Available Water Holding Capacity and Available Water Storage Capacity

Alice you are correct that the text is distinguishing the water available to the plant per rooting depth.

Fundamentally they are very similar except:

In the lecture notes we are distinguishing Available water storage capacity AWSC = FC - PWP [cm3 water / cm3 soil]. The assumption is made that water is applied evenly over space i.e. per cm2 (similar to the idea of rainfall; we report rainfall in mm or cm not cm3] Thus if we know the cm of water per cm of soil (vertically), we can multiply by the rooting depth (in cm) to get the cm of water available to the plant.

re: Box 5.3 (or Box 5.2 in the 2nd edition) - Calculation of the total amount of water available to a plant growing in the field

AWSC = FC - PWP

If moisture content is in g/g (mass basis) i.e. mg water per mg soil, you need to then convert this value into a volumetric water content (cm3 water / m3 soil). To do this you multiply by the ratio of the bulk density of the soil / density of water.

Once you have a volumetric water content (cm3/cm3), to determine the amount of water in the rooting zone, simply multiply by the rooting depth (20 cm in your example) = AWC (in the rooting zone)

I like being very clear with my units, i.e. cm water / rooting zone