Water Harvesting

Clouds building over the Mogollon Rim and storms along the Sierra Madre indicate that the monsoon rains will soon arrive and now is the time to make sure that you are ready to capture, store and use storm water to irrigate garden and landscape plants.

Water harvesting it is called, and in the desert, water is such a precious resource that those who live here are always looking for good ways to make every drop count.

Each year during the winter wet season and the summer monsoon, many gallons of water fall onto hard surfaces like roofs, carport driveways and patios.  A water catchment system to harvest this often wasted water is a good way to save money on the water bill.  It is also helps us be good stewards of a very precious resource.

For centuries, people have struggled to survive in water-scarce environments. In so doing, unique systems have been devised to manage those limited water resources. One strategy has been to capture and concentrate rainfall to irrigate crops or to supply water for people and animals. This practice, known as water harvesting, offers some excellent possibilities for the home landscape.

The most important component of a rainfall harvesting system is rain.  Desert environments are always short of this precious commodity and the drought cycles make it even more difficult.  However, every little bit that can be captured can help keep plants healthy. 

In Arizona, water harvesting for crop production is best suited in areas that receive more than 10 inches of rain annually and where summer temperatures do not exceed 100 degrees F. In Pinal County, annual rainfall averages between 8 and 12 inches annually, starting from the west side of the county and working east. Casa Grande averages 8.2 inches a year, Florence 9.8, and the San Pedro Valley between 10 and 12 depending upon the elevation.

Temperature is important to consider because evaporation is directly tied to how hot it gets during the day.  The hotter the day, the faster water will evaporate from a surface.  Closed storage systems help prevent evaporation losses while the water is being kept for future use. 

The other major component of a water harvesting system is a series of hard surfaces coupled with a delivery system to bring the water to the root zones of landscape plants.  In higher rainfall areas, the storage facility becomes even more important as it accumulates and protects the excess harvested water.

Water harvesting systems capture water in three principal ways. These are described as water-spreading systems, diversion/terrace systems, and micro catchment systems. The first two probably do not have application in an urban setting as they involve diverting water from desert washes and applying it to cropping areas. The early cultures of Arizona were masters at these techniques and used them to good advantage.

Residential water harvesting systems utilize variations of the micro catchment system. The amount of water that can be collected depends upon the amount of rainfall received and the square footage of the catchment surface.   There are many factors to be considered, including the influence of wind on how the rain falls and the pitch of a roof but a good rule of thumb is to expect about 0.623 gallons of water per square foot of hard surface during a one inch rain.

Using this conversion factor, a flat roof 1000 feet square could collect 623 gallons of rain water during a rainfall event of one inch.  This would be enough to irrigate a line of plants along a well prepared drainage channel in a desert landscape.  If a drip irrigation system with a good filter and a gravity pressure system were  available, it could irrigate a larger area.

A typical residential water harvesting system uses several different types of hard surfaces to collect or channel rainwater.  Rooftops, garage roofs, driveways and carports, sidewalks, tennis courts or other play surfaces and the ground surface itself can all be used. 

Rainfall that is captured from rooftops can be concentrated with eave gutters and down spouts before being diverted to garden or storage areas.  If the storage tank is at a high point on the property, a manual outlet valve could be opened to drain water by gravity flow but, if this is not possible a small sump pump will help deliver water to where it is needed.

Another benefit of roof top water harvesting is the cleanliness of the water.  Roofs are usually fairly free of sand and other debris that could clog emitters in drip irrigation systems.  The water coming from the roof should be checked, however, to make sure that it has not picked up anything that could damage the system.  If so, the water will probably require just a minimum of filtering before it is used.

Surfaces at ground level can be constructed in such a way that water can be directed to appropriate locations within the landscape, such as the beginning of a water channel or to the wells around trees and shrubs.  The complexity of the system will be directed by the terrain and type of plants in the landscape.

Land surfaces can be shaped and contoured to encourage runoff during rainstorms.  If the planting is some distance from the collection area, planning and careful engineering can take the captured water to the desired location.

Micro catchment systems offer excellent possibilities to the home gardener and variations allow a diverse range of applications depending upon the particular need of the location. Water catchment can be as simple or as complicated as the landscape requires but good planning will always be a key to success when installing these systems.

Effective water harvesting concentrates rainwater runoff for use in irrigating landscape plants.  Not only are they fun projects to occupy part of our gardening time but, once in place, we get the added benefit of seeing our landscape plants get watered automatically while we sit in our easy chairs and watch it rain.