By: Graham Towerton
What is rainwater harvesting?
Rainwater harvesting is simply the collection of rainwater from any surface or area and directing it into a storage tank for later use; or into an area of land where it can be used directly in the soil for watering plants. This article will discuss these two main methods used within the scope of permaculture.
Why harvest rainwater?
Certain climates are very prone to drought and the collection of rainwater will enable plants, livestock, farms, people and communities to survive through the drought. Very arid areas may only have a few rainfall events every year and the collection of rainfall 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 having a storage of rainwater allows a supply to plants and animals to cover those periods. Even in areas of high rainfall, these storage methods may be beneficial to ensure the water is directed to where it most needs to be used, maximizing plant yields, or to protect certain plants and areas from having too much water which could be harmful to plants or soil.
What are the uses of harvested rainwater?
Water is essential to all human, animal, plant and microbial life but also is the medium by which so many natural and human events occur. The dissolving of rocks and minerals into water provides plants and water borne microbes with nutrients. Rivers are used for transportation of people and goods in both directions. Streams and lakes provide habitat for many species.
Harvested rainwater can provide many beneficial uses within the context of a home, whether rural or urban; a homestead or a farm. Uses of rainwater can be for human consumption, household uses (laundry, dishwashing, bathrooms), watering plants and livestock, supplementing ponds with fresh water make-up, nutrient solution preparation (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.
How do you collect rainwater?
Rainwater can be collected from any hard surface, provided that the surface is free from contaminants that may be harmful to plants, animals and people; or provided that the water can be suitably treated to remove any harmful contaminants. Rainfall in different locations can have very different levels of contamination by absorbing and dissolving pollutants from the air. For example, prior to action by Canadian and US government environmental agencies, the phenomenon of acid rain was caused by dissolving sulfur oxides from pollution sources into rainwater which then became acidified to the point where rainfall was destroying maple forests in the north-eastern part of the American continent.
Different surfaces that collect rainwater can also create contamination. Whether dust, animal excretions, dissolved metals or chemicals from roofing materials, surface microbial populations – all rainwater should be considered as having some form of contamination that may need to be treated; or which requires use of the water in specific applications where the contaminants are of no concern.
From the roof areas of buildings, the typical method of collection is a rain gutter and downspout system. Over the ages these have taken many forms including hollowed out wood, terracotta clay, stone, cast iron, galvanized metal, stainless steel, copper, aluminum, and plastic. Likewise, the roof area itself through the ages has consisted of a similar array of materials including also thatched roofs made from plant materials.
What do you use to store rainwater?
Once the rainwater is collected in a gutter and downspout, it can be directed to a specific part of the garden directly (using surface or underground piping) or into a storage tank for later use. Again, storage tanks through the ages have included a great many materials for above ground tanks and below-ground cisterns.
The size of storage systems really depends on several factors:
The period of time where rainwater will be used to cover drought or dry periods between rain events. In arid areas the “dry period” may be for a large percentage of the year, so a larger tank will be required. In wet areas, with dry periods of a few weeks, a smaller tank can be used.
The uses of rainwater and the columns required to support those uses. Homes using rainwater for all domestic uses will need a much larger storage tank, whereas properties only using rainwater for small gardens and small livestock needs will require smaller tanks.
The amount of rainfall in your location which defines your potential total volume available.
The amount of rainfall and available collection surface area can be used to calculate the annual volume available. This chart provides approximate volumes of annual rainfall based on annual inches falling on 1000 square feet.
|Annual Rainfall||Total Possible Collection per 1,000 sqft surface area (85% efficiency)|
Suitable rainwater storage tanks can be obtained in a variety of ways:
Purchase of 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 provide kits for constructing round galvanized metal tanks with butyl plastic liners which become far less expensive for larger storage volumes of 20,000 gallons and above.
Re-use of water storage tanks purchased from prior owners through various social media markets. Please note the cautions of ensuring that these have only been used for water storage.
Construction of your own tank. Options include stone, cement, ferro-cement, brick and wood – which was the original material used for gutters and downspouts!
An open top “tote” or Intermediate Bulk Container (IBC) converted into a rainwater tank collection water directly off a barn roof without gutters. If using a used IBC, only use containers that have previously stored food products or food grade products with triple rinsing of the containers. Do not use any containers that have held pesticides, herbicides, pharmaceuticals, fertilizers or other synthetic chemicals that could be harmful.
A 1500-gallon (5,600 liter) HDPE or High-Density Polyethylene, aka “poly” tank collecting water from a building via cutter and downspout. This is then distributed by hose via gravity to water plants downhill from the tank.
The large surface area of this roof (over 2000 square feet or 215 square meters) provides a large surface for water collection and many downspouts for rainwater capture. Some of these downspouts are directed straight to garden areas to provide moisture into the soil at the base of trees and plants. With 36” of rainfall in this location, the potential exists for over 36,000 gallons of rainwater to be collected from this roof on an annual basis.
A small 500-gallon poly tank with overflow directed into a smaller 400-gallon fiberglass tank which in turn overflows into a catchment swale at the top of the fruit orchard so that all excess water is directed to the soil. The owner uses hoses (by gravity) and watering cans to take water from storage to where it is needed. Alternatively, a pump system could be used to deliver water into an irrigation system.
Direct Storage In-Ground
Within permaculture designs there are three main methods for water capture and storage:
Ponds and dams which capture water from streams (and/or from rooftops) and other hard surfaces (roads, concrete) and store the water for use later, typically using pumped irrigation systems.
Swales, which intercept water as it travels down a slope and captures the water in a long horizontal shallow trench where the water then percolates into the soil for plant use. Trees and shrubs are then grown along the swale ridge where they can directly access the water as it percolates from the swale.
Rain gardens, which are specifically designed arrays of garden beds which improve soil water penetration so that the water percolates into the ground instead of pooling or running off.
Two beautifully designed rain gardens planted at the low end of a gradient and where water would traditionally pool and flood the areas. Now all water is captured, percolates into the soil and provides all of the water requirements for these pollinator plants.
Graham’s Social Media Links: