Complete Primitive Water Collection and Rain Harvesting: From Cloud to Cistern
Rainwater harvesting provides a reliable, clean water source independent of wells or streams. This campaign covers collection surfaces, storage systems, filtration, and calculation of water yield.
Chapter 1: Collection Surfaces
Surface
Collection Efficiency
Contamination Risk
Cost
Durability
Area Needed
Metal roof (galvanized)
90-95%
Low
Moderate
Excellent
Standard roof
Metal roof (painted)
85-90%
Low-moderate
Moderate
Very good
Standard roof
Tile roof
80-90%
Low
High
Excellent
Standard roof
Asphalt shingle
75-85%
Moderate (chemicals)
Low
Good
Standard roof
Thatch roof
60-70%
Moderate (organic)
Very low
Moderate
Standard roof
Concrete/stone
80-90%
Low
Moderate
Excellent
Ground catchment
Plastic sheeting
90-95%
Very low
Very low
Low (UV damage)
Any size
Rock outcrop
70-85%
Low
Free
Permanent
Natural
Water yield calculation: 1) Measure roof area in square feet. 2) Multiply by annual rainfall in inches. 3) Multiply by 0.623 (converts to gallons). 4) Multiply by collection efficiency (0.80-0.95). 5) Example: 1,000 sq ft roof x 40 inches rain x 0.623 x 0.90 = 22,428 gallons per year. 6) One inch of rain on 1,000 sq ft = approximately 560 gallons. 7) This is often more water than a family needs. 8) Key: adequate storage to capture wet-season surplus for dry-season use.
Chapter 2: Gutter and Conveyance
Component
Material
Function
Sizing
Gutters
Metal, PVC, bamboo, wood
Collect water from roof edge
5-6 inch for standard roof
Downspouts
Metal, PVC, bamboo
Carry water from gutter to storage
3-4 inch diameter
First flush diverter
PVC pipe with valve
Diverts first dirty water
1 gallon per 100 sq ft roof
Leaf screen
Wire mesh, screen
Prevents debris entry
Cover all gutter openings
Settling tank
Barrel or tank
Allows sediment to settle
50-100 gallons
Overflow
Pipe from tank
Prevents tank overflow damage
Same size as downspout
First flush diverter: 1) First rain washes dust, bird droppings, and debris off roof. 2) This "first flush" is the dirtiest water. 3) Divert first 1 gallon per 100 sq ft of roof area. 4) Simple design: vertical pipe below downspout with ball valve at bottom. 5) Pipe fills with dirty water first (ball floats up, sealing pipe). 6) Clean water then flows to storage tank. 7) Drain first flush pipe after each rain event. 8) Dramatically improves water quality with minimal cost.
Chapter 3: Storage Systems
Storage Type
Capacity
Cost
Durability
Material
Best For
Rain barrel
55-100 gallons
Very low
Good
Plastic (HDPE)
Small gardens
IBC tote
275 gallons
Low
Good
Plastic/metal cage
Moderate use
Polyethylene tank
500-10,000 gallons
Moderate
Very good
Plastic
Household supply
Ferrocement cistern
500-10,000+ gallons
Low (DIY)
Excellent
Cement, wire mesh
Large-scale, permanent
Underground cistern
1,000-50,000+ gallons
Moderate-high
Excellent
Concrete, fiberglass
Large-scale, hidden
Stone/brick cistern
500-5,000 gallons
Moderate
Excellent
Masonry
Permanent, traditional
Chapter 4: Water Quality
Contaminant
Source
Treatment
Prevention
Bird droppings
Roof surface
First flush diverter, filtration
Screen gutters
Dust/pollen
Atmosphere
Settling, filtration
First flush diverter
Leaf debris
Trees
Gutter screens, filtration
Trim overhanging branches
Mosquito larvae
Standing water
Sealed tanks, screen vents
No open water surfaces
Algae
Light + nutrients
Opaque tanks, dark storage
Block all light
Bacteria
Various
UV treatment, chlorination, filtration
Sealed system, regular cleaning
Chapter 5: System Design
Household Size
Daily Need
Monthly Need
Minimum Storage
Recommended Storage
1 person
5-10 gallons
150-300 gallons
500 gallons
1,000+ gallons
2 people
10-20 gallons
300-600 gallons
1,000 gallons
2,500+ gallons
4 people
20-40 gallons
600-1,200 gallons
2,500 gallons
5,000+ gallons
Small farm
50-100 gallons
1,500-3,000 gallons
5,000 gallons
10,000+ gallons
Reference Card
One inch of rain on 1,000 square feet equals 560 gallons (most roofs collect far more water than people realize; calculate your yield). 2. First flush is the dirtiest (the first rain washes contaminants off the roof; always divert the first flush away from your storage). 3. Opaque tanks prevent algae (light promotes algae growth; use dark or opaque storage containers and block all light). 4. Screen everything (mosquitoes breed in standing water; screen all openings, vents, and overflows to prevent mosquito entry). 5. Gravity is free pressure (elevate your tank and gravity provides water pressure; 1 foot of elevation equals 0.43 PSI). 6. Size storage for dry season (calculate how many days between rain events in your driest season; size your tank to bridge that gap). 7. Ferrocement is the cheapest large tank (a DIY ferrocement cistern costs a fraction of a commercial tank and lasts decades). 8. Rainwater is soft water (rainwater has no minerals, making it excellent for washing, gardening, and with treatment, drinking).
