Water Table & Drain Before Waterlogging Cuts Yield
Computes SEW₃₀ stress index
A water table that rises into the root zone suffocates roots and cuts yield — enter your crop, the current table depth and how long it has been high to get the SEW₃₀ stress index, the yield loss and exactly how deep you must drain.
Enter your crop & water table
Next: act now: the table at 20 cm has been high for 5 days, giving SEW₃₀ 50 cm·day (Safe) and roughly 15.2% yield loss. Drain hard — lower the table by 100 cm to reach Maize's 120 cm target, using surface drains, subsurface drains or pumping.
SEW₃₀ = Σ over days of max(0, 30 cm − water-table depth in cm), the standard drainage-criteria stress index (Sieben 1964; Wesseling 1974). Yield loss is a saturating function of SEW₃₀ scaled by the crop's tolerance. Critical & target depths from FAO-29 / ICID drainage guidance. Planning values — confirm with a piezometer and local drainage data.
Waterlogging & drainage — key facts
- SEW₃₀
- Σ days of (30 cm − table depth)
- Safe SEW₃₀
- below ~100 cm·day
- Yield loss starts
- table above crop critical depth
- Rice critical depth
- 0 cm (needs standing water)
- Cotton critical depth
- ≈ 100 cm
- Sensitive crops drain to
- 110–150 cm
- Drawdown needed
- target depth − current table
- Twin risk
- shallow table → salinity
- Drainage methods
- surface · subsurface · pumping
- Privacy
- Runs in your browser; nothing uploaded
Crop waterlogging tolerance & drainage depths
Whether a high table hurts depends entirely on the crop. Tolerant crops shrug off a shallow table for weeks; sensitive crops lose yield within days. These FAO-29 / ICID values set each crop's critical depth (where loss begins), the days it tolerates a high table, and the target depth you should drain to.
| Crop | Tolerance | Critical depth (cm) | Target drained (cm) | Tolerance days | Max loss |
|---|---|---|---|---|---|
| Rice (paddy) | tolerant | 0 | 30 | 30 | 35% |
| Sugarcane | tolerant | 60 | 100 | 12 | 55% |
| Alfalfa | sensitive | 120 | 150 | 3 | 80% |
| Wheat | moderate | 80 | 110 | 7 | 60% |
| Maize | sensitive | 90 | 120 | 4 | 70% |
| Cotton | sensitive | 100 | 130 | 3 | 75% |
| Soybean | moderate | 80 | 110 | 5 | 60% |
| Sorghum | moderate | 75 | 100 | 6 | 55% |
| Potato | sensitive | 100 | 130 | 3 | 75% |
| Tomato | sensitive | 90 | 120 | 3 | 75% |
| Pulses (gram) | sensitive | 100 | 130 | 2 | 85% |
| Groundnut | moderate | 80 | 110 | 5 | 60% |
Sources: SEW₃₀ index — Sieben (1964), Wesseling (1974); FAO-29 "Drainage of irrigated lands"; ICID drainage guidelines; extension waterlogging yield-loss studies. Planning values — confirm with a piezometer and local drainage data.
How a high water table cuts yield
Roots breathe. They need air in the soil pores to respire, take up water and nutrients, and grow. When the water table rises into the root zone, those pores fill with water and the oxygen is gone within hours. Roots stop growing, nitrogen uptake collapses, and after a few days toxic compounds and disease pressure build in the anaerobic soil. The damage depends on two things together: how shallow the table is, and how long it stays there — which is exactly what the SEW₃₀ index measures by summing the excess water above 30 cm over each day.
That is why a drainage decision needs both numbers. A table at 15 cm for two days is a scare; the same table for ten days is a serious loss. This calculator combines your water-table depth and the days it has been high into one SEW₃₀ figure, compares it to the crop's tolerance, estimates the yield loss, and then tells you the target depth to drain to and how far the table must be lowered. A shallow table also feeds capillary rise and surface salt accumulation, so draining protects against waterlogging and secondary salinity at once.
How to use it
- 1Choose your crop to load its critical depth, tolerance days and target drained depth.
- 2Enter the current water-table depth below the surface in centimetres (from a piezometer or observation well).
- 3Enter how many days the table has been at that high level.
- 4Read the SEW₃₀ stress index, the stress band and the estimated yield loss.
- 5If drainage is needed, lower the table by the drawdown shown to reach the crop's target drained depth.
Frequently Asked Questions
What is the SEW₃₀ index and how is it calculated?+
SEW₃₀ (Sum of Excess Water above 30 cm) is the standard drainage-criteria stress index: SEW₃₀ = the sum over days of (30 cm − water-table depth in cm), counting only days when the table is shallower than 30 cm. A table sitting at 10 cm for 5 days gives (30−10)×5 = 100 cm·day. Crops generally tolerate up to roughly 100–200 cm·day before yield declines noticeably.
How shallow can the water table be before my crop loses yield?+
It depends on the crop's critical water-table depth — the depth above which waterlogging begins. Rice tolerates a table at the surface; sensitive crops like cotton, maize, potato and pulses need the table 90–100 cm or deeper. This tool stores each crop's critical depth and warns you when the current table is shallower than it.
How much yield will waterlogging cost me?+
The calculator estimates yield loss from the SEW₃₀ index using a saturating curve scaled by the crop's tolerance: loss rises fast at first, then plateaus at a crop-specific maximum (about 35% for rice, up to 75–85% for sensitive crops under prolonged flooding). For example, maize with SEW₃₀ of 100 cm·day loses roughly 25% of yield.
Which crops are most sensitive to a high water table?+
Sensitive crops include cotton, maize, potato, tomato, pulses and alfalfa — they suffer within 2–4 days of a high table and need it drained to 110–150 cm. Moderately tolerant crops include wheat, soybean, sorghum and groundnut. Rice and sugarcane are tolerant; rice in fact requires standing water. The tool labels each crop's tolerance class.
How deep should I drain my field?+
Drain to the crop's target drained depth — the design depth that keeps the table safely below the root zone: about 30 cm for rice, 100 cm for sugarcane, and 110–150 cm for sensitive crops. The tool reports the target and the drawdown needed = target depth − current table depth, so you know how far to lower the table.
Why does a high water table reduce yield?+
A shallow table saturates the root zone, displacing soil air. Roots need oxygen to respire and take up water and nutrients; under saturation they suffocate, root growth stops, nutrient uptake (especially nitrogen) collapses, and toxic compounds build up. The longer and shallower the saturation, the worse the damage — which is exactly what SEW₃₀ captures by combining depth and duration.
Does the number of days at a high table matter, or just the depth?+
Both — that's why SEW₃₀ multiplies the excess depth by the days. A table at 15 cm for two days (SEW₃₀ 30) is far less damaging than the same depth for ten days (SEW₃₀ 150). Crops have a tolerance-days window; beyond it, loss accelerates. The tool combines your depth and days into a single SEW₃₀ figure and verdict.
What's the difference between critical depth and target drained depth?+
Critical depth is where yield loss begins — the table must stay below it during the season. Target drained depth is the deeper, design figure a drainage system aims for, giving a safety margin below the critical depth so the table doesn't bounce back into the root zone after rain or irrigation. The tool reports both for your crop.
Is a shallow water table linked to soil salinity?+
Yes. A high table feeds capillary rise to the surface, where evaporation concentrates dissolved salts in the root zone — secondary salinisation. This is a common twin problem in canal-irrigated areas: waterlogging and salinity together. Lowering the table by drainage tackles both, which is why drainage criteria target keeping the table well below the root zone.
How do I lower a high water table?+
Three main methods: surface drains (open ditches) to remove ponded water fast; subsurface drains (buried perforated pipes or tile) to draw the table down across the field; and vertical drainage (pumping from tubewells) where an aquifer allows. The right choice depends on soil, the drawdown needed and the cost. This tool tells you how much drawdown the crop requires.
Is SEW₃₀ of 150 cm·day bad for cotton?+
Yes — cotton is sensitive, with a critical depth around 100 cm and a low SEW₃₀ tolerance, so 150 cm·day already puts it in the high-stress band with a meaningful yield loss. The tool flags this and recommends draining now: lower the table to cotton's 130 cm target drained depth before the loss compounds.