Commentary
The word: يَنَابِيعَ (yanabi in verse 21: فَسَلَكَهُ يَنَابِيعَ فِي الْأَرْضِ (made it penetrate into the earth [ and gush forth ] in the form of springs) is the plural form of: یَنبُوع (yanbu) which means springs that gush out from the soil. The sense is that the act of sending down water from the sky is by itself a great blessing, but also crucial was the arrangement to conserve it underground. But for this arrangement to save this blessing of water, its users would have derived benefit from it only at the time of rains, or for a few days following it. Although, on water depends their life, and it is the kind of need one cannot stay free from, even for a day. Therefore, Allah Ta’ ala did not consider it sufficient to just send down this blessing, instead, made elaborate and very unique arrangements for its conservation. Some of it gets deposited in ditches, ponds, tanks and reservoirs. Then a huge supply is turned into ice and made to sit on mountain peaks and its ridges, an arrangement that takes care of the danger of water going bad. Then ice melts and water travels through veins in the mountains until it reaches the land and gushes out in the form of streams, all over, on its own, without any human input, and finally finds its way through the land in the form of rivulets and rivers. Rest of the water keeps flowing underground which can be retrieved by digging a well almost anywhere.
Details of this water supply system as they appear in the noble Qur'an have been given in the commentary of Surah Al-Mu'minan under the verse: (then We lodged it in the earth, and of course, We are able to take it away - Al-Mu'minun, 23:18). (Please see Ma ariful-Qur’ an, Volume VI, under 23:18, pages 311 to 313).
Later in verse 21, it was said: مُّخْتَلِفًا أَلْوَانُهُ (the crops of different colours). At the time the crops grow and ripen, colors keep changing from one to the other. Since these colors change, therefore, the word: مُّخْتَلِفًا (mukhtalifan), in terms of its grammatical analysis, has been used in the form of: حَال (ha state, circumstantial condition) which denotes change.
In the last sentence of verse 21, it was said: إِنَّ فِي ذَٰلِكَ لَذِكْرَىٰ لِأُولِي الْأَلْبَابِ (Surely, in that, there is a lesson for the people of understanding), that is, in this process - when water is sent down, is conserved, is made available to human beings to grow all sorts of crops and trees the colors of which change following which they turn yellow and dry making grains separate from chaff - there is a great lesson for people of understanding, because they provide the proof of the infinite power and wisdom of Allah Ta’ ala. These are visible signs that could lead human beings to discover the reality behind their own creation, and that in turn, could become the means through which one succeeds in recognizing his or her own creator and master.