Agriculture Reference
In-Depth Information
Rain Harvesting Into the Soil
While cisterns and ponds are a better option than a rainwater barrel, we should mostly be
focusing on the ultimate storage solution, the soil. The soil is the best way to hold onto wa-
ter and makes rain harvesting using a rainwater barrel look like child's play.
A loamy sand without organic matter, which is 70-85% sand, and not very good at hold-
ing water, can hold at least two inches of water. Of course, just how much depends on how
wet it is already, the health of the soil food web, compaction and so on. This loamy sand
could potentially store that inch of rain, plus the extra inch from our roof, if we could
somehow direct that inch evenly over 1,000 square feet, which we probably can't, and if
the infiltration rate of the soil is high enough, which it may or may not be.
Soils higher in silt and clay can hold four to five inches of water, but infiltration rates on
these soils are generally much lower than a downpour of one inch of rain per hour, so they
may have enough to handle with the rain alone, unless it comes gradually over a few days.
So what to do?
There's one thing 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 bonanza. Various research has tried to
determine how much water organic matter can hold, often concluding it can hold tens or
even hundreds of times more water and nutrients than the same amount of soil. Even if it
holds only four 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 just 1.5%, and this is easily doable.
A study in the
Journal of Soil and Water Conservation
found that regardless of the type
of soil, “as organic matter increased from 1-3%, the available water capacity approximately
doubled.” That means we can now be harvesting two or three inches of rain from the roof,
20-30 times more than a 60 gallon rainwater barrel can hold.
Organic matter can be brought in as compost and mulch. Even incorporating two inches
of good compost into the top 12 inches of a new garden bed will often increase the organic
matter content by 2-3%. It won't increase the stable humus that much, but I'm just talking
about any organic matter here. Mulch goes on top of the soil, but it holds a lot of water, as
well. Your soil should always be covered in mulch, and when possible, plants. This im-
proves water infiltration and decreases evaporation. Now all that's left is to direct the run-
off to your gardens with gutters, and perhaps slight grading of the ground, using techniques
such as swales and berms.
