Making a Water Budget

Okay, so you have decided to take a good, hard look at how much water you are using on your garden or landscaping and see if there is a better way to do it. The first step is to figure out how much you are getting and how much you are going to need. If it is starting to sound like basic budgeting, you are right. It is time to make a water budget. My city, Prescott, Arizona, USA, has a really good worked example for the locals here (link downloads a .pdf file). The basics are actually very simple.

First, you need to separate your planted areas by water requirements. Are they xeriscape, garden, turf, or native landscape plants? Garden and lawn take the most water and are generally considered oasis areas. Non-native landscape plants and fruit trees typically take less water than an oasis area, but more than native vegetation. Native plants and xeriscape plants usually need additional water only in a drought. Then you need to figure how much water you are going to need for each area. Prescott's numbers are probably pretty good as a starting point. Prescott is above 5,000 feet in elevation in Arizona, so it doesn't get excessively hot here (typically 5 or so days a year over 100 degrees F), but it is very dry, typically in the 10-30% humidity range. We do also get normal winters, being in USDA Zone 7. So you may have to adjust water requirements just a little to fit your area: up for hotter, down for cooler, down for more humid. It may take some trial and error. For Prescott, oasis areas require 8" of water a month during the growing season and 1" per month during the dormant season. Trees require 45 gallons a month during the growing season and 4.5 gallons a month during the dormant season.

Secondly, you will want to look up your rainfall data. I get mine from the Weather Channel page, which has monthly averages. Given the nature of climates, much more accurate data than that isn't actually going to be more accurate.

Lastly, you just calculate how many inches of rain you are going to need for your landscape in a given month and subtract from that how many inches you are likely to get. The difference between the two is the amount you have to add to your garden or landscaping to keep it alive and allow it to thrive. That water can come from your tap, gray water, or from captured rainwater.

There really isn't much to say about tap water. You turn on the faucet and it comes out. It is super easy and reasonably cheap, but not necessarily the best solution. If you are in an area that is short on water, and most are these days, it drains the local supply and increases shortages. Also, while water doesn't cost too much, neither do vegetables. How much water does it take in your garden before you are paying more for the water to grow your veggies than it would have cost to get them at the store?

Gray water involves double using SOME of the water from your house. I will cover that in detail in a future post.

Capturing rainwater is another good way to get water and there are two basic ways to do it. The first is to shape your land so that it collects water and the second is to collect rainwater off of the roof of your house. I'll also cover those in upcoming posts.

Finally, remember that just because you get more water than you need in a given month, it doesn't mean that it will arrive when you need it. Tap water is a really great backup system.