Why does the Torah insert "Harim"? What is the advantage of mountains over flat terrain?
Rashi: 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: Refer to 11:10:5:1.
Rashi (in the commentary on the prayer for rain of R. Elazar ha'Kalir that is attributed to Rashi, citing the Sifri): It can be compared to king who drove past a poor man (with reference to other countries) lying on the street, who ordered his servant to sustain him; but when he subsequently drove past 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.
This is also the opinion of the Seforno, who comments on the current Pesukim that Eretz Mitzrayim 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.