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Getting the most out of gray (grey) water and rainwater harvesting

Archive for the ‘Gray (grey) water’ Category

ETo (Evapo Transpiration) and calculating optimum irrigation efficiency

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ETo (Evapo Transpiration)

Greywater goes a lot further than you think!

The most important aspect of how much water is required to irrigate your garden is not the type of soil, but the climate of your region and the type of plants you want to grow.

This discussion does assume drip irrigation is used to water the garden, as it has the highest efficiency of all irrigation methods (it is also no more expensive than other methods such as branched drain and mulch basin irrigation.

Evapo Transpiration tells us how much water is required per month to properly account for soil moisture loss via plant growth and plant moisture evaporation.

An excellent ET guide is provided online by  California Irrigation Management Systems (CIMIS).

The image above shows a section of an ETo chart for California.

Large images are available at the CIMIS website.

Contact your local garden shop or irrigator to find out the ETo value for your area.


The amount of rainfall per month is subtracted from the ETo to determine the monthly water shortfall (ie how much additional water is required).

Plant Coefficient

The ETo value is an estimate of how much water is used by a well watered, full cover grass surface, 3-6 inches in height; used as a reference point for determining water use by other plant types through the application of a plant coefficient.

Plant Coefficient              Plant Type

0.13                            Very low water use
0.26                            Low water use
0.45                            Medium water use
0.65                            High water use

Converting inches per month to gallons per month per sq ft

The ETo value represents inches per sq ft required per month, but we are working backwards – we need to know how far a certain amount of water will go for the month.

The multiplication factor is 0.62
(144 cubic inches per sq ft per inch of rain / 231 cubic inches per gallons)

Capillary Irrigation Efficiency

The ETo value is based on 100% irrigation efficiency. With standard spray irrigation, much water is lost to surface evaporation and overspray. Typically only 50% efficiency is achieved, so the calculated ETo value would be doubled to account for this.

Capillary Irrigation (mentioned earlier in this eBook) has a very high efficiency – over 90%.

The ETo rate should be multiplied by 1.1 to account for the 10% inefficiency.

The last step is to calculate the correct garden area that can be irrigated.


G  = number of people x graywater per person, per day x 30
ET = Evapo Transpiration rate for your area.
R   = Rainfall for the month being considered.
P   = Plant coefficient

Irrigation area (sq ft) = G divided by ( (ET-R) x P x 0.62 x 1.1 )

The following shows the calculations made for Tucson, AZ.

ETo & Rainfall values for TUCSON, AZ



Inches required per month (medium water use)


Convert to Gallons per sq ft needed per month


Final value, based on 25 gallons per person per day



A 2 person household, with a graywater diversion and irrigation system, can grow a vigorous 470 sq ft garden space with medium water requirement plants, without any additional water during the hottest / driest months.

By selecting low water requirement plants, the area can be increased to 813 sq ft. Very low water requirement plants increase the area to 1,627 sq ft.

A 4 person household, with a graywater diversion and irrigation system, can grow a vigorous 705 sq ft garden space with medium water requirement plants, without any additional water during the hottest / driest months.

In all cases the garden space can be singular, or a group of smaller garden spaces adding up to the total amount.

To access our online calculator go to http://www.besthomewatersavers.com/pages/Graywater-Irrigation-Coverage-Calculator.html


Written by Waterharvesting

July 1, 2010 at 8:26 am

Understanding Irrigation Efficiency

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It is tempting to believe that whichever way you have decided to reuse rain or graywater, that you are saving at least as much water as you are placing in the garden.

Over the years, several methods of reusing graywater, based on mulch basins, have become popular. The sad fact is that simply placing graywater into a mulch basin surrounding a tree (a mulch basin is basically a big hole around the tree covered in mulch) isnt very efficient at all.

If you put 10 gallons of water into this mulch basin, it will have the same effect as using 5 gallons of tap water into the hole at a more measured pace. Yes you have saved 5 gallons of tap water (because you ddn’t need to supplement with tap water), but you could have saved 15 gallons of tap water by using the 10 gallons of graywater to greatest effect.

The following discussion is focussed on graywater reuse, however the same principles apply for rainwater reuse.


These are the commonly accepted methods of reusing untreated graywater:

  • Buckets
  • Branched Drain (or similar)
  • Laundry to Landscape
  • Gravity Dripperline
  • Pumped Dripperline

Before going in to the pro’s and con’s of each method, the concept of how much water is actually saved needs to be understood.

If you put 10 gallons of water in one 4′ hole in the ground every day, you have not saved 10 gallons per day. In Tucson, Arizona, that hole only needs 12 gallons for the whole of July for medium water use plants.

If this was done daily, then 300 gallons has been irrigated over the month, instead of the required 12 gallons.

The actual amount saved is 12 gallons. The irrigation efficiency is 12/300 or 4%.

This may sound extreme, but I have seen quite a few branch drain systems set up this way (the rest of the water goes down into the subsoil and is wasted, unless trapped by clay in which case the roots of the plants may rot).

So while on the surface some methods may appear to be very inexpensive for the amount of water diverted, the amount of water actually saved needs to be reviewed.

Irrigation Efficiency Rates for Different Methods:

Irrigation Method Efficiency
Buckets 50%
Branched Drain 30%
Laundry to Landscape 40%
Gravity Dripperline 90%
Pumped Dripperline 90%

The efficiency rates listed for branched drain and Laundry to Landscape methods are extremely generous, based on absolute best practice installation methods by highly experienced installers.

Cost / Benefit Analysis of the different methods.

After applying the irrigation efficiencies of the methods, determining how much potable water is no longer needed to irrigate the garden, these charts can now be presented.

If you would like the maths behind the following charts, please contact us via the contacts page for more information.


Cost vs Gallons of Potable Water saved over 5 years


Paid labour has been used for installation of all methods except for bucketing of water. A small amount has been added to the bucketing methods, for 1 chiropractic visit (this isnt a joke, it’s an onging issue in Australia).

While voluntary labor (you) can be used to reduce the cost, the efficiency of Branched Drain and Laundry to Landscape methods will typically halve. Those interested in installing their own Branched Drain network should go to our videos page and review the branched drain installation video (8 minutes)


Cost per gallons of water saved over 5 years use.

This chart illustrates how Dripperline irrigation is far more cost effective over 5 years (at about 0.5 cents per gallon) than even Laundry to Landscape at about 2.5 cents per gallon.

For more information about the pro’s and con’s of different graywater re-use methods, go to part 2 of this article http://www.besthomewatersavers.com/pages/Different-ways-to-reuse-graywater-%28part-2%29.html

How will Arizona find 100% more potable water in the next 30 years?

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It is fascinating to correlate states that have commenced water harvesting code development (graywater and rainwater) against their population growth rates.

Here is a chart (from google.com) showing population growth over the last 30 years for selected states:

Link = http://www.google.com/publicdata?ds=uspopulation&met=population&tdim=true&dl=en&hl=en&q=population+usa#met=population&idim=state:04000:01000:06000:12000:32000:35000:48000:53000:49000:13000

So while the population growth for the US averages about 1% per year (over the last 30 years), California had 54% growth, and Arizona 132% growth!

While there is scope for saving water in existing homes – currently Tucson residents average over 140 gallons per day in personal water use, compared to 40 gallons per day personal water use in places such as Melbourne, Australia, it will not be an easy path.

Aggressive water saving initiatives within the home will help – water efficient appliances, showers, change in habits etc; in warmer states over approximately 60% of personal water use is external (ie irrigating the garden, washing cars etc.

At some point authorities will have seriously tackle the water issue. The US economy relies on real growth, not debt growth, and will falter unless infrastructure can support the growth.

Certainly politicians are beginning to take note, with attempts to promote water re-use, although once the code actually hits the ground competing interests fight for turf.

Water Purveyors do need to sit back and work out how they are going to support this growth without massive impost on the consumer. Several purveyors now find they do not want water to be saved at the home – they need every drop to fulfill water reclamation contracts (ie water treatment) that are already in place. Examples of this are parts of Nevada and Arizona. This would be fanatastic if the reclaimed water was being used sensibly, however in many cases the water is simply re-used on golf courses and sporting grounds.

Even if the re-claimed water is presented back to the home, the intial sales pitch was your home could now have a guaranteed supply of water for the garden, at minimal cost. So instead of encouraging home owners to conserve this reclaimed water, the water purveyor wants them to use as much as possible, so they can improve the bottom line.

Drainage enginners don’t want you to use less water, because they are concerned you will be too efficient and cause flushing issues within the sewer and storm networks. It would make sense to design new subdivisions with this in mind. As I mentioned in the previous post, common sense and reality dont always come together.

If you live in one of the states listed on the chart, I recommend visiting your local water purveyor, and ask  how they plan to cope with growth for the next 30 years, based on the past 30 years. Let me know what you find out.  The many dicsussions I have had across the country are worrying.

What we do know is there is no more water to be found in the Colorado basin. Even if global warming is a non event, there will not be a 50%+ increase in rainfall over Western USA to support growth.

Hats off to Tucson Water and the City of Tucson. Their code requiring compulsory greywater stubouts on new houses is a great start. They have much more work to do developing a framework and impleneting the policy but they are on their way.

Water Harvesting – it’s more than just Rain

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This blog is about extending the concept of water harvesting from simply rainwater harvesting, to harvesting all the water across the home.

Rainwater is extremely valuable, especially as water utilities start raising the price of water to cope with infrastructure upgrades required now, not just the future.

Relying on rainwater alone to help reduce water consumption in the home is risky business (unless you live a cool, moist environment). Over 60% of water use in the average American home is in the garden, and rainwater collection (and storage) can help supply this water.

However if you live in an area that has rainfall for 6 months of the year, and dry for the remaining six months, relying on a rainwater harvesting system can be very expensive.

Lets say your garden area requires 3,000 gallons of water per month for irrigation. Lets assume you have a six month dry period (It could be worse – San Diego has a 9 month dry period).

To ensure garden water supply from rainwater harvesting alone, the tank / cistern would need 6 x 3,000 gallons, or 18,000 gallons capacity.  Apart from the expense involved (many thousands of $$), tanks / cisterns this size require a huge area, and land is expensive.

So the purpose of this blog is to explore the many different methods of water harvesting, such as rainwater harvesting, graywater (also know as gray water, greywater and grey water) re-use, reclaimed water, alternate water supplies; and most importantly place these methods into the appropriate context.

I promise to be straightforward and forthright in this blog. Water politics is not just confined to America, it is a world-wide phenomenon.

Water is a vital resource, second only to air, and as a result is big money. Vested interests include (but aren’t limited to):

– Water Utilities / purveyors
– Water fixture manufacturers
– Water treatment manufacturers and irrigators
– Water re-use designers and manufacturers (be kind – I am one of these!)
– Environmental quality jurisdictions
– Federal / State / City / Country code officials, planning consultants, and inspectors
– Plumbers / irrigators / landscapers

– and the most important stake holder of all; YOU.

I now have over 7 years of water re-use experience in Australia, and 2 years experience here in the US – at local, state and federal levels. Based on what I have learnt so far, the challenges facing America far outweigh those experienced in Australia.

The water / money relationship is much bigger in the US, and the stakeholders above all recognise the issues are serious, they just havent worked out how to work together on it yet. It is likely they will not come to agreement willingly, after all money is involved.

In the end, YOU, the consumer of water will ultimately drive how water is delivered to your home, how much you will pay for it, how you will use it (and re-use it, hopefully over and over again). You may not have control over how much you pay per gallon for water supply and sewerage disposal, but you will be able to control how much you potable water you consume.

Of even more concern to the regulators, is that YOU also decide whether to adhere to codes after the house is built, and they know code violations are everywhere. A pragmatic approach would be to develop codes that were sensible to the typical consumer, or change core infrastructure to cope with code violations.

In future posts, I will provide examples of how pragmatic solutions are rarely, if ever, found in the water regulation industry.

This primary focus of this blog is to raise awareness of what can be done, and what the impediments are.  But it would be stupid of me not to promote two of my websites:

To read more about rainwater harvesting vs graywater go to: http://www.besthomewatersavers.com/pages/Rainwater-Guide.html

Written by Waterharvesting

June 12, 2010 at 7:40 am