deviation (in phi) and a median grain size in mm.

In order to generate a distribution of particle sizes you would also need a mean particle size. How does PTM (or perhaps an SMS pre-processor) calculate a mean grain size from a median grain size and a distribution?

As an example I have been running a simple model to look at what the grain size distribution in the model is like and I get results like this.

Model Input:

median grain size: 0.088 mm

St Dev. (phi): 1.22

Statistics of Model Output

median: 0.0879 (mm)

mean: 0.1497 (mm)

How is the mean calculated?