RHS

Ecology & Environment

Rainwater Harvesting Scenario Calculator

Compare two rainwater systems by gross annual harvest, demand coverage, tank-fill equivalents, harvest difference, and capacity difference.

Scenario A catchment area (m²)Scenario A annual rainfall (mm)Scenario A runoff efficiency (%)Scenario A tank capacity (L)Scenario B catchment area (m²)Scenario B annual rainfall (mm)Scenario B runoff efficiency (%)Scenario B tank capacity (L)Entered annual nonpotable demand (L)

Scenario A annual harvest potential (L)-

Scenario B annual harvest potential (L)-

Scenario A gross demand coverage-

Scenario B gross demand coverage-

Scenario A equivalent tank fills-

Scenario B equivalent tank fills-

Scenario B minus A harvest (L)-

Scenario B minus A tank capacity (L)-

Decision view

Rainwater system scenario comparison

Rainwater system scenario comparisonHarvest, storage, equivalent fills, and gross demand coverage are paired by scenario.

Exact scenario comparisonScenario B annual rainfall (mm) changes while all other entered assumptions remain constant.

Scenario B annual rainfall (mm)	Scenario A annual harvest potential (L)	Scenario B annual harvest potential (L)	Scenario A gross demand coverage	Scenario B gross demand coverage	Scenario A equivalent tank fills	Scenario B equivalent tank fills	Scenario B minus A harvest (L)	Scenario B minus A tank capacity (L)

How to use Rainwater Harvesting Scenario Calculator

Enter area, rainfall, efficiency, and tank size for both scenarios.
Enter common annual demand.
Compare gross harvest and storage separately.
Use a monthly or daily balance model for final tank sizing.

Calculator guide

Understanding Rainwater Harvesting Scenario Calculator

Annual rainwater potential depends on catchment area, rainfall depth, and runoff efficiency. Storage capacity is a separate timing constraint, so annual litres and tank size must not be treated as interchangeable.

Climate Rainfall depth enters explicitly.

Geometry Area converts depth to volume.

Efficiency Losses reduce gross capture.

Storage Tank size remains a separate constraint.

Calculation method

How the calculation works

Compare two rainwater-harvesting catchment, rainfall, runoff, and storage designs while keeping annual demand common and clearly labeled. Use the identity 1 mm over 1 m² = 1 litre, multiply area by annual rainfall and runoff fraction, then compare annual harvest separately with demand and tank capacity.

Detailed calculation process

Compare two catchment-and-storage scenarios

The default compares A: 160 m², 850 mm, 88%, 12,000 L with B: 220 m², 700 mm, 82%, 18,000 L against 95,000 L annual demand.

General formula: H_A = A_AR_A(e_A/100)H_B = A_BR_B(e_B/100)C_A = 100H_A/DC_B = 100H_B/DF_A = H_A/T_AF_B = H_B/T_BDelta_H = H_B-H_ADelta_T = T_B-T_A Millimetres of rain over square metres convert directly to litres before runoff losses. Demand coverage is an annual gross ratio; equivalent tank fills do not simulate rainfall timing or overflow.

What each symbol means

A_A, A_B Scenario catchment areas (m²).

R_A, R_B Annual rainfall depths (mm/year).

e_A, e_B Runoff efficiencies (percent).

H_A, H_B Gross annual harvest potentials (L/year).

D Common annual nonpotable demand (L/year).

C_A, C_B Gross annual demand coverage (percent).

T_A, T_B Tank capacities (L).

F_A, F_B Equivalent annual tank fills.

Delta_H, Delta_T Scenario B minus A harvest and capacity differences (L).

Worked substitution with the default inputs

1. Calculate scenario A harvest H_A = 160(850)(88/100)H_A = 119680 L/year The rainfall-area product is litres before applying 88% runoff efficiency.

2. Calculate scenario B harvest H_B = 220(700)(82/100)H_B = 126280 L/year B has more area but lower rainfall and runoff efficiency.

3. Compare annual demand C_A = 100(119680/95000) = 125.9789%C_B = 100(126280/95000) = 132.9263% Coverage above 100% means gross annual volume exceeds demand, not that demand is met every day.

4. Compare storage scale F_A = 119680/12000 = 9.9733 fillsF_B = 126280/18000 = 7.0156 fills Tank-fill equivalents are volume ratios and do not model actual fill-and-spill events.

5. Reconcile scenario differences Delta_H = 126280-119680 = 6600 L/yearDelta_T = 18000-12000 = 6000 L Harvest advantage and installed capacity advantage are reported independently.

Scenario B yields 126,280 L/year versus A's 119,680 L/year and has 6,000 L more tank capacity, but timing remains unmodeled.

Purpose-built visual

Compare paired catchment and tank capacities

Two system lanes show gross harvest, annual demand, and tank capacity on clearly labeled scales so annual supply is not confused with instantaneous storage.

Harvest bars Annual collectible volume.

Demand line Common annual requirement.

Tank blocks Installed storage.

Difference B minus A comparison.

Worked situations

Practical examples

Scenario A gross harvest is 119,680 L/year.
Scenario B is 6,600 L/year higher.
Both gross annual coverage ratios exceed 100%.

Better inputs

Useful tips

Use local rainfall records.
Deduct first flush and system losses in the efficiency input.
Size tanks from time-series inflow and demand.

Before relying on the result

Limitations and common mistakes

Rainfall timing, drought sequence, overflow, carryover, first flush, water quality, and regulation are excluded.
Annual coverage does not guarantee reliable daily supply.
Potable use requires appropriate treatment and compliance.

Reference

Key terms

Catchment: Surface contributing rainfall runoff.
Runoff efficiency: Share of incident rain assumed collectible.
Gross harvest: Annual potential before time-sequence storage losses.
Tank-fill equivalent: Annual harvest divided by tank capacity.

Important note

Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.

Frequently asked questions

Why does mm × m² equal litres?

One millimetre is 0.001 m; over one square metre that is 0.001 m³, or one litre.

Does 126% coverage mean no shortages?

No. Rain and demand timing can still empty the tank.

Why can fewer tank fills still be better?

A larger tank changes the ratio; it does not by itself reduce annual harvest.

Can this size a potable system?

No. Water quality, treatment, regulation, and time-series reliability require separate design.