Available Water Holding Capacity and Available Water Storage Capacity

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