Why does the Torah insert "Harim"? What is the advantage of mountains over flat terrain?
Rashi #1: Refer to 11:10:5:1.
Rashi #2: In one Beis-Kur of flat terrain one can plant one Kur, whereas on a mountain one can plant five - four on the slopes and one on top.
What is the significance of the fact that Eretz Yisrael's water supply comes from the rain of the heaven?
Rashi #1: Refer to 11:10:5:1.
Rashi #2 (in his commentary on the prayer for rain of R. Elazar ha'Kalir that is attributed to Rashi, citing the Sifri): It can be compared to a king who passed a poor man (with reference to other countries) lying on the street, and who ordered his servant to sustain him; but when he subsequently passed a poor man from a respectable family (with reference to Eretz Yisrael) lying in the street, he sustained him personally from his own treasury.
Refer to 11:10:1:4.
Ta'anis, 9b (according to R. Yehoshua) 2 : This Pasuk proves that rain comes from the heaven.
This is also the opinion of the Seforno, who comments on the current Pesukim that Eretz Egypt does not need rain, whereas Eretz Yisrael does - since there are insufficient rivers to provide all its needs - and continues - with a comment on the following paragraph - that the supply of rain depends on whether we keep the Mitzvos or not.
As opposed to R. Eliezer, who mantains tht the rain-water comes initially from the sea. See Torah Temimah, note 3.