It is not intended to delve too deeply into the subject here, but it is important to understand the basics. The soil particles themselves only change over longer time scales than we are interested in here, but it is important to know as much as possible about your soils, down to at least a metre. In practice this is very difficult to identify without good measuring equipment, as often some layers of soil have finish draining while others will continue to drain for some time. A very important 'ingredient' in soil is oxygen as it is needed for roots to operate efficiently. It is only when one layer is 'overfull' will water continue down to the next.
Soil water holding capacity is easiest to understand in percentages of water.
If we measure a soil moisture content at multiple depths we can then arrive at the total amount of water in that soil. Also we can see that the top 30cms is holding 70mms. Closed Channel equipment Sprinklers, pipe, pumps, drip equipment, center pivots, travelers.