Water Table & Drain Before Waterlogging Cuts Yield

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.