The resource is a flow, not a photograph.
A foggy scene does not specify a water yield. The incoming liquid-water content, the air volume crossing the collection boundary and the duration determine the liquid available to intercept. Local wind, droplet sizes, seasonal occurrence and terrain all influence that resource.
Begin with a clearly drawn boundary. For a collector, that can be its inlet face and outlet drain. For a pond, it includes the receiver surface and all inflows and outflows. For an industrial process, it must include the equipment and operating changes that make recovery possible.
ṁin = LWC × Q
ṁdelivered = ηcapture ηdrain LWC × QQ: airflow (m³/s); U: face-normal speed (m/s); A: cross-section (m²); LWC: liquid-water content (kg/m³). Efficiencies are fractions between 0 and 1 and must be measured for the operating conditions. The calculator uses one combined delivery fraction.
Interception, drainage and usable water.
Water may arrive on the hardware yet remain as a film, evaporate, escape with the air or drain outside the intended receiver. Separating captured mass from delivered mass reveals whether the next improvement belongs in particle transport or in the surface and drainage system.
Useful water is a further step. Its composition can reflect the incoming air, collection materials and accumulated contamination. The intended use sets treatment and verification requirements. A collection rate does not establish drinking-water suitability or compatibility with a cooling loop.
Three Weatherhill applications.
- FogFarm: establish a local fog resource, a passive collection baseline, storage delivery and the relationship to actual water demand. An annual total must account for periods without suitable fog.
- PondFall: investigate whether an intervention increases delivery into a specified receiver. Direct deposition must be measured against natural deposition, evaporation and other flows; moving droplets near a pond is not proof that they enter it.
- Industrial recovery: study a known mist or plume stream with its process owner. Identify what is liquid, what is vapor, what equipment changes heat rejection and where recovered water can be used.
Cooling towers: four different water pathways.
Evaporation removes heat by changing liquid water into vapor. Drift carries liquid droplets out with airflow. Blowdown removes concentrated circulating water to manage dissolved solids. Leaks and overflow are other losses. These routes have different remedies. DOE’s cooling-tower guidance provides the accounting framework. Source S11 →
Steady storage is assumed. A changing basin inventory must be included as a storage term. A droplet collector acts on entrained liquid; recovering vapor requires condensation or another suitable separation process.
Cooling performance must be measured alongside water savings. Added pressure drop can change fan demand or airflow. A treatment that impairs heat rejection is not adequately evaluated by its collected-water volume. Water chemistry, scaling, corrosion, biological control and maintenance constrain any proposed change.
The energy denominator matters.
Incremental specific energy = added electricity / positive added usable waterReport kWh per m³ of water, the baseline, the operating duration and every included load. Zero or negative incremental water gain cannot be assigned a finite favorable incremental efficiency.
A discharge-only energy figure excludes other equipment unless it is explicitly included. An honest comparison includes fans, pumps, power supplies, conditioning and relevant treatment. It also records maintenance and downtime instead of assuming continuous ideal operation.
If an intervention adds water but evaporation and withdrawals remain larger, the receiver still loses storage. The useful claim is the measured incremental benefit under matched conditions, with the overall balance reported alongside it.