Here's a very nice explanation of how an irrigation system works from Rainbird. Who needs an irrigation system in the rainy Northwest, you might say? Pretty much anyone who wants a green lawn and non-stressed plants at the end of September – unless you're really into dragging hoses and sprinklers around your yard. Did you know the average rainfall for Seattle is only about 3" for July through September? Hardly makes up for the other nine months, but we'll take what we can get.