Water Harvesting (II)

Types of water harvesting systems

Harvested rainwater is a renewable source of clean water that is ideal for landscape use.  Water harvesting systems provide flexible solutions that can effectively meet the needs of new and existing, as well as small and large sites.  The two main types of water harvesting systems, passive and active, can be used in combination to provide the solution most suitable for your needs.

In a passive water harvesting system, no water storage containers are used, and rainwater is diverted immediately to the soil, where it is stored for direct use by the plants.  In active rainwater harvesting systems, rainwater is stored in containers for use in the dry season.  Active water harvesting systems consist of four main elements: catchment areas, water storage containers, conduits connecting catchment areas to water storage containers, and conduits that distribute water from storage areas to the landscape.

Catchment areas include any surfaces from which water can be harvested such as rooftops, paved areas, and driveways.  Hard smooth surfaces are the most suitable for catchment areas.

Harvested rainwater can be stored in different types of containers, the choice of which depends on your needs, preferred materials, placement, capacity, and cost.  Water storage containers can be placed underground or aboveground.  Underground water storage cisterns are expensive to build, but are visually unobtrusive and occupy almost no space in the garden.  They can be built of concrete block, cast-in-place concrete, building rock, or steel.  Aboveground containers, on the other hand, are less expensive to install than cisterns, but occupy space in the garden.  They can be made of the same materials as underground cisterns, or one can use prefabricated smaller containers made of steel, polyethylene, or clay.

Water storage containers should be located as close as possible to the points of collection and use, and are better situated at an elevated level to take advantage of gravity flow for distribution.  This will decrease or even limit the need for pumps and therefore conserve energy.  Make sure to seal the water containers to keep out organic materials and sunlight, and therefore prevent evaporation and bacterial growth.  The inlet to the container should be provided with a filtering device (which at its simplest can consist of a window screen) to keep leaves and debris out of the container.  Also, it is advised to divert the first rainfall away from the storage area so as to wash away the dust and debris that collects on the catchment area during the dry period.

A great variety of water conduits can be used.  These include gutters, downspouts, channels, and ditches. Gutters and downspouts can be either concealed inside the wall or attached to building exteriors, and have the potential of forming distinguished visual elements.  The proper sizing of these elements is important to efficiently direct the harvested rainwater.  To maximize the efficiency of water collection, space downspouts a minimum of 6 meters, and a maximum of 15 meters apart.  Also provide about 7 square centimeters of spout area for every 10 square meters of roof area.  Gutters should be provided with a minimum slope of 2% to insure the proper flow of water.

Water can be distributed from storage containers to the planted areas through a variety of conduits including hoses, open channels, or through solid or perforated pipes.  Drip irrigation systems are the most effective in terms of achieving significant water savings.

To find out how much rainwater you will be able to harvest multiply rainfall (meters) by catchment areas (square meters) by the runoff coefficient, and the result would be the quantity of harvested rainwater (in cubic meters). Note that the runoff coefficient is the percentage of the rainfall that can be harvested from a specific material.  It varies from 0.95 - 0.90 for asphalt and smooth concrete to 0.10 - 0.05 for flat, sandy soil.

Once you’ve added a water harvesting system to your garden, compare your water bills before and after installing the system to accurately assess its effectiveness.  Also test your system during the rainy season and implement the necessary adjustments to increase its efficiency.  Keep in mind that a water harvesting system will require some maintenance.  This includes keeping storage areas, gutters, and downspouts free of debris, and cleaning the different conduits of your system and repairing damaged ones.

Make sure that no water escapes your property. By this you not only are saving water, but also are contributing to preventing urban flooding and overflowing the municipal sewage system.

The upcoming article of this series will discuss the principles of plant selection for your landscape.