What does Isaiah 22:9 mean?
The prophet is probably laying out what the king and the people of Jerusalem did to prepare for the siege by the Assyrian army. Another possibility is that this refers to the siege by the Babylonian army at a much later date—well after Isaiah's life. He has already said that they checked on, and likely distributed, the weapons from the armory (Isaiah 22:8).Next, he describes an assessment of Jerusalem's walls. Many spots in the wall were discovered to need repair. The people figured out how to redirect water from outside the gates into Jerusalem. This was essential so that the siege would not deprive them of drinking water. This tactic would also limit the water supply available to the invaders.
King Hezekiah had famously dug a long tunnel all the way from the spring of Gihon in the Kidron Valley into the city itself (2 Kings 32:30). This king specifically dug this tunnel to provide water to the city during any future siege.