Yesterday I was walking by a tree and heard a hissing noise coming from underground. That was odd. So I started to dig. Three hours later I was able to diagnose precisely what had happened - a tree root (Leylandi Cypress) roughly 6 inches (15 cm) in diameter had grown around a 1 1/4 inch (3.2 cm) plastic irrigation pipe and over the last 15 years slowly fused together until it squeezed the water pipe closed. The hissing noise was the water trying to squeeze through the narrow opening which remained. This sort of explains the poor water pressure on that side of the property.