Farm Drainage Discharge Calculator
Enter the drainage coefficient (mm/day) and farm area (ha) to compute daily discharge Q = coefficient × area × 10 (m³/day) and L/s for drainage design.
輸入資料
計算結果
重點速覽：Farm drainage discharge is the excess volume a field or catchment must shed per unit time — the core number for sizing drains, perforated pipes, collectors and pumps. Farmland needs drainage because a saturated root zone (waterlogging) drives out air, starving roots and microbes of oxygen, stunting growth or causing rot; in arid/semi-arid irrigated areas poor drainage raises the water table and brings salts to the surface (salinisation). The design flow comes from two factors: the drainage coefficient (mm/day, the drainage intensity needed) and the farm area. 1 mm over 1 ha = 10 m³, so Q(m³/day) = coefficient(mm/day) × area(ha) × 10, convertible to L/s by ×1000 ÷ 86400. Example: coefficient 12.5 mm/day, area 20 ha → Q = 12.5 × 20 × 10 = 2,500 m³/day ≈ 28.9 L/s.
計算公式
Daily discharge: Q (m³/day) = coefficient (mm/day) × area (ha) × 10.
Conversion: 1 mm over 1 ha = 10 m³; factor is 10.
Flow: Q (L/s) = Q (m³/day) × 1000 ÷ 86400.
$$Q = DC \times A \times 10 \;(\text{m}^3/\text{day})$$$$Q_{L/s} = \dfrac{Q \times 1000}{86400}$$使用說明
- Set the drainage coefficient (mm/day) from crop tolerance, soil and design rainfall.
- Enter the farm / catchment area to drain (hectares).
- The tool returns daily discharge (m³/day) and flow (L/s) for channels, pipes and pump design.
Farm drainage discharge examples
| Item | Value | Note |
|---|---|---|
| Drainage coefficient | 12.5 mm/day | Moderately tolerant crop |
| Farm area | 20 ha | Design catchment |
| Conversion factor | ×10 | 1 mm × 1 ha = 10 m³ |
| Daily discharge | 2,500 m³/day | = 12.5 × 20 × 10 |
| Discharge flow | ≈ 28.9 L/s | = 2500×1000/86400 |
Coefficient depends on crop, soil and design rainfall; peak instantaneous flow may exceed the daily mean — add a safety factor.
理財情境案例
Design flow for a 20 ha field
Vegetable field, coefficient 12.5 mm/day, area 20 ha.
Q = 12.5 × 20 × 10 = 2,500 m³/day ≈ 28.9 L/s.
Size the main drain section by slope and roughness, or pick a pump matching this flow.
Sensitive crop raises the coefficient
Same 20 ha, switched to waterlogging-sensitive fruit trees, coefficient raised to 20 mm/day.
Q = 20 × 20 × 10 = 4,000 m³/day (≈ 46.3 L/s), 60% more.
Shows crop tolerance directly drives the coefficient and works scale — confirm crop and code first.
常見問題
How is the drainage coefficient decided?
The drainage coefficient (daily depth to remove per unit area, mm/day) reflects crop tolerance, soil permeability, design rainfall (what return-period storm must be drained) and allowable ponding time, per local farmland drainage design codes. Fields commonly use 10–25 mm/day: tolerant crops like rice take lower values, waterlogging-sensitive vegetables and fruit trees take higher; rainy areas and impermeable clay also take higher. Too low floods in storms; too high wastes works — set it per code.
Why multiply by 10?
The coefficient is a depth (mm) and area is in hectares (ha); their product must become volume (m³). 1 mm = 0.001 m, 1 ha = 10,000 m², so 1 mm over 1 ha = 0.001 × 10,000 = 10 m³. Hence each 1 mm/day × 1 ha = 10 m³/day, and the factor 10 in Q(m³/day) = coefficient(mm/day) × area(ha) × 10 is exactly this unit conversion. Using m² or metres changes the factor — keep units consistent.
How is the discharge used in design?
The daily discharge (m³/day) and flow (L/s) are the design flow for the drainage system: size open-channel sections and slopes (e.g. via Manning's formula), pick perforated-pipe diameter and spacing, size pump capacity for low/poldered land, and cascade多级 systems (field → lateral → main → outlet) accumulating area per reach. Add a safety factor for above-design rainfall.
Is daily discharge the same as peak flow?
No. This formula gives a steady, uniform design daily discharge — spreading the daily total evenly. Real storm drainage is unsteady; a heavy burst concentrates inflow and the instantaneous peak can exceed the daily mean. For safe passage without overflow, use rainfall–runoff–drainage simulation (curve number, unit hydrograph) or amplify the daily flow with a safety factor. This tool suits preliminary estimation and scaling.
How does drainage relate to irrigation demand?
They are two sides of farmland water management: irrigation adds water to the root zone in deficit, drainage removes excess to keep good aeration and water table. Dry periods rely on irrigation (see irrigation water calculator); rainy periods or over-irrigation rely on drainage to prevent waterlogging. In irrigated areas drainage also controls the water table and leaches salts. Plan both (irrigation and drainage) together.
相關工具
參考資料
Content reviewed by the Hong Kong Calculator science team. Q(m³/day) = drainage coefficient(mm/day) × area(ha) × 10.