Green Man Publishing

Using rainwater

I grew up in rainy England, where rain was so frequent (and dampened so many outdoor activities) that it was hard to appreciate its many virtues. Having spent the last 30 years in California and seen what a low rainfall landscape looks like, I have finally come around to understanding the miracle that is water falling from the sky.

It is easy to look upon rain as an inconvenience, a waste product to be disposed of as quickly, efficiently and safely as possible, but it brings life and the fact that it is raining is nearly always good.

To increase your awareness of the rain that falls on your garden, you could use it to make a living sculpture with a series of linked water features that come alive every time it rains. Gargoyles and rain chains on the roof, could pour water into tubs and bowls, which then runs along canals or rills and over a waterfall into a pond and finally out through a spillway into the soil. In areas with year round rainfall, it can be even more appreciated as some of it will fall in the summer when you are out in the garden.

Rainwater infiltration

In dry climates improving rainwater infiltration should be a high priority. To do this you need to understand how water moves across your land and where it goes.

In most cases the land will already retain most of the water that falls on it, without any problem. You just have to deal with those areas where it runs off or causes erosion for some reason. Go out in the rain and these places should be obvious because you will see accumulated water running downhill.

Soil varies enormously in its ability to absorb water. A couple of days ago we had 5 inches of rain overnight in an early fall rain, yet when I dug around I found places where the ground was still dry only 3 inches down. When rain falls very rapidly on to sloping ground, it quickly saturates the surface soil until no more can enter (until it has percolated down). Further rain then just runs off straight downhill. To prevent this you need to make the water stand still long enough to soak in.

Enhancing rainwater infiltration

Nature has always taken care of rainwater infiltration on your land and will continue to do so unless you mess it up by creating bare soil, paths, driveways and roofs. These become a problem because they allow water to accumulate in volume and move quickly. Of course when you start landscaping you will often do these very things, and may have take steps to prevent erosion and enhance infiltration (almost the same thing). Water becomes a problem when it accumulates in volume and starts to move.

In very dry climates water is too precious to just let it run off down storm drains, so you need to enhance the soils ability to absorb it. You do this by creating obstacles to slow down the flow, so it has time to soak into the soil. You may also want to redirect the natural flow of water, so it soaks into the soil where it can be of the most use. By giving runoff water somewhere to go, you prevent it becoming an erosion problem lower down. You should also have a provision for handling any overflow water that makes it through the whole system.

Water travels from the highest to the lowest spots by the most direct (steepest) route downhill. It moves more quickly on slopes and slows down or stops on the flat. Your aim is to slow down the water flow long enough for it to soak in to the soil, or to direct it to low spots where it can soak in.
Steep or unstable slopes should never be encouraged to absorb water, as it can cause them to slide. Instead water should be drained away from these areas.

Permeable paving should be used in most places as it allow water to go right through it. Impervious surfaces are used when you want to concentrate and collect the water (and have somewhere for it to go).

Rainwater storage

Rainfall is often erratic even in humid areas; it might rain for weeks and then be dry for weeks. It may dump 2 inches of rain in an hour and then be dry for a month. If you are to have this water available when you need it, you have to store it in some way.

An inch of rain equals .623 gallons of water per square foot, so an inch of rain falling on an acre of land gives you 27000 gallons of water. The average winter rainfall at my house is around 50 inches, which means my 2.5 acres receives over 3 million gallons of water every year.

The only practical way to store large quantities of water is in the soil. This is known as passive storage because it doesn’t require you to build or do anything. On good soil this happens naturally without your involvement, but on poor soil you might want to help the water to infiltrate into the soil.

The soil is an enormous sponge that can absorb and holds huge quantities of water almost indefinitely with no evaporation. Water held by the soil doesn’t go stagnant, breed mosquitoes or cost anything to store; it just sits there, available for plants to use in the hot dry days of summer. Many trees, shrubs and perennial plants can send down roots far enough to tap into this ground water, so we don’t even have to pump it out of the ground. My established fruit trees go through 6 rainless summer months (and fruit heavily) just on the rainwater held by the soil.

The easiest way to use rainwater is to let it percolate into the soil and then pump it back out via a well. Of course this is how most rural properties get their water. If you don’t have a well, or live in a city and have to rely on expensive tap water, you should investigate active storage systems.

How much water can you collect?

To estimate how much rainwater you can collect from your roof, multiply its footprint (looking down directly from above, not its actual surface area) in square feet by the average yearly rainfall in inches. Divide this number by 12 to get the number of cubic feet of water. Then multiply this by 7.5 to get the number of gallons.

For example, my roof is approximately 800 square feet and we get about 50 inches of rain per year. This works out to 40000, which divided by 12 equals 3333. Multiply this by 7.5 and you get 25,000 gallons of water. If only storing it were as easy as calculating it!

Runoff from larger surfaces such as roads, paths and hillsides might also be collected. Rainwater knows no boundaries, so you might even be able to collect water from neighboring land. A paved driveway could be shaped to collect water and deliver it to a pond or tank. Be aware that driveway runoff may be contaminated unless you take precautions. Concrete is the best material for this and it should be kept clean, with no oil or other chemicals on it. It is possible to clean dirty water with wetland plants or even fungi, but this is perhaps a little too complex for the moment.

Active storage

Water in barrels, tanks or ponds is called active storage because it is immediately available for use This should ideally be stored high up on the property, so it can be gravity fed to where it is needed, without electricity or pumps. The drawback to active storage is that it usually costs money and to store a lot of water can cost a lot of money. The strategy you adopt will depend upon your climate, rainfall pattern and the depth of your pocket.

In dry areas with rain throughout the growing season, storage is pretty simple. All you need to do is store enough water from recent rains to see you through any rainless stretches. If you run out of stored water you will just have to resort to tap water, no big deal. A few rain barrels might be all the storage you need, or perhaps an old discarded hot tub or a couple of old water beds. Concrete or galvanized metal culvert sections can also work well.

Rainwater collection is particularly useful in arid areas that get infrequent but heavy summer showers. In such areas rainwater can provide for most of your irrigation needs.

I live in an area that is more problematic. We get massive amounts of rain in winter (40 – 100 inches) and none at all in summer. In this situation a storage system has to be big to have any significant effect. Large tanks are expensive to buy, though they may save you money in the long run if you pay monthly water bills. It is possible to make your own tanks out of concrete or ferrocement.

One of the cheapest ways to collect a significant amount of rainwater is in an above ground swimming pool (the vinyl may leach toxins into the water so don’t drink it). A more durable alternative is a hole in the ground, lined with pond liner. This should be covered in summer to reduce evaporation and to keep out mosquitoes and children.

Even bigger storage would be a pond or reservoir, which can be of almost unlimited capacity. This doesn’t have to be very complicated, it can be simply a large hole in the ground lined with pond liner (essentially a swimming pool). Keep the depth fairly shallow and you don’t have to worry about the sides collapsing. In some areas there are restrictions on doing this, so you may want to check with your local building department (it could also present the same drowning hazard as a swimming pool)). Also be aware that large water storage brings with it the possibility of large problems.

Water storage should be placed as high up as you can get it (for gravity flow). It should also have an overflow to carry away any  excess water and dispose of it safely (this could be an overflow pipe attached to a perforated pipe, or section of soaker hose.

 Rainwater collection systems

Rainwater that lands on buildings is easily collected for garden use, as it falls high up and can be gravity fed to a storage place without the use of pumps. A rainwater collection system usually includes:

• A clean catchment area. In most cases the roof already fills this requirement, it is relatively clean and comes with ready made piping in the form of downspouts. You could also collect rainwater using waterproof fabric awnings.

• Pipes and gutters. These must be large enough to collect the maximum flow of water without overflowing and deliver it to a storage tank. These should be kept free of leaves and other debris that might clog them and cause them to overflow. You can even push water uphill somewhat, so long as the top of the tank is lower than the gutter.

• A storage tank. This should be covered to reduce evaporation and should have screened vents to keep out insects. It should also have an overflow to direct excess water into the soil. It should be dark inside to prevent the growth of algae. The inlet for the tank should have a screen to remove any debris.

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