Jamieson, Fausset and Brown on 2 Chronicles 32:2-8

2-8. when Hezekiah saw that Sennacherib ... was purposed to fight against Jerusalem--An account of the means taken to fortify Jerusalem against the threatened siege is given only in this passage. The polluting or filling up of wells, and the altering of the course of rivers, is an old practice that still obtains in the wars of the East. Hezekiah's plan was to cover the fountain heads, so that they might not be discovered by the enemy, and to carry the water by subterranean channels or pipes into the city--a plan which, while it would secure a constant supply to the inhabitants, would distress the besiegers, as the country all around Jerusalem was very destitute of water.