YielixRainwater Harvesting for Farm Irrigation
Start with a supply you can measure
A barn, packhouse or greenhouse roof can turn recorded rainfall into an on-farm water supply. This is a planning record, not a full-year water budget, storm promise or crop-water recommendation.
The Rainwater Harvesting Calculator keeps catchment, rainfall period, loss and matching-demand assumptions editable.
Measure horizontal catchment and collection losses
Use effective horizontal roof area draining to collection, not sloped surface area. Collection efficiency is an editable loss assumption for first flush, gutters, screens, splash, overflow, leakage or drainage layout—not a measured property of every roof.
Keep periods consistent: an event depth estimates that event and a monthly depth estimates that month. Apply first-flush diversion by event rather than subtracting an annual allowance.
Formula and units
potential captured water [L] = horizontal area [m²] × rainfall [mm] × efficiency [fraction]
1 mm × 1 m² = 1 L before collection losses
The calculator also supports ft², inches and US gallons; the physical identity is unchanged after conversion.
Worked example: a small equipment-shed roof
Use the retained Metric fixture: 100 m² horizontal catchment, 50 mm April rain and 80% collection efficiency.
100 m² × 50 mm = 5,000 Lbefore loss.5,000 L × 0.80 = 4,000 Lpotential capture.- 4,000 L = 4 m³, approximately 1,056.69 US gallons.
- May at 25 mm with the same assumptions captures 2,000 L; supplied months total 6,000 L, not a yearly estimate.
Estimate roof capture from your measured catchment, rainfall period and loss assumption
Rainwater Harvesting Calculator
Estimate potential roof runoff from supplied rainfall, catchment area and collection efficiency, with optional monthly demand coverage and coarse tank balance.
Compare matching periods and read storage as a sequence
Compare demand only with the same supplied rainfall periods. Missing rainfall or demand is not zero. Roof supply is also not effective field rainfall: use the Effective Rainfall Calculator and Irrigation Water Requirement Calculator for crop-side questions.
For retained tank-balance values—500 L starting storage, 4,000 L capture, 1,000 L demand and a 3,000 L tank—available water is 4,500 L. The tank ends at 3,000 L with 500 L overflow and no unmet demand. This approximate monthly sequence is not tank design. Roof water is not automatically potable or suitable for every crop-contact use.
available [L] = starting storage [L] + captured water [L]
used [L] = min(available [L], demand [L])
ending storage [L] = min(capacity [L], max(0, available [L] − used [L]))
Assumptions and limitations
- Entered roof area drains to collection, records represent named periods, and efficiency is the user's best loss assumption.
- The model does not infer missing weather or demand, water quality, field infiltration, tank chronology, pump performance, permits or legal permission to collect and use water.
- Daily or event inflows and outflows are needed for storage sizing and operational overflow prediction.
Common mistakes
- Using sloped roof area: use horizontal plan area unless a verified drainage design provides another basis.
- Treating efficiency as measured: retain and improve the loss assumption.
- Filling missing months with averages: keep omissions distinct from explicit zero.
- Equating volume with safe or free water: check quality, treatment, pumping, cost and rules separately; use the Water Cost Calculator for a separate cost scenario.
Sources
- University of Arizona Cooperative Extension, Harvesting Rain in Cochise County — roof-catchment and rainwater-harvesting planning context.
- Utah State University Extension, Rain Barrels in Utah — roof-runoff arithmetic and collection context.