Harvesting rainwater to use for growing vegetables makes a great deal of sense. Unfortunately, the most common method of rainwater harvesting isn’t the most effective. Typically, gardeners invest in a rain barrel — which holds only 50 or 60 gallons of water — and then dole out the captured water to plants as needed, hopefully emptying the barrel before the next storm.
But 50 gallons is only a small fraction of the water you could be harvesting each time it rains. During a 1-inch shower, more than 900 gallons of water flow off the roof of a 30-by-50-foot house or barn. Instead of catching just a little bit of it in a rain barrel, why not capture it all? You can do just that with a simple setup that diverts rain from your downspouts directly to your garden. We’ll tell you more about how to do this in a minute, but first, we’ll explain why we think it’s such a good idea.
How Soil Stores Water
Even many experienced gardeners have trouble comprehending just how much water soil can hold. Except in areas with consistently high rainfall, your garden soil’s moisture level will seldom be at “field capacity.” That’s the term scientists use to describe the maximum amount of water a soil can hold. When it rains or when we irrigate, gravity pulls the water down into the soil. After a heavy rain, some of the water may move all the way down to the water table or the bedrock, but a large amount of it is held by capillary forces that cause water to coat each soil particle and partially fill the spaces between particles. (An example of capillary action is the way a paper towel absorbs liquid.) That capillary water is what your crops use as they grow.
Each soil’s field capacity varies depending on how much sand or clay is in it. One cubic inch of coarse sand may contain 125,000 particles, while the same amount of the finest silt could contain 15.6 trillion particles! Soil particles have an astonishing amount of surface area. One cubic inch of an ordinary soil (with a mix of sand, silt and clay particles) could have a surface area of 25 square feet.
What those numbers mean is that many soils can hold 2 to 3 inches of water in each foot of soil depth, and garden soils that contain lots of organic matter can hold even more. Crop roots can reach down 4 feet — sometimes even 8 feet deep — to tap this capillary water. To be sure crops get the water they need, gardeners would ideally want to keep their soil moisture near field capacity to a depth of at least 4 feet. During peak growth, crop transpiration together with surface evaporation can draw as much as a half-inch of water per day. The more water you’ve stored in your soil, the less you will need to provide supplemental irrigation