Links: The Gift of Rain

The Gift of Rain
RAINWATER HARVESTING FOR LANDSCAPING

by Frank Clemente
Earth-Friendly Landscape Designer

 Have you ever looked out your window during a storm and thought of using rainwater to irrigate your landscaping?

Has seeing rainwater flow off your property and down the street like a river seem a waste?

Have you ever wondered how many gallons of water fall on your property?

If you are a homeowner, rancher, farmer or gardener that pays to have your property irrigated, consider the effective and less expensive strategy of water harvesting. It is a rainwater collecting method made possible through re-patterning the land in a way that holds the rainwater on site as it reduces runoff.

Rainwater harvesting is a smart and sustainable way to reduce our use of potable water to irrigate plants, and its free, and pure, and it works as nature intended.

Capturing rainwater and holding it on site decreases erosion, reduces flooding, minimizes the spread of pollutants and when collected in bio-retention zones prevents mosquito breeding.

How does rainwater harvesting work? We know that water flows from high to low points. In a residential lot, rainwater flows from roofs to the ground and from higher elevations on the lot to the lower areas. Rainwater is collected by re-patterning the yard.

We begin the collecting process at the highest elevation on the lot and at the roof runoff systems. All water is directed to the lowest parts of the property through a combined interconnected network of bio-basins (concave water catchments filled with pants and swales (shallow channels). The soil and plants in the bio-basins absorb the rain water. The depth of the basins is based on mathematical calculations and the percolation rate (the amount of time it takes for the soil to absorb water) of the soil. Any overflow, usually about 5 percent with water harvesting is released to the city gutters. The result is that we keep about 95 percent of the rainfall on site to irrigate the plants.

Does it rain in Phoenix? Yes, and it varies. Some regions collect 6 inches of rain per year and others 13 inches per year. The official rate is between 7 and 8 inches per year.

Imagine a 10,000 square foot, rectangular lot in the Phoenix Metro area. How much water will it collect in one year based on 7 inches of rainfall?

Here is a simple rule of thumb method for calculating rainfall volume on a catchment (collection) surface.

*Rule Of Thumb says that for every inch of rain that falls on a 1,000 square foot catchment surface you can collect 600 gallons of water.

How much rainwater will a 10,000 square foot catchment area collect in a neighborhood that receives 7 inches of rain per year?

1. 10,000 divided by 1,000 square feet = 10 (each 1,000 square feet is equal to 600 gallons of water per inch of rainfall).
   
2. 10 x 600 gallons = 6,000 gallons of water per inch of rain over the entire 10,000 square foot area (lot and house).
   
3. 7 inches of rain per year x 6,000 gallons of water = 42,000 gallons of water falls on a 10,000 square foot lot (catchment area) per year.
   

The next question is how many trees or plants will 42,000 gallons irrigate in one year? For simplicity lets consult a plant watering list published by the Arizona Landscape Irrigation Guidelines Committee - 1999, which shows the estimated water requirements for a Mesquite tree with a 20 foot canopy for Phoenix, Arizona. The following is based on The Desert Adapted Plants-Natives Chart. It shows the months of January - December. Based on this chart it is estimated that the tree will use 4,531 gallons of water per year. Evapotranspiration (Eto) rates are not included.

We continue by dividing the 42,000 gallons by 4,431 gallons = 9.26 Mesquite trees. We round this off to 9 trees that can be watered by rainfall harvested throughout the year. Initially, the trees will need more water to get them established.

When we design a rainwater harvesting system we enhance soil fertility, grow food with less pollutants, boost our wildlife habitats, and we become empowered and water self-sustaining.

Harvesting rainwater is safe, productive and helps us become less susceptible to drought. It reduces the impact of dry seasons because it acts as a buffer from changing climates due to global warming and climate extremes while it makes our land more resilient.

Water harvesting is used in several ways:

1. Above ground cistern (tank) for gravity flow. Water is collected by roof gutters and directed to the tank.
2. Below ground cisterns. These require a pump to deliver the water to the plants. (Exception: when tank and collecting systems are uphill.)
3. In cold climates, a tank is placed inside the home where water is sheltered from freezing. A dual tank system is possible. A large tank is outside and a smaller tank is in the home to provide water for up to one week.
4. Gray water systems combined with water harvesting. Note: gray water is never used for vegetable gardens.
5. Drip irrigation and water harvesting. Eventually it is possible to reduce the amount of water supplied by the drip system by using native plants.
6. Rooftop gardens and landscaping

For mosquito controls include the use of Biopesticides such as Bacillus thuringiensis var. isrealensis (Bti). There is a similar product called Mosquito Dunk. See website lists below.

If water conservation through water harvesting is something you would like to investigate, please call Frank at Clemente Design, the Earth-Friendly Landscape Designer at 602-540-3773 or send him an email at clementedesign@mindspring.com.

To receive Frank's Quarterly Water Harvesting and Green Landscaping tips - email your requests to frank3773@msn.com.

*Rainwater Harvesting for Drylands, by Brad Landscaster

To learn more please access these websites:

www.HarvestingRainwater.com

www.lid-stormwater.net/biolowres_specs.htm

www.cals.Arizona.edu/pubs/water/az1344.pdf

http://RainwaterHarvesting.tamu.edu/index.html

www.ext.colostate.edu/pubs/insect/05556.html

www.ghorganics.com/MosquitoDunks.html