I love rain harvesting.

My rainwater barrel is super cool and a good metaphor for water conservation. But I find it interesting to actually run the math and see how little water they hold, and I wonder if they are taking the limelight away from more important organic gardening practices.

Rainwater Barrel Mathematics 101

If your roof is 1000 square feet (100 square meters) and you get 1 inch of rain on a spring Friday afternoon (don’t worry, it had to get it out of its system before the sunny, warm weekend ahead), you will have 625 gallons of water coming off that roof.

I don’t know about where you live, but where I live, 1 inch of rain is common.

Let’s say you are rain harvesting into a 60 gallon rainwater barrel that will get nice and full and you can use it when organic gardening to water your tomatoes another day. But 90% of that water is going elsewhere, maybe into your city’s sewer system. Your barrel can only take 1/10 inch of rain from that 1000 square foot roof.

So while I like the 60 gallon rainwater barrel, I think there are better solutions.

Rain Harvesting Into Cisterns

A 600 gallon rainwater cistern could handle almost a full inch and a 1500 gallon cistern could easily take 2 inches. Think of it as a giant rainwater barrel. I think every house should have one of these, the size dependant on the amount of rainfall in your area.

They can be a bit tricky to fit into the landscape and some people think they are an eyesore, but they can be cleverly hidden or even buried.

Rain Harvesting Into Ponds

A more attractive organic gardening solution for rain harvesting would be to build a small pond to catch the water. A pond the size of a king size bed and 2 feet deep would hold your inch of water and if constructed thoughtfully, would provide drinking water for insects and animals.

Rainwater barrel
This awesome pond holds way more water than a rainwater barrel

Every organic garden should have some open water in it for this purpose. Mosquitoes aren’t a problem if you keep the water moving and put some effective microorganisms in it, but that’s for another article.

While your rain harvesting into a pond, you might as well bring your greywater into it to be cleaned and used as well, rather than piping it to the sewer or septic system. Your sink, shower and laundry water can all be put to use in the garden. Make sure to use non-toxic soaps, shampoos and detergents.

Rain Harvesting Into Soil

What I think we should mostly be focusing on is the soil in our organic gardens. The soil is the best way to hold onto water and makes rain harvesting into a rainwater barrel look like child’s play (not that there’s anything wrong with child’s play).

Let's look at how it works using 1 inch of rain as an example. If your roof is 1000 square feet (100 square metres) and you get 1 inch of rain, you will have 625 gallons of water coming off that roof.

A loamy sand, which is 70-85% sand (hence not very good at holding water), can hold 1.1 to 1.2 inches of “plant available water” (plant available water refers to the water that can actually be taken up by plant and microbes and also lost to evaporation; the soil can hold more water than that, but some of it is so tightly held by the soil that it can’t be used).

This loamy sand will already be fairly saturated from our 1 inch rain, though, so it won’t take much of the water from the roof.

A silt loam, which is 75-90% silt, is great at rain harvesting as it can hold the most “plant available water” at 2 to 2.5 inches. It can handle all of the 1 inch rain if you can spread the roof runoff over 1000 square feet. That might be possible with some smart engineering.

A clay soil, which has at least 55% clay, actually holds less plant available water than the silt loam because while it can hold more water, it is held so tightly that plants and microbes can’t get it. It holds 1.2 to 1.5 inches of water, again not much of our roof runoff.

So what to do?

Organic Matter: The Magic Ingredient

There is one thing that we can put in and on our soil that will hold the extra water, and that is organic matter. Organic matter is a rain harvesting genius. I don’t know exactly how much water organic matter can hold, but an article on the USDA website says it can hold 10 to 1000 times more water and nutrients than the same amount of soil minerals.

Even if organic matter holds only 4 times its weight in water, you can hold nearly an extra inch of water if you can increase the organic matter content of your soil by 1.5%, easily doable.

Rain Harvesting
Rain harvesting into the soil is the best option

A study in the Journal of Soil and Water Conservation found that regardless of the type of soil (sandy loam, clay, etc.), “as organic matter increased from 1 to 3%, the available water capacity approximately doubled. When organic matter content increased to 4%, it then accounted for more than 60% of total.”

That means we can now be rain harvesting 2 or 3 inches of rain from the roof, 20-30 times more than your rainwater barrel. This is what organic gardening is all about.

Organic matter can be brought in via compost or mulch. Incorporating 2 inches of good compost into the top 12” of a new garden bed will easily increase the organic matter content by 2-3% (although you might as well go for 6 inches of compost while you’re doing it).

Mulch goes on top of the soil, but it too holds a lot of water.

Now all that’s left is to direct the runoff to your gardens with gutters, and perhaps slight grading of the ground and swales and berms.

Rain Harvesting Summary

The rainwater barrels are a nice idea, but they don’t go very far in rain harvesting from the roof. Cisterns and ponds can hold more and should be used, but the winner in this event is the soil and especially organic matter.

Most soils are low in organic matter and if that’s the case for yours, bringing in lots of compost and mulch will transform your soil into the ultimate storage tank.