1)
What is the difference between the Reisha and Seifa?
1.
Rashi, from Ta'anis 9b: He gives Matar to Eretz Yisrael, and sends water via a Shali'ach to other lands.
2.
Devar Eliyahu: Geshem in its place is called Matar. After reaching the land, if it goes elsewhere (e.g. via rivers), it is called Mayim. Malbim - some lands, e.g. Egypt and some places in Africa, are irrigated via rivers that bring water from elsewhere. This shows His Hashgachah, that water not lack in any place prepared for human settlement.
