See Part 1 of this series here.
...Now that’s all nice to see annual totals, but if we want to invest in the infrastructure to use this resource of ours wisely then we are going to have to get specific. The best way to do this is to find out exactly how much of this water you are going to use.
...Now that’s all nice to see annual totals, but if we want to invest in the infrastructure to use this resource of ours wisely then we are going to have to get specific. The best way to do this is to find out exactly how much of this water you are going to use.
Part 2: How can I tie it in to my current system
or maintenance routines?
To answer this question let's break it down into three parts: collection, storage and delivery.
Collection Method
Here we evaluate the effectiveness of 4 things - Material, Delivery, Climate and Other:
Material of roof
- Is the roof constructed of standing seam metal, ethylene propylene diene monomer (EDPM), slate or tile? (different materials have different runoff coefficients i.e. metal is .95, concrete and asphalt is .9, tar and gravel is .8-.85)
- Is the roof or exterior building walls capable of withstanding load or attachment of a "delivery system" (see below)?
- Does it direct all catchment into a secure delivery system (gutters, eaves trough, piping/tubing, down spouts, chains, free-fall)?
- Are there any leaves, debris or sediment being transported with the water?
- Does the water need to be filtered? (debris or sediment traps?)
- Is the whole delivery system above (height) the storage location?
- What is the differential grade? (must be a negative grade from roof to storage)?
- Is the delivery system winter proof?
- Will it crack if frozen?
- Does it have exposure to prevailing winds?
- Should I cut overhanging branches down to prevent damage to the system?
- Will it’s aesthetics come in to consideration?
- Will it’s footprint effect my operations in any way?
- How much maintenance will this require? (Time, Labor, $)
- Any permits for building this infrastructure?
Storage Method
Catchment area and rainfall determine supply, and demand and loss dictates required storage capacity, so with that said lets figure out how much storage you need at your site.
The Turfhugger Rainwater Harvesting Calculation Table can help you evaluate the effectiveness of your potential project.
You can view a larger html version here, can download an excel version here, or access the blank Google Docs version here. I’ve included below an explanation for the table, calculations, listed some additional resources and gave some tips for the successful completion of the table. The info in this sample is meant for demonstration purposes only.
Total Roof Footprint
Remember that we want the footprint not total surface area. A flat top and a church can have the same footprint but the church would have a larger surface area...right? Right.
Have any doubts on how to collect this info? Use the http://pro.scribblemaps.com/ polygon measuring tool or ruler tools to determine length (X) and width (Y) like the image here, or walk it with a measuring wheel - either way, be accurate!
Then times X by Y. For example:
X = 30.62m x 3.280 = 100.43ft
Y = 18.39m x 3.280 = 60.31ft
100.43 x 60.31 = 6056.93 Sq ft = HRA
Landscape Area in Sq Ft
This is your area of intended use. It doesn’t have to be landscape, it’s just an easy case study to make. You could use this water for filling sprayers, washing equipment, drinking!
Rainfall Coefficient
Different materials have different runoff coefficients i.e. metal is .95, concrete and asphalt is .9, tar and gravel is .8-.85
Plant Factor
I use the following chart to figure out my relative plant factor:
| Plant Type | Percent Range | |
| High | Low | |
| Low Water Use | 0.26 | 0.13 |
| Medium Water Use | 0.45 | 0.26 |
| High Water Use | 0.64 | 0.45 |
Municipal Charge per 100 Cubic Feet
This is how much water is costing you per 100 Cubic Foot (CCF).
If you want to leave this out just type 0.
Average/max precipitation for your region.
Check these links for average monthly precipitation in American & Canadian cities. Alternatively try Googling “monthly average precipitation” for your city. It’s important to consider maximum rainfall levels, but because we want to make an economic case for the purchase it is best to use the average as the base.
Rainfall Harvest in Gallons
To Calculate your “monthly yield” or Rainfall Harvest use this formula:
RH = [Harvestable Roof Area X Amount of Rainfall X Rainfall Coefficient (material and slope)]
For example: 6056.93 x 162.7 x .95 = 936189.38mm
1 milliliter = 0.000 264 172 052 36 gallon [US, liquid]
Therefore 936189.38mm = 247.315 Gal.
Actual Demand in Gallons
For landscape water demand I use the formula of Demand = (ET x Plant Factor) x area x 7.48.
ET (go here for historical ET of American Zip Codes, sorry no Canadian:(, remember it is important to use a rate that is accurate)
Plant Factor (see above chart)
Area = Area in square feet
7.48 = the conversion of Cubic feet in to US gal. Most water companies provide totals in cubic feet so by using this unit in your estimates a comparison will be kept easy. This is the total amount of water that could be left in the storage tank and includes the total amount that fell during the month and whatever was not used from the month prior.
Historical ET
Remember to leave ET rate out of months where you do not intend on using this supply.
Go here for historical ET of American Zip Codes, sorry no Canadian:(, remember it is important to use a rate that is accurate.
Monthly Supply Surplus/Deficit in Gallons
This is the monthly total of unused or overused supply. This is figured out by subtracting Actual Demand in Gallons from the total Rainfall Harvest for each individual month.
Cumulative Storage Gallons
This is the total amount to date that has not been used. The highest amount will be the size of tank you’ll want! Using our example here we see you’d want at very least a 700 Gal tank. Remember to always round up as you don’t want to opt down in storage capacity.
Municipal Usage in Gallons
How much water did you have to buy?
1 cubic foot = 7.480 519 480 5 gallon [US, liquid]
Total $ in Municipal Source
This is how much money you would have to spend to cover your demand.
It’s best to fill out this table with data from the last three years to get an accurate picture of what the supply could be. Remember, for the initial year you’ll be operating out of a deficit because your using the tank for the first time, similar to if you must winterize your storage by emptying or disconnecting.
"Delivery" will be in Part 3 of this series.
EmoticonEmoticon