Tuesday, August 10, 2010

Rain Water Harvesting Basics

by Joseph Tyson
RainTankDepot
Mr. Tyson is a member of ARCSA

How much rain may I expect to harvest off my roof?
There are several factors to consider in calculating the water storage potential of your system.
>Average annual and monthly rainfall totals
>Median annual and monthly rainfall totals
>Rainfall Intensity
>Rainfall frequency

For most cities, rainfall data is available from the National Climatic Data Center at http://hurricane.ncdc.noaa.gov/cgi-bin/HPD/HPDStats.pl

Don’t forget to consider your climate – many northern locales are subject to rainwater harvesting systems being shut off in order to prevent freezing. Large sections of the US experience precipitation in the form of snow and ice. You’ll need to know the amount of annual runoff in addition to precipitation levels.

Intensity of rainfall is measured in inches of rain per hour (IPH). IPH is an important factor in designing your system as it will affect choices of first flush, filters, gutter and downspout sizes and collection techniques. You will find IPH info along with other similar rainfall data

Consider how frequently you will experience rainfall. For example, areas of Washington State may have some rain fall for as many as 80 or 90 days consecutively. Conversely, sections of the Southwest may experience zero precipitation for many days on end, followed with rainfall intensity rarely experienced elsewhere.

Rainfall return period indicates the inverse of the probability that a precipitation event of a certain magnitude will occur in a one year period. For instance, a storm with a 10 year return period has a 10% chance of occurring in any given 1 year period. You may find more information here: http://wrcc.dri.edu/pcpnfreq.html

The simplest calculation of the number of gallons of water that may be collected off any catchment area during a rainfall is as follows:

Catchment (roof) area in SF x Rainfall in inches x 0.623 x Runoff Coefficient* x Safety Factor **

Runoff coefficient – different surfaces, based on texture and porosity, will affect the amount of water that will successfully runoff into your collection system. Gravel, rocks, asphalt shingles, concrete, for example, will all retain some of the water that you aim to collect. Rainwater harvesters uses this table in calculating catchment potential:

Surface

Characteristics

High

Low

Roof

Metal, Gravel, Shingle

0.95

0.75

Paving

Concrete, Asphalt

0.95

0.70

Paving

Brick

0.85

0.70

Paving

Gravel

0.70

0.25

Safety Factor- any design makes allowances for an overestimation. The safety factor that you use will be reflective of how important the water that you collect is. The family relying on harvested rainwater for drinking water, crop irrigation and the like will use a safety factor much greater that folks who are using harvested rainwater solely to irrigate landscaping. Consider a safety factor between 0.65 (on the high side) and 0.95 (on the low side).

So, let’s look at a simple example-

Roof area (always measured in square feet at the drip line) = 2,500sf
Rainfall in inches = 2.75
Runoff Coefficient = 0.95 (for our example we will assume a high runoff coefficient)
Safety factor = 0.95 (we will be using our harvested water to irrigate our landscape)

2500 x 2.75 x 0.623 x 0.95 x 0.95 = 3,865 gallons

Surprised? Most folks are.

Let’s take a look at how much water a San Antonio, TX homeowner might expect to harvest on an annual basis. First, I go here http://weather.msn.com/monthly_averages.aspx?wealocations=wc:USTX1200 to get monthly average rainfall data and use it to compile a table (MS Excel is an outstanding tool for this exercise)

January

1.83

February

1.81

March

2.42

April

2.53

May

4.06

June

3.85

July

2.48

August

2.27

September

2.75

October

3.22

November

2.16

December

1.96

As you can see, the total rainfall in San Antonio, TX averages just 31.34 inches of rainfall annually. Let’s take a look at how much water the roof of a 3,000 sf ranch home in San Antonio can expect to harvest.
Our home is 60 feet by 50 feet, with a 2 foot overhang all the way around. So we calculate our catchment area as 64 x 54 = 3,456 sf. Our roof is a Spanish tile. We will be using harvested rain water for landscape irrigation and, perhaps, to wash our automobiles. Let’s calculate:

3,456 x 31.34 x 0.623 x 0.95 x 0.95 = 60,899 gallons!

Once we determine that we have the potential to collect over 60,000 gallons of water in a year, the next thing we must determine is how much storage capacity do we require?

We'll discuss that in the next post

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