Do you have a big enough roof? (Strategic Water Reserve for Any Garden Endeavor) Part 2
Never die of thirst.
Never again lose crops or animals because of lack of water. Never again be vulnerable to drought periods. The solution is literally over your head - catching rainwater from your rooftops.
Here is what you need to do to make it work.
In this edition, in 5 minutes or less:
#1 The Strategy of Water
#2 How Much Water Do You Need to Survive and Thrive (Household + Garden + Animals)
#3 How to Make the Calculations - Is Your Roof Big Enough?
The Strategy of Water
Water strategy is crucial to life, and even people who live in a city and get their water from a tap are dependent on some water strategy. Normally, this strategy is determined by government and influenced by macroeconomics and politics.
We need water for drinking, cooking, sanitation, food production (vegetables and animals), and industrial production across all fields. Many times water gets polluted as a consequence of its use, causing a chain effect where the pollutant spreads out, rendering our living space bad for us and the once-drinkable water poisonous. We, as people, don’t control water use, but as voters, we theoretically do have control. It seems that the necessity of fresh clean water for normal people to drink and avoidance of pollution is placed below other priorities. Building strategic water reserves through private initiative is crucial to be able to actively choose the intended use and how to use water.
On a broad scale strategy, that means building dams, ponds, earth banks, and swales. On a human scale, that means harvesting rainwater in water tanks.
We are going to focus on the minimal viable product of drinkable water for a household and its home garden.
How Much Water Do You Need to Survive and Thrive (Household + Garden + Animals)
You can actually choose your household water use level (per person per day):
- Minimal survival: 15–25 L/person/day (drinking, cooking, basic hygiene)
- Modern, complete service with efficient fixtures/appliances: 150–170 L/person/day (drinking, cooking, hygiene, clothes washer, dishwasher)
- Modern, complete service without efficient fixtures/appliances: 230 L/person/day (drinking, cooking, hygiene, clothes washer, dishwasher)
You might have a 100m² home garden:
- Cool spring/fall: 10–15 mm → 1,000–1,500 L/week (140–215 L/day) unmulched or 600–1,125 L/week (85–160 L/day) mulched
- Warm summer: 25–30 mm → 2,500–3,000 L/week (360–430 L/day) unmulched or 1,500–2,250 L/week (215–320 L/day) mulched
- Hot/dry or sandy: 35–45 mm → 3,500–4,500 L/week (500–645 L/day) unmulched or 2,100–3,375 L/week (300–480 L/day) mulched
When you have a home garden, you might benefit from an animal-powered nursery, which houses 4 chickens or 10 guinea pigs plus their babies, as their minimal viable version:
- Guinea pigs (10 adults + babies): Adults (typical 0.8–1.2 kg each) drink 50–150 mL/kg/day; Nursing pups (<3–4 weeks) drink mostly milk but still offer 0–10 mL/pup/day of water; Weaned pups (0.25–0.5 kg) drink 50–150 mL/kg/day
- 10 adults + babies → 1.0–1.5 L/day (plan 1.5–2.0 L/day in heat)
- Chickens: 0.2–0.35 L/hen/day (up to ~0.5 L in hot weather)
- 4 hens → 0.8–1.4 L/day (plan 1.0–2.0 L/day for safety)
Warning! These are good numbers to start with, but you should take them with a grain of salt.
We have watered our 100 m² home garden just twice last year when we had a very severe midsummer drought, and not once this year. Linda Woodrow, author of “The Permaculture Home Garden,” usually doesn’t water her garden even once, even though she lives in subtropical Australia. Pay attention to the fact that you can design efficient water uses, like using greywater to water a garden, further decreasing your water needs.
You can design a lot around water, except how much fresh water you need to drink.
How to Make the Calculations - Is Your Roof Big Enough?
- How much can your roof catch in your climate?
- Size your storage (tanks and/or ponds) for your local drought window.
- Size safe overflows (spillways) so storms don’t damage your system.
- Bonus: size swales to infiltrate water into your soil and paths.
1. Roof area (in m²) × 0.9 (coefficient of loss) × total precipitation of a rain event (in mm) = Total water volume per rain event in Liters
Consult your local data to discover:
- The historical max rain event - the 100-year flood
- The average rain event
- The normal max rain event
- The minimal rain event
You can then calculate your average total monthly potential for water harvesting, which equals the size of the water tank.
2. Total water usage per day × how many days between rain events = The strategic water volume needed
As you have already seen, you can choose how much water you will use.
Don’t forget that there are some non-negotiables, like drinking water for you. The point of all this is having reliable water reserves and being drought-proof, so include in your calculations water for your animals and your garden.
You will become invulnerable to drought.
Consult your local data to discover:
- The historical max days without rain
- The average days without rain
- The normal max days without rain
- The minimal days without rain
Compare the number you got from calculating how much you potentially can harvest from your roof with the number that tells you how much water you have to have stored.
- Do you catch enough water for daily needs?
- Do you have surplus water?
- Do you need a bigger roof (bigger catchment area)?
- Remember, check if the rainwater is clean (relates directly to air quality)
3. Roof area (in m²) × Max precipitation during 24 hours (in mm) ÷ 24 ÷ 60 ÷ 60 = received volume in Liters per second = required discharge rate of Liters per second of the spillway
You want to discover the biggest inflow of water coming into your system to be able to size the spillway in a way that prevents blowouts.
At this moment, you should extrapolate the number and add 15% extra capacity to future-proof the system.
That is an obligatory security measure.
Direct the spillway to a gravel bed to dissipate the water’s strength.
You can improve the design by directing the overflow to a swale on contour, where the water can infiltrate your garden bed. You can also create multifunctional pathways around your yard and garden beds that are actually swales.
You can always iterate and tweak.
4. Cross-sectional area of your swale (you can consider it a semicircle) × swale length = Water volume your swale can hold
Be careful with units!
If you calculate using meters, you will get the volume in m³. 1m³ = 1000L; 1L = 1000 mL
You can create multifunctional pathways that function like a swale so that you can increase your water-holding capabilities without a roof.
The function of the swale is to soak in the water, so it won’t be a water reservoir, but it will charge the soil with water. You can also waterproof the swale, turning it into a canal.
You will also need to install security elements on a swale, like the spillway, and a proper freeboard (which is a vertical distance between the maximum water level and the crest of the swale).
There is a whole range of possibilities.
Your rooftops are your water-harvesting net.
You are not using them, just like a fisherman who has a net and refrains from casting it to catch enough fish. Is your net big enough for your needs? How much does it rain in your location? Is the air clean enough? Can you leverage other rooftops (even from your nursery)?
You may spend one day making the calculations and setting up the infrastructure on your roof to become self-reliant on water for the rest of your life.
