Or, because the soil of a mountain is light and that of a plain heavy, I might think that the waters would drive the soil from mountain to valley, and the valley, (having become "caked") would lack water; it is, therefore, written "a land of mountains and plains," each (nourished with water) in its own way. And thus is it written (Zechariah 10:11) "Ask the L-rd for rain in the time of late rain, etc."
Or, because Eretz Yisrael is surrounded with mountains, I might think that what is revealed (i.e., exposed) drinks (rain), and that what is concealed does not; it is, therefore, written (Devarim 11:11) "From the rain of the heavens you will drink water" — both the revealed and the concealed. And thus is it written (Job 37:11) "Af-Bri (the plenipotentiary of rain) will load the cloud. He (Af-Bri) will scatter the cloud of His rain, and he around (the mountain) (will guide it) with his stratagems." So that clouds surround it and water it from every side.