Rainwater harvesting refers to the collection and storage of rain. Collection is usually from rooftops, and storage in catchment tanks. Stored water can be used for non-potable purposes such as irrigating lawns, washing cars, or flushing toilets. Rainwater harvesting systems can range from a simple barrel at the bottom of a downspout to multiple tanks with pumps and controls. Before the creation of public water utilities, rainwater harvesting provided water for many American homes. It is still popular in places with limited water resources such as island communities like Hawaii. While we think of ourselves as living in a rainy climate, recent drought conditions throughout Oregon remind us how quickly we can run short of water.
Using purified potable water for purposes like flushing toilets or irrigating landscape is a waste of a valuable resource. Portland residents are asking more questions about the role of conservation in extending the supply of drinking water. Stored water can substitute for piped drinking water for many uses where a high level of purity is not required.
Rainwater harvesting is also effective in reducing stormwater runoff pollution. When rain falls, it is clean, but it immediately picks up pollutants from rooftops and pavement. This pollution is carried into storm drains and then into streams. Collecting stormwater from rooftops and directing it to storage tanks so it can later be used for irrigation or flushing decreases the volume and rate of runoff.
Rainfall. Rainwater runoff refers to rainwater which flows off a surface. If the surface is impervious (water cannot penetrate it), then runoff occurs immediately. If the surface is pervious (water can penetrate it), then runoff will not occur until the surface is saturated. Runoff can be harvested (captured) and used immediately to water plants or can be stored for later use. Several factors affect runoff, the most important being the amount of rainfall. Rainfall duration refers to the length of time the rain falls, the longer the duration, the more water available to harvest. The intensity of the rainfall affects how soon the water will begin to run off and also how fast it runs off. The harder it rains and the longer it lasts the more water there is for harvesting. The timing of the rainfall is also important. If only one rainfall occurs, water percolates into the dry soil until it becomes saturated. If a second rainfall occurs soon after the first, more water may runoff because the soil is already wet.
Plant Water Requirement. The type of plants selected, their age and size, and how closely together they are planted all affect how much water is required to maintain a healthy landscape. Because rainfall is scarce in arid regions, it is best to select plants with low water requirements and control planting densities to reduce overall water need. Native plants are well-adapted to seasonal, short-lived water supplies, and most desert-adapted plants can tolerate drought, making them good choices for landscape planting.
Water Collection and Distribution System. Water harvesting systems range from simple to complex. In a simple system the rainwater is used immediately. Most homeowners can design simple water harvesting systems to meet the needs of their existing site. Designing water harvesting systems into new construction allows the homeowner to be more elaborate and thorough in developing a system. In the case of very simple systems, the pay back period may be almost immediate.
A simple system usually consists of a catchment area, and a means of distribution, which operates by gravity. The water is deposited in a landscape holding area, a concave area or planted area with "edges" to retain water, where it can be used immediately by the plants. Water collects on roofs, paved areas or the soil surface. A good example of a simple system is water dripping from the edge of the roof to a planted area or diversion channel directly below. Gravity moves the water to where it can be used. In some cases, small containers are used to hold water for later use.
A catchment area is any area from which water can be harvested. The best catchments have hard, smooth surfaces, such as concrete or metal roofing material. The amount of water harvested depends on the size, surface texture, and slope of the catchment area.
The distribution system connects the catchment area to the landscape holding area. Distribution systems direct water flow, and can be very simple or very sophisticated. For example, gutters and downspouts direct roof water to a holding area, and gently sloped sidewalks distribute water to a planted area. Hillsides provide a perfect situation for moving water from a catchment area to a holding area. Channels, ditches, and swales all can be utilized to move water. Elaborate open channel distribution systems may require gates and diverters to direct the water from one area to another. Standard or perforated pipes, and drip irrigation systems can be designed to distribute water. Curb cutouts can channel street or parking lot water to planted areas. If gravity flow is not possible, a small pump may be required to move the water.
Landscape holding areas store water in the soil for direct use by the plants. Concave depressions planted with grass or plants serve as landscape holding areas, containing the water, increasing water penetration, and reducing flooding. Depressed areas can be dug out, and the extra soil used to berm (a bank of soil used to retain water) the depression. With the addition of berms, moats, or soil terracing, flat areas also can hold water. One holding area or a series of holding areas can be designed to fill and then flow into adjacent holding areas via spillways (outlets for surplus water).
Soil erosion can be a problem with water moving quickly over the soil surface. Basins and spillways help reduce this. Crescent-shaped berms constructed around the base of the plant on the down-hill side are useful on slopes for slowing and holding water. Gabions (a stationary grouping of large rocks encased in wire mesh) are widely used to contain water and reduce erosion. French drains (holes or trenches filled with gravel) can also hold water for plant use. And lastly, pervious paving materials, such as gravel, crushed stone, open paving blocks, and pervious paving blocks, allow water to infiltrate into the soil to irrigate plants with large, extensive root systems, such as trees.