Calculator Inputs
Formula Used
Runoff inflow:
Qin = (Rainfall intensity / 1000) × Catchment area × Runoff coefficient
Constant head permeability:
k = (Q × L) / (A × h × t)
Falling head permeability:
k = (a × L / (A × t)) × ln(h1 / h2)
Effective permeability:
ke = k × clogging factor × wetness factor / safety factor
Infiltration capacity:
Capacity = (ke / 1000) × Recharge area
Temporary storage needed:
Storage = max(0, runoff volume - infiltrated volume during storm)
How to Use This Calculator
- Select manual, constant head, or falling head method.
- Enter the soil permeability value or test data.
- Add rainfall intensity, duration, catchment area, and runoff coefficient.
- Enter the planned recharge area, storage depth, and porosity.
- Use safety, clogging, and wetness factors for practical design.
- Press Calculate to view the result above the form.
- Use CSV or PDF download for project records.
Example Data Table
| Case | Soil | Rainfall mm/hr | Catchment m² | Recharge Area m² | Expected Result |
|---|---|---|---|---|---|
| Small roof recharge | Sandy loam | 30 | 100 | 12 | Usually workable with storage |
| Courtyard soakaway | Loam | 40 | 180 | 20 | Needs careful drain time check |
| Clay site recharge | Clay | 25 | 120 | 25 | Likely needs overflow route |
| Coarse sand trench | Coarse sand | 50 | 250 | 30 | High intake potential |
Understanding Soil Permeability
Soil permeability shows how easily water moves through soil voids. It matters for every rainwater harvesting system that sends runoff into the ground. A permeable soil accepts water quickly. A tight soil accepts water slowly and may cause ponding, overflow, or long emptying times.
Why It Matters For Harvesting
Recharge pits, soakaways, trenches, and infiltration basins need a safe intake rate. Roofs and paved yards can produce runoff faster than soil can absorb it. The calculator compares inflow from rainfall with the effective infiltration capacity of the selected soil area. This helps size recharge area and temporary storage.
Inputs That Shape The Result
Rainfall intensity controls how fast water arrives. Catchment area sets the contributing surface. Runoff coefficient adjusts for losses, splash, and surface type. Soil permeability is found from a lab test, field test, or direct site value. Safety, clogging, and wetness factors reduce the ideal rate to a realistic design rate.
Reading The Outputs
The effective permeability is the working rate after adjustments. Inflow is the runoff volume entering the harvesting feature each hour. Infiltration capacity is the volume the soil can absorb each hour. If inflow is higher, storage is needed for the excess water. Drain time shows how long the stored water may remain after rainfall stops.
Good Construction Practice
Use this estimate during early planning, then verify it with local tests. Test at the proposed depth, not only at the surface. Avoid placing recharge structures near weak foundations, septic areas, polluted ground, or unstable slopes. Add pretreatment, such as silt traps and inspection chambers, because sediment can reduce permeability over time.
Design Notes
A conservative design protects the site during heavy storms. A lower safety factor is not always better. It may understate risk. Use larger infiltration area, cleaner filter media, overflow routes, and maintenance access when soil conditions are uncertain. The goal is not only to capture water. The goal is to capture it without flooding, erosion, contamination, or structural damage.
Document assumptions, test dates, and observed water levels. Recheck the design after excavation, because hidden clay seams or compacted layers may appear. During operation, inspect early after storms and remove trapped silt before it blocks inlet media or stone voids.
FAQs
What is soil permeability?
Soil permeability is the rate at which water moves through soil pores. It helps estimate whether rainwater can soak into the ground safely.
Why is permeability important for rainwater harvesting?
It shows whether a recharge pit, trench, or soakaway can absorb runoff. Low permeability can cause overflow, ponding, or slow emptying.
Which permeability unit does this calculator use?
The main result uses millimeters per hour. The calculator also converts lab test inputs into the same unit for easier design comparison.
What runoff coefficient should I use?
Use higher values for roofs and paved surfaces. Use lower values for rough, vegetated, or partly absorbent areas. Values must stay between 0 and 1.
What does safety factor mean?
The safety factor reduces the ideal soil rate. It allows for test uncertainty, future clogging, wet soil, and construction variation.
Can clay soil be used for recharge?
Clay soil often has very low permeability. Recharge may still be possible, but it usually needs larger area, more storage, and reliable overflow control.
What is drain time?
Drain time estimates how long stored rainwater may remain before it infiltrates. Shorter drain time usually means safer harvesting performance.
Should I verify results on site?
Yes. Always confirm with site testing, local rules, soil profile checks, and professional review before final construction.