Albert Barnes Commentary Isaiah 22:9

"And ye saw the breaches of the city of David, that they were many; and ye gathered together the waters of the lower pool;" — Isaiah 22:9 (ASV)

Ye have seen also the breaches - You who are inhabitants of the city. That such breaches were actually made, see (2 Chronicles 32:5).

Of the city of David - Of Jerusalem, so called because it was the royal residence of David. Zion was usually called the city of David, but the name was given also to the entire city.

And ye gathered together... - That is, Hezekiah and the people of the city collected those waters.

Of the lower pool - (For a description of the upper and lower pool, see the notes at Isaiah 7:3). The superfluous waters of the lower pool usually flowed into the valley of Hinnom, and from there, into the valley of Jehoshaphat, mingling with the waters of the brook Kedron. It would seem from the passage here that those waters were not usually retained for the use of the city, though it was possible to retain them in case of a drought or a siege. At present, the lower pool is without the walls, but Hezekiah appears to have extended a temporary wall around it so as to enclose it (see the note at Isaiah 22:11). This he did, probably for two purposes:

1. To cut off the Assyrians from the supply of water; and
2. To retain all the water in the city to supply the inhabitants during the siege; see (2 Chronicles 32:4), where it is expressly declared that Hezekiah took this measure to distress the Assyrians.