Charles Ellicott 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 ... —The prophet paints the hasty preparations for defense. So in 2 Chronicles 32:5: Hezekiah built up all the wall that was broken, and raised it up to the towers, and added an outer line of defense. The "city of David" is, of course, the fortress of Zion.

The waters of the lower pool. —This was the Lower Gihon, now the Birket-es-Sultan. The operation is described more fully in 2 Chronicles 32:3–4. Its object was to stop the outflow of the streams, and gather them into a reservoir, partly, of course, for the supply of the inhabitants during the siege, but still more that the Assyrian armies might find little or no water in the immediate neighborhood of the city. Sargon, in his inscriptions, describes like preparations at Ashdod (Smith, Assyrian Discoveries, p. 291).