Normally when it rains rainwater pours off the roof, down through gutters, off the driveway and other impermeable surfaces and runs into the street or other areas of the property. Rainwater harvesting is the practice of collecting and using this rainwater. Rainwater harvesting systems can be as simple as using a barrel to capture water or infiltrate the water into soil storage or it can be as complex as installing a large underground cistern or storage tank.
Always start at the top of the "watershed". Think of the term watershed in this instance to be part of the larger natural watershed but more specifically the property boundaries that the rainwater harvesting system is being used for. In the urban or residential setting the top of the watershed is often the roof.
The basic principal of rainwater harvesting is to channel rainwater from a catchment area (roof or other surface), through a conveyance system (gutters, downspouts and pipes/channels) to storage -whether that be in the soil or in containers such as a barrels or tanks. Often there may be rainwater filtration of some type to filter debris or clean out pollutants. Rainwater distribution take the water to desired areas of the landscape for use as irrigation or passive infiltration for nearby trees and shrubs . Finally there is the overflow system that ensures surplus rainwater is used beneficially in the landscape and prevents flodding and erosion.
Catchment Area
The total area of a landscape that drains to a particular site or drainage. As used in rainwater harvesting practice, catchment area refers to the area, such as a roof, that drains to a rainwater storage system -be that in a tank or in the ground.
The highest water quality comes from smooth, impervious catchment surfaces. Impervious materials improve the efficiency of rainwater catchment systems by absorbing less water and reducing the chance for microbes and debris to collect in the pores and/or seams of the catchment surface. Smoother surfaces also allow the rainwater to flow faster and thus “clean” the catchment area. Metal, clay, and concrete tile roofing will generate the cleanest water.
Catchment areas are prone to water loss to the environment know as a "run-off coefficient" -which is used when calculating the volume of water one can collect from a specific catchment area.
Conveyance System & Filtration
Rainwater is usually conveyed from a catchment area to some type of storage (containers, infiltration basins or rain gardens) by gutters connected to downspouts. Downspouts with the aid of piping help move rainwater further. The number of downspouts needed depends on the surface area of the roof and intensity of rainfall.
Downspouts can be fitted with filtration devices such as leaf eaters and first flush devices which exclude leaves and debris and filter out the first few gallons of water. Many types of first-flush diverters are readily available; all must be installed “upstream” of the storage tank.
Storage
Passive rainwater harvesting systems utilize storage systems such as mulch basins, swales and other earthworks to collect and hold rainwater until it infiltrates into the ground.
Active rainwater harvesting systems utilize manmade containers such as tanks, barrels and cisterns to store rainwater for use at a later time.
See the next section for more information on Passive and Active Systems.
Distribution Systems & Use
Passive system distribution of rainwater happens
before storage and can be simple overland flow of water using gravity and water's least path of resistence. Piping from downspouts to earthworks like infiltration trenches can also be constructed to move rainwater where we want it to go -such as mulch basins or raingardens to irrigate specific plants.
In an active system water distribution happens
after storage and includes all piping, pumps, and other devices that moves water from storage to the point-of-use in the landscape -such as the vegetable garden.
Overflow
You cannot turn off the rain! Once your storage is full it's vital to have a plan in place for where the water will go. Design the rainawater overflow to be a resource so it can passively flow or spill over to mulched/vegetated areas, mulch basins or another series of earthworks (as shown in the photo above). Spillways (also called level sills) are low points in a berm-basin or swale system and are level surfaces to allow water to flow evenly across the land to another point of use. Diversion drains are a gradually sloping drains and can also be used for overflow; they are on a slight slope (<1%) to direct water to another area.
Overflow routes should always be stabilized with rock, dense vegetation or other durable materials so fast moving water will not erode the land. Make sure the overflow route and spillway are large enough to handle all surplus water. The last and lowest point in your site's "watershed" should be directed to a natural drainage or in the case of urban settings, the stormwater system in the street. Whenever possible try to make sure all rainwater that falls on your land can stay on site to be fully maximized in the landscape.