Transpiration Bags and Solar Stills

Transpiration Bags

Plants constantly lose water through their leaves through a process called transpiration. A sealed clear plastic bag over a leafy branch traps this moisture, which condenses and collects at the bag's lowest point.

What you need: Clear plastic bags (2-4 gallons works well; more surface area = more collection), something to tie the bag closed.

Setup:

Best plants for transpiration bags:

• Willow and cottonwood — abundant transpiration
• Any broad-leaved deciduous tree
• Green desert shrubs with many leaves

Avoid: Pine, juniper, and other conifers (resins from leaves can concentrate in the bag and contaminate the water). Plants with milky sap.

Realistic yield: 0.2-0.5 liters per bag per day in warm sunny weather. Multiple bags running simultaneously are needed for meaningful collection.

The water is clean — transpiration is pure distillation through the plant. You do not need to treat transpiration bag water if you've avoided toxic plants.

Ground Solar Still

A solar still uses the sun's heat to evaporate moisture from soil and vegetation, condense it on a plastic sheet, and drip it into a collection container.

What you need:

• Clear plastic sheeting (a large trash bag cut open, heavy-duty clear plastic, or a dedicated solar still bag) — minimum 6 feet square
• A container (cup, pot, or bottle) to collect water
• A small rock or weight
• Optionally: flexible tubing to drink from the container without dismantling the still

Site selection — the most important factor:

• Dig in areas with the most moisture: dry streambeds, low depressions where water has collected, places with greenish soil
• Full sun exposure throughout the day
• Downhill from any vegetation that indicates groundwater

Construction:

Improving yield:

• Larger pit surface = more collection area
• More moisture-bearing material inside = more evaporation
• Building multiple stills simultaneously
• Clear (not colored or opaque) plastic — maximum solar transmission
• Sealed edges — any leak destroys efficiency
• Fresh dig area each time (exhausted soil produces progressively less)

Realistic daily yield: 0.5-1 liter in ideal conditions. Less if soil is dry, sky is overcast, or plastic has leaks.

Treating the water: Water from a solar still is distilled — essentially pure. Treatment is not required if the collection container is clean.

When Are These Worth Building?

Yes, build them when:

• You're in a fixed position awaiting rescue with time to construct
• No other water source is accessible
• You have the materials and can spare the energy
• The digging/setup effort won't cause more dehydration than the water recovered

No, don't bother when:

• You need to be moving toward a known water source
• Energy conservation is critical and the yield is unlikely to cover the construction cost
• Better water-finding options exist (springs, seeps, dry wash digging)

Prioritize: Multiple transpiration bags over a single solar still if you have vegetation. Multiple solar stills over one if you have the material and energy.

Contaminated Water in a Solar Still

A solar still can also purify contaminated water. Pour contaminated water (including urine, contaminated surface water, chemically contaminated water) into the pit around the collection container. The evaporation-condensation cycle separates pure water from contaminants.

This works for:

• Saltwater
• Chemically contaminated water
• Muddy or biologically contaminated water
• Urine (last resort)

This does not work for: Volatile organic compounds (VOCs) — these evaporate and condense just like water, carrying over into the distillate.

The practical implication: if you can add contaminated water to a solar still, you can stretch dry-soil yields significantly.