From: Graham Towerton

What is rainwater harvesting?

Rainwater harvesting is simply collecting rainwater from any surface or area and directing it to a storage tank for later use; or to an area of land where it can be used directly in the soil to water plants. This article will discuss these two main methods used in permaculture.

Why harvest rainwater?

Some climates are very drought prone and rainwater harvesting will help plants, livestock, farms, people and communities survive the drought. Very arid areas may experience only a few rainfall events each year and rainfall harvesting is an essential requirement for human existence. Even in areas of moderate rainfall, there may be extended periods of a few weeks without rain, and rainwater storage provides a supply for plants and animals to cover these periods. Even in high rainfall areas, these storage methods can be beneficial to ensure that water is directed where it is needed most, maximizing plant yields, or to protect certain plants and areas from having too much water that could be harmful to plants or soil.

What is rainwater harvesting used for?

Water is essential to all human, animal, plant and microbial life, but it is also the medium through which so many natural and human events occur. The dissolution of rocks and minerals in water provides plants and waterborne microbes with nutrients. Rivers are used to transport people and goods in both directions. Rivers and lakes provide habitat for many species.

Harvested rainwater can provide many beneficial uses in the context of a home, whether rural or urban; a house or a farm. Rainwater uses can be for human consumption, domestic uses (laundry, dishwashing, bathrooms), watering plants and livestock, supplementing ponds with fresh water, preparing nutrient solutions (e.g. compost tea), aquaponics, and aquaculture. Each of these uses has different quality requirements and considerations for water treatment options to ensure that quality is achieved.

Rainwater can be collected from any hard surface, provided that the surface is free of contaminants that may be harmful to plants, animals and people; or provided that the water can be properly treated to remove any harmful contaminants. Precipitation at different locations can have very different levels of contamination by absorbing and dissolving pollutants from the air. For example, prior to the intervention of Canadian and U.S. government environmental agencies, the phenomenon of acid rain was caused by the dissolution of sulfur oxides from pollution sources into rainwater, which then acidified to the point where the rain destroyed maple forests in the northeastern part of the American continent.

Various surfaces that collect rainwater can also create contamination. Whether it is dust, animal excretions, dissolved metals or chemicals from roofing materials, or surface microbial populations, all rainwater should be considered to have some form of contamination that may require treatment; or that requires the water to be used in specific applications where contaminants are not a concern.

From the roofs of buildings, the typical method of collection is a gutter and downspout system. Over the ages, these have taken many forms, including hollowed out wood, terra cotta, stone, cast iron, galvanized metal, stainless steel, copper, aluminum, and plastic. Likewise, the roof area itself through the ages has consisted of a similar range of materials, also including thatched roofs made from plant materials.

What do you use to store rainwater?

Once rainwater is collected in a gutter and downspout, it can be directed directly to a specific part of the garden (using surface or underground piping) or into a storage tank for later use. Again, storage tanks throughout the ages have included a wide range of materials for above ground and underground tanks.

The size of storage systems really depends on several factors:

The length of time the rainwater will be used to cover the drought or dry periods between rain events. In arid areas, the "dry period" can last a large percentage of the year, so a larger reservoir will be needed. In wet areas, with dry periods of a few weeks, a smaller tank can be used.

Rainwater uses and the columns needed to support those uses. Homes using rainwater for all domestic uses will need a much larger storage tank, while properties using only rainwater for small gardens and small livestock needs will require smaller tanks.

The amount of rainfall at your location that defines your total potential available volume.

How do you harvest rainwater?

The amount of rainfall and available collection area can be used to calculate the annual volume available. This chart provides approximate annual rainfall volumes based on annual inches falling on 1000 square feet.

Appropriate rainwater storage tanks can be obtained in a variety of ways:

Purchasing new poly tanks from a wide variety of online suppliers for tanks between 50 and 15,000 gallons. As an alternative to poly tanks, there are a variety of manufacturers that offer kits for building round galvanized metal tanks with butyl plastic liners that become much less expensive for larger storage volumes of 20,000 gallons and more.

Reuse of water storage tanks purchased from previous owners via various social media markets. Please note the precautions to make sure these were only used for water storage.

Building your own tank. Options include stone, cement, ferro-cement, brick and wood - which was the original material used for gutters and downspouts!

Direct Soil Storage
In permaculture designs, there are three main methods of capturing and storing water:

Ponds and dams, which capture water from streams (and/or roofs) and other hard surfaces (roads, concrete) and store the water for later use, usually with pumped irrigation systems.

Swales, which intercept water as it moves down a slope and capture the water in a long, shallow horizontal trench where the water then seeps into the soil for use by plants. Trees and shrubs are then grown along the ridge of the swale where they can directly access the water flowing from the swale.

Rain gardens, which are specially designed networks of beds to improve water penetration into the soil so that water seeps into the ground instead of pooling or running off.

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