Rashi: Just like I pour water on the thirsty (Radak - a thirsty place), so I will pour My spirit on your seed. Also Yo'el said "Eshpoch Es Ruchi Al Kol Basar" (3:1). This refers only to Yisrael - "v'Nib'u Bneichem u'Vnoseichem" (ibid). The Nefesh of Yisrael in Galus is thirsty for Nevu'ah, which ceased many years ago -"Lo Ra'av la'Lechem v'Lo Tzamei la'Mayim Ki Im Lishmo'a Devar Hashem" (Amos 8:11). Hashem promised to quench the thirst, like water satiates a thirsty place - "Etzok Ru'ach Al Zar'echa."

Malbim: Divine influence comes only on those who are prepared for it, and only where it is needed. Just like Hashem sends rain only for Sedei Terumos that need it, in any case once He sends influence for those who are prepared and need it, it happens to fall also for those who are not prepared for it, just like rain sent for grain fields comes also on other places. Even though I pour water for the thirsty, for land and vegetation thirsty for water, it will pour also on the entire land.

Radak: It is dry and thirsty. The entire land is called Yabashah as a Klal - "va'Yikra Elokim la'Yabashah Eretz" (Bereishis 1:10). In particular, it refers to a dry place.

Radak: The Navi spoke to his generation, and promised them that the salvation will come in their children's lifetime.

Malbim: Zar'echa are old children; Tze'etza'echa are small children. I will pour Ru'ach Kodshi on Zar'echa, who are prepared for Nevu'ah, and on Tze'etza'echa, who are not prepared. Also they will be blessed!

Radak: The bodies of Yisrael in Galus are thirsty for salvation and all the good that Hashem promised them when they will return to their land.