

Q: How do I calculate how much water is needed for an area? 

A: This is a question I get sometimes, and not often enough. A lot of times folks buying small farms, or starting them don’t calculate this out before they start  big mistake. I’m answering the question below for the Northwest US where we have an irrigation season that’s about 22 weeks long.
The simplest way to estimate water usage is to assume 1" per week for all of the space, for the irrigation season. Here the irrigation season is typically the last week of April to the first week of October (although that varies from year to year). I usually think of a water source of 5GPM as sufficient for 1 acre of irrigation. For a 22 week irrigation season you'd be looking at 22" of water. The conversion for 1" of water to gallons depends on the area you're covering. For 100 square feet you'd be looking at about 62.5 gallons (per week  1375 gallons per season for just 100 square feet). This number is a good average for calculating water use, but there are a lot of variables. If you're watering inefficiently the number can go up (overhead water at inappropriate times, lots of overspray, leaky systems, etc). The number can go down if not all of the beds are in production all of the time (therefore not being watered all of the time, less likely in a situation where you need maximum production from the space). When calculating out the space you're watering you may or may not include the paths. Drip systems are considered in the range of 95% efficient (multiply by 1.05), overhead might be more like 60% efficient (multiply by 1.67) depending on overlap and other considerations. What you're doing is estimating though so you might look at the range of possibility. For the fastest estimate, just multiply your total square feet of garden space by 13.75 and that will give you a high end estimate for gallons used over an entire season.
This should give a very rough idea, there a lot more detail that you could go into, but for an initial estimate it’s probably not necessary.
July 2011 

© joshvolk.com 
