In the spring, when the soil is moist and the surface is warm, roots near the surface show the most growth. As the soil dries out, roots near the surface die and new roots grow deeper in the soil. As the season progresses, the depth of active root growth moves progressively deeper and deeper in the soil. With each rain or irrigation new roots appear near the surface. These new roots absorb water and nutrients and then die.
Lets go back to the analogy of the upside down tree to represent the root system.
We can imagine that instead of one trunk that there are several and the trunk and branches of this root system are composed of woody roots. Now imagine that the leaves on this tree are not green and flat like an apple or a grape leaf but long and thin like the needles of a pine and white not green. I find that thinking of white roots as leaves helps me to put the process of root growth in a visual framework that I can understand. The white roots really are like leaves. They are temporary structures that absorb water and nutrients.