🏠 Roof Rainwater Harvesting Replenish Calculator

Estimate the volume of water supplied from roof rainwater harvesting systems using a daily tank water balance. Rainfall over the roof is converted to tank inflow through an adjustable collection efficiency, routed through tank storage against a defined daily demand, and the volume actually drawn from the tank is reported as the replenish estimate. A single installation can be described directly, or many installations across different buildings uploaded as a CSV schedule.

Preliminary modelled estimate
ℹ️
The replenish estimate is tank yield, not roof capture. Water that enters the tank and later overflows, leaks or evaporates delivers no benefit, and water is only drawn from the tank when there is both demand and stored water available. The headline figure is therefore constrained by whichever of rainfall, roof area, tank capacity or demand binds first, and the tool reports which one does. All populated parameter values are illustrative defaults and must be replaced with site-specific evidence.

📁 1. Upload daily rainfall

CSV format: Date, Rainfall (mm). Additional numeric columns are read as further named rainfall series, so several gauges can be supplied in one file. Daily, consecutive, dated records are required; at least three representative years are recommended.

🏘️ 2. Installations

Model one installation from the parameters below, or upload a schedule of many installations (houses, schools, clinics, factories) to run as a portfolio against the same rainfall record.

🧱 3. Roof, capture and tank

Descriptive only; it does not affect the calculation.
Horizontal projected area of the roof draining to the tank, not the pitched surface area.
Combined roof runoff coefficient, gutter and downpipe losses, and filter or screen losses. Typically 0.75–0.90 for pitched metal or tile.
Depth discarded or retained on the roof before flow reaches the tank. Applied as an initial abstraction each day with rainfall.
Usable storage between the draw-off point and the overflow, not the nominal vessel volume.
Starting the tank empty avoids crediting water the project did not supply.
Leakage and evaporation from storage. Leave at zero for a sealed tank in good condition.

🚰 4. Demand and accounting

Demand the tank is intended to serve on an operating day, not total site demand.
Demand is zero on non-operating days; storage still fills.
Comma-separated month numbers with no demand, for school holidays or seasonal shutdowns. Leave blank for none.
YAS draws demand from the previous day's stored volume before the day's inflow is added; YBS allows same-day inflow to meet demand.
Counterfactual test: the share of tank yield that avoids an actual withdrawal from the water body or aquifer the claim relates to.

📐 Method, inputs and limitations

1. Roof capture and collection losses

Daily inflow to the tank is calculated from rainfall depth, the horizontal projected roof area, an initial abstraction and a single multiplicative collection efficiency:

I = max(0, P − Ia) × Aroof × C / 1000

Here I is inflow in m³, P is daily rainfall in mm, Ia is the first-flush and wetting loss in mm applied on each day with rainfall, Aroof is roof plan area in m², and C is the collection efficiency. The division by 1000 converts mm·m² to m³.

  • Roof plan area is used because rainfall is measured on a horizontal plane; pitch increases the wetted surface but not the catch.
  • C bundles the roof runoff coefficient with gutter overshoot, downpipe and filter losses. Splitting these apart is not warranted at screening level, but a single value should be evidenced rather than assumed.
  • The initial abstraction is applied per rain day regardless of intensity, so many small rain days are penalised more than the same depth falling in fewer events. Where intensity data exist, sub-daily modelling will give a different and usually higher capture.
  • Gutter capacity is not modelled. High-intensity rainfall that overshoots the gutter is only represented insofar as it is embedded in C.
2. Daily tank water balance and operating rule

Storage is tracked at a daily time step. Any tank loss is removed from the opening stored volume first. The order in which yield and spillage are then resolved is a modelling choice with a material effect on the result, so both standard behavioural rules are offered.

YAS: Yt = min(Dt, St−1)    then    St = min(St−1 − Yt + It, Ctank) YBS: Yt = min(Dt, St−1 + It)    then    St = min(St−1 + It − Yt, Ctank)

Yield after spillage (YAS) does not allow the day's inflow to meet the same day's demand and is the conservative choice recommended for design; yield before spillage (YBS) typically reports 10–15 per cent more yield for the same system. Overflow is the volume in excess of tank capacity after the rule has been applied, and unmet demand is the shortfall met from another source.

The balance is reconciled and reported: opening storage plus total inflow must equal total yield, plus overflow, plus tank losses, plus closing storage. The residual is shown as a closure error so the result can be checked rather than taken on trust.

3. Demand pattern

Demand is a constant daily volume applied on operating days only. Two masks are available: a day-of-week pattern, and a list of closed months. This matters because a system serving a school accumulates storage through holidays and then draws it down, which a constant-demand model would misrepresent in both directions.

  • Within-day and within-week demand variability is not represented; a daily time step cannot resolve whether demand is met at the moment it occurs.
  • Demand is not reduced in wet weather, although in practice irrigation and some cleaning demands are.
  • Demand is treated as fixed and independent of supply. Where users ration in dry periods, modelled unmet demand overstates the shortfall actually experienced.
4. Replenish definition and additionality

The replenish estimate is the volume actually drawn from the tank over the record, annualised. Water that enters the tank and subsequently overflows or is lost is excluded, as is demand that the tank cannot meet.

Net replenish = Σ Yt / years × displacement fraction

This is a withdrawal-reduction claim in VWBA 2.0 terms rather than a supply-augmentation claim: the benefit is the withdrawal that did not occur because the demand was met from the roof. The displacement fraction is the counterfactual test, and a value of 100 per cent has to be justified. It should be reduced where the demand is new rather than substituted, where the alternative source is not the water body the claim relates to, or where the alternative source is unstressed.

  • Intercepted roof runoff would otherwise have reached the ground, and some fraction of it would have infiltrated or reached a watercourse. Where that pathway matters locally, the displaced withdrawal is not the only term in the catchment balance.
  • A stormwater attenuation benefit may also exist but is a different claim and is not quantified here.
  • The estimate is a modelled potential. It assumes the system is commissioned, connected to the intended demand, and maintained.
5. Storage sensitivity and the limiting regime

Tank yield is recalculated across a logarithmic range of tank capacities with all other inputs fixed. Three limiting regimes are distinguished from the resulting curve and the modelled balance:

  • Demand-limited — reliability is essentially complete and the curve is flat. Yield is set by demand; a larger roof or tank adds nothing, and the replenish estimate is insensitive to the capture parameters.
  • Supply-limited — overflow is a small share of inflow and demand is not met. Yield is set by rainfall and roof area; additional storage adds little, and the estimate is sensitive to the collection efficiency.
  • Storage-limited — the system both spills and fails to meet demand, and the curve is still rising at the design capacity. Additional storage converts spill into yield, and the marginal yield per additional cubic metre of storage is reported.

Where spill and unmet demand occur in different seasons and the marginal return on storage is small, the system is described as seasonally limited: bridging a dry season requires storage of a different order from that required to smooth between storms.

6. Important exclusions
  • Sub-daily rainfall intensity, gutter and downpipe hydraulic capacity, and first-flush device behaviour.
  • Water quality, treatment requirements, and fitness of the yield for its intended use.
  • Pump energy, backup mains top-up controls, and any switching logic between sources.
  • Capital and maintenance cost, and the behaviour of the system once maintenance lapses.
  • Stormwater runoff attenuation, sewer discharge reduction and associated benefits.
  • Whether the avoided withdrawal addresses the relevant shared water challenge in the right place and season.
Sources
  • WRI et al. (2025), Volumetric Water Benefit Accounting 2.0, withdrawal-reduction accounting and counterfactual treatment.
  • Fewkes, A. and Butler, D. (2000), Simulating the performance of rainwater collection systems using behavioural models, Building Services Engineering Research and Technology 21(2), 99–106 — the YAS and YBS operating rules and the case for YAS in design.
  • Jenkins, D., Pearson, F. and Moore, E. (1978), Feasibility of rainwater collection systems in California — origin of the yield-before and yield-after spillage formulations.
  • Mitchell, V.G. (2007), How important is the selection of computational analysis method to the accuracy of rainwater tank behaviour modelling?, Hydrological Processes 21, 2850–2861.
  • BS EN 16941-1:2018, On-site non-potable water systems — Part 1: Systems for the use of rainwater, for roof runoff coefficients, filter efficiencies and system definitions.