See 2 Chronicles 32:3 note. Either then or afterward, Hezekiah conducted the water of this spring by an underground channel down the Tyropoeon valley to a pool or reservoir (marginal reference).
Related Commentaries of 2 Chronicles 32:30
2 Chronicles 32:30
30 This same Hezekiah also stopped the upper watercourse of Gihon, and brought it straight down to the west side of the city of David. And Hezekiah prospered in all his works.