WebToolArcPractical guide and verification
Use the tool first, then apply these checks to verify inputs, interpret the result, and hand it off without displacing the primary workflow.
Distinguish gross rainfall from collectable water
Rainfall depth multiplied by area gives the theoretical volume that fell on that surface. A real harvesting system captures less because of wetting, splash, evaporation, first flush, gutter losses, filters, and overflow. Keep gross volume visible and apply a separate runoff or collection coefficient when planning storage or usable supply.
Use the horizontal catchment footprint
For roof harvesting, rainfall is normally based on the horizontal projected area draining to the collection point, not the larger sloped roof surface. If several roof sections feed different gutters or tanks, calculate only the sections that actually reach the same system. Measure the catchment boundary before fine-tuning efficiency assumptions.
Choose the runoff coefficient as an assumption, not a constant
Smooth impervious roofs can shed a high fraction of rainfall, while rough, porous, dirty, or landscaped surfaces lose more. Coefficients also vary with storm size and system condition. Record the selected coefficient and test a lower-efficiency scenario instead of presenting one harvest estimate as guaranteed output.
Compare inflow with free tank capacity
A tank can capture only the volume of empty storage available when the storm arrives. Subtract any planned first-flush diversion, then compare the remaining inflow with free capacity and report potential overflow separately. Collection volume does not establish potability; treatment, local rules, and intended use remain separate design questions.