Az-Zumar: (21) "Have you not seen that..."
{He sent down water from the sky}
It is rain. It has also been said: Every water on earth descends from the sky to the Sakhra (the foundation rock), then Allah distributes it.
{And He caused it to flow}
Meaning: He introduced it and channeled it.
{Into springs in the earth}
Meaning: Into fountains, pathways, and conduits, like veins in bodies.
{Varying in its colors}
Meaning: Its forms—green, red, yellow, white, and others—and its varieties, such as wheat, barley, sesame, and so on.
{Then it withers}
Meaning: It reaches its state of dryness. According to Al-Asma'i: Because when its dryness is complete, it is time for it to be uprooted from its place of growth and pass away.
{And you see it turning yellow}
(Note: This refers to the verse's continuation).
{Then He makes it debris}
Meaning: Crumbling and broken fragments.
{Indeed, in that is a reminder}
Meaning: A reminder and an alert that there must be a Wise Creator, and that this exists by design and management, not by abandonment and neglect. It is also permissible that this is a parable for the worldly life, as in His saying: {The example of the worldly life is but like...} (Yunus: 24), and {Present to them the example of the worldly life} (Al-Kahf: 45). It has been recited as: (musfarran - turning yellow).
{So is one whose breast Allah has expanded to [accept] Islam and he is upon a light from his Lord [like one whose heart is hardened]? Then woe to those whose hearts are hardened against the remembrance of Allah. Those are in manifest error.}