Why Catchment Calculations?
To fully grasp just how much water impervious surfaces shed, easy catchment calculations
are utilized to analyze the water resources potential. Below are a series of calculations seen in images captured from slide shows. They reflect the site analysis of my folks place which is a mini-suburban educational spot and thriving garden in Cincinnati, OH, USA: Parkwalk Permaculture. I learned the calculations in my Permaculture Course but revisited them heavily when I took my Keyline Course with Darren Doherty and read Rainwater Harvesting from Brad Lancaster after also studying with him. Use these catchment calculations for the following reasons:
- Size tanks and cisterns to specific parts of the roof going into particular gutters. Its one thing to know the whole roof amount but tanks are most likely associated with one or two gutters.
- Size earthworks such as swales, ponds, or rain gardens to catchment basin or roof size and particular pitches.
Here is a bit of a cheat sheet to do rough catchment calculations in the field found in Brad Lancaster Rainwater Harvesting book.
Catchment Calculations: Finding Catchment Area
Simple math of length times width reveals the first step which is to find the square footage or meters of the house or catchment area. GIS maps, google or bing maps, deeds and property search software help to find the size of a building. Simply walking the perimeter with pacing or a measuring tape can work as well. Satellite or aerial maps help greatly because they can help you examine not only total size but particular pitches corresponding to particular gutters and downspouts. In field observation also allows you to observe which downspouts yield the most runoff if this technology isn’t present.
Catchment Calculations: Finding Volume
With the square footage of the catchment area calculated, volume is found through another multiplication. The multiplication either calculates annual totals, an average rainfall event, or a storm rainfall amount producing heavy downpours. This depends on how much you enter as the rainfall in feet or mm. Convert inches into feet by diving by twelve. For example 1 inch becomes 1/12=.083 ft. Rainfall is most often measured in cm’s so divide by 1o to get rainfall totals in mm’s.
On the below figure, the square footage and meters are displayed after the initial calculation for the one story ranch style house. Then with the above calculation, the below figure also reflects our average annual rainfall of 43 inches or 1.092 meters in our humid temperate climate in the Ohio River Valley. Metric converts the volume easily but in the imperial system you use the figure of 7.48 to generate volume in gallons from cubic feet. With these simple calculations you begin to see the astounding figures of how much water comes off of roofs especially in these more humid regions.
Catchment Calculations: Finding Runoff Coefficient
What is your roof surface– clay tiles, tar and gravel, or metal? Different surfaces shed different amounts of water thus creating another multiplication element of a catchment coefficient. This essentially rates how impervious the surface is or how much absorption capability it has. Each surface has more or less edge allowing more water to absorb or to be shed.
Design with Catchment Calculations
This allows you to most accurately gauge true potential of the catchment possibility. Doing this for individual rainfalls really helps with designing an earthwork such as a swale or rain garden to create an appropriate volume of
water holding capacity corresponding to the size of the earthwork. Again you can examine this in the context of average rainfalls (1 in or 2.5 cms) or more extreme ones (3 in or 7.5 cm) and size them accordingly. This is also important with tanks because of seasonality of rainfall. To withstand drought in Berlin or Washington DC is different than Lisbon or Los Angeles. The drylands areas only receive rainfall in certain months making tanks larger because of a larger space between rainfall.
Once these factors are known, then you can go further with your designs. This fits in with the assessment stage of your design if buildings are already present or the master planning phase of a building design.
If you are unable to capture all the water off of your roof in tanks or earthworks, should you be allowed to build the building? This is a fundamental question of development that wasn’t asked many years ago. Had it been, housing developments, urban planning, and farm design would all look different. These all have a positive or negative impacts on the environment and in particular the hydrological cycle. The color of our rivers and the quantity of life in waterways and water bodies can be positively impacted. The failure to address this in the design phase also has financial implications because of poor water quality damaging drinking water supplies and the flood and drought syndrome of modern day.
Furthermore, when examining landscapes and their ability to shed water, the following factors should be considered:
- Steepness of slope
- soil type
- organic matter percentage
- Vegetation type
Steeper slopes tend to produce heavier runoff amounts but is balanced by soil types. Clay soils and those that have been impacted detrimentally from over plowing, use of chemicals, farm equipment, and overgrazing are sure to give more water runoff. Also as the organic matter % drops so does the ability for soils to infiltrate and absorb rain thus producing more runoff. Diverse perennial vegetation such as well-managed grasslands, savannah, or forests all hold runoff better than degraded systems. Lawns tend to shed lots of water but food forests much less. Industrial agriculture is a major contributor to poor water quality and degradation of social systems alike showing once more our innate connection as humans to water and our settlements. Unfortunately, as homogenization of culture occurs, our concern for the hydrological cycle wanes.
As a practical observation of my own, I have seen there is very little overland flow at the family land on the Ohio River side of the valley where we have Sandy
Loam soils and mostly flat, slightly undulating land. The only real potential to catch water into earthworks is from the roofs of the two buildings and then also the road. The landscape itself tends to absorb much of the water but these compacted road surfaces shed a lot of water. In Portugal at one of our projects, Terra Mae, the road runoff is funneled into a 64,000 L water tank which sustains the project in the extended dry period. Thus our design, implementation, and ongoing management should aim to make the site a sponge of water giving resilience to the landscape and local ecosystem.
To see a video on the idea of catchment calculations and getting water to infiltrate, check out this short homemade video I made some years ago about the earthworks we did on a small piece of land in a village to deal with all of the roof and street runoff that was naturally running onto our property. In conclusion, know the impact of impervious surfaces and begin to take the steps needed to match catchment size and type to strategies and techniques for infiltration or storage.
Written by Doug Crouch
Header Art by Anita Tirone