Use these sample scenarios to understand typical inputs and outputs.
| Scenario | Area (m²) | Rain (mm) | Slope (%) | Soil | Cover | Impervious (%) | Expected risk |
|---|---|---|---|---|---|---|---|
| Mulched beds on gentle slope | 30 | 20 | 3 | B | Mulched beds | 5 | Low to Moderate |
| Garden beds after heavy watering | 60 | 35 | 6 | C | Garden beds | 10 | Moderate to High |
| Bare soil on steeper patch | 40 | 25 | 12 | D | Bare soil | 0 | High to Very High |
| Path runoff draining to beds | 80 | 30 | 4 | B | Gravel path | 35 | High |
Real outcomes vary with soil structure, mulch depth, and storm intensity.
Runoff depth is computed from event depth and a curve number.
- S = (1000/CN − 10) inches
- Ia = 0.2 × S
- If P ≤ Ia → Q = 0
- Else Q = (P − Ia)² / (P + 0.8S)
This tool converts mm↔inches internally.
Runoff depth approximates as a fraction of event depth.
- Runoff depth: Q ≈ C × P
- Intensity: i = P / (duration in hours)
- Peak flow: Qₚ = 0.00278 × C × i × A(ha)
Useful for comparing changes in slope and surface cover.
Runoff volume: Liters = runoff depth (mm) × area (m²).
- Estimate your drainage area in square meters.
- Enter rainfall or irrigation depth for the event window.
- Select slope, soil group, cover type, and condition.
- Choose moisture based on recent rain or watering.
- Press Calculate to view results above the form.
- Use CSV or PDF export to save the outcome.
For best results, measure slope and confirm soil group using a local guide.
Rain depth and intensity inputs
Event depth is the total water applied in the chosen window. Intensity matters: 30 mm in 60 minutes equals 30 mm/hr, and short bursts often overwhelm compacted soil first. If you irrigate, try two cycles with a soak period; spreading the same depth across time can reduce surface flow and improve root-zone storage. For storms, use measured gauge totals when available.
Soil group and infiltration behavior
Soil groups A through D reflect infiltration potential, from fast to very slow. Sandy, well-structured soil can absorb higher rates, while clayey or compacted areas may pond quickly. If uncertain, start with Group B and compare C. Choose Wet moisture after rain within 48 hours for conservative results. Adding organic matter and reducing traffic typically improves infiltration over weeks to months.
Cover, compaction, and curve selection
Cover type and condition adjust runoff potential in both methods. Mulch and dense vegetation protect aggregates, slow overland flow, and create macropores. Bare soil and compacted paths increase sealing and runoff. A practical field target is 50–80 mm mulch in exposed beds, plus consistent groundcover on edges and walkways. “Good” condition implies low crusting and limited compaction.
Slope and connected hard surfaces
Slope increases flow speed and reduces infiltration time. Around 2% is gentle, 5–10% is moderate, and above 10% often needs contour beds or terraces. Impervious areas matter most when connected; even 20% hard surface draining to one outlet can dominate runoff. Break long slopes with small berms so water slows and infiltrates. Disconnect downspouts and spread flow into infiltration zones.
Interpreting outputs for garden protection
The calculator reports runoff depth (mm), volume (L), runoff share, and a 0–100 risk score. Remember: 1 mm over 50 m² equals 50 liters. If runoff share exceeds 40% or risk is High, prioritize mulch, buffers, and split watering. Moderate scores still merit inspections after storms for rills and sediment. This reduces nutrient losses from beds. Save CSV/PDF results, then rerun after improvements to compare scenarios.
Q1: Which method should I use for a home garden?
A: Use the Curve Number method to estimate runoff depth from a single event. Use the Coefficient method when you also want a quick peak flow estimate and are comparing intensity across different durations.
Q2: What if I do not know my soil group?
A: Start with Group B for loamy soils, then compare results for A and C to see sensitivity. If your soil puddles, stays wet, or is heavily compacted, Group C or D may be closer.
Q3: How does “Antecedent moisture” change results?
A: Wet conditions increase runoff because pores are already filled. Dry conditions reduce runoff because more storage is available. Choose “Wet” after several rainy days or heavy irrigation within 24–48 hours.
Q4: How accurate is the runoff volume in liters?
A: Volume is derived from depth and area using 1 mm over 1 m² = 1 liter. Accuracy depends on input realism and site complexity. Use it as a planning estimate, not a measured discharge value.
Q5: Why can small hard areas raise risk so much?
A: Hard surfaces shed water quickly and often concentrate flow to one point. When connected to a downslope bed, that concentrated runoff can exceed infiltration and cause rills, splash erosion, and nutrient transport.
Q6: What is a practical target to lower risk?
A: Aim to keep runoff share below 20–30% for routine storms by improving cover, loosening compaction, and spreading inflow. For higher slopes, add terraces or swales so water slows and infiltrates.