Allocating scarce well water before maize tasselling
A farmer has 2 ha of maize on sandy loam with a 3 L/s tube well that runs 8 hours a day, giving about 86 m³/day for the farm. Maize is at 45 days after sowing, tasselling begins in 10 days. Crop evapotranspiration is 6 mm/day; effective rainfall is negligible. The soil's available water capacity is 120 mm/m, root zone 0.6 m, and current depletion is 55%. The farmer also has 0.5 ha of mustard at 30 days after sowing and a 1 ha fodder sorghum plot. He can irrigate only one plot per day and has 10 days before tasselling. He asks whether to skip the next maize irrigation to save water for mustard.
1. Calculate the maize irrigation requirement and allocate the well water over the next 10 days among maize, mustard and fodder sorghum. Show your reasoning.
Read the worked answer
Maize water balance: root-zone available water = 0.6 m × 120 mm/m = 72 mm. At 55% depletion, the deficit is 0.55 × 72 = 39.6 mm. At 6 mm/day, the remaining 32.4 mm would last about 5.4 days, so irrigation is due now.
Volume needed: 39.6 mm over 2 ha = 39.6 × 10 m³/ha/mm × 2 = 792 m³. The well supplies 3 L/s × 8 h × 3600 s = 86.4 m³/day. Time to deliver 792 m³ = 9.2 days. In other words, maize alone needs nearly the full 10-day output just to refill the root zone to field capacity.
Allocation: do not skip the maize irrigation. Tasselling is the most drought-sensitive stage; stress then can reduce yield by 30-50%. Irrigate maize for about 9 days, then use the remaining 0.8 day (about 69 m³) on the most stressed part of the mustard, not the whole 0.5 ha. Mustard at 30 days is vegetative and can tolerate mild stress better than maize at tasselling. Fodder sorghum is the most expendable and can be harvested early if necessary. If the farmer insists on spreading water, apply 30 mm to maize (600 m³ in 7 days) and 30 mm to mustard (150 m³ in 2 days), but this leaves maize 10 mm short at flowering and risks yield loss.
2. If the well yields only 70 m³/day, what deficit-irrigation strategy would you use?
Read the worked answer
At 70 m³/day for 10 days, the available volume is 700 m³, or 88% of the maize requirement of 792 m³. The maize deficit would be 700/(2 × 10) = 35 mm instead of 39.6 mm, leaving a shortfall of about 4.6 mm, roughly 0.8 day of evapotranspiration. That is a mild deficit if it is managed.
Strategy:
- Prioritise maize: apply the 700 m³ to the 2 ha in alternate furrows and close the furrows after irrigation. Mulch with crop residue to cut evaporation. Irrigate in the evening to reduce losses.
- Do not spread water thinly: wetting the whole surface with a shallow layer is less useful than wetting the root zone of the most sensitive crop. A 35 mm application refills most of the deficit.
- Delay mustard: at 30 days it can wait until pre-flowering; a mild stress now is less damaging than stress to maize at tasselling. If mustard shows wilting, give it a small 20-25 mm irrigation after the maize is secured.
- Sacrifice fodder sorghum first: harvest it early for green fodder rather than irrigate it.
- Plan next season: reduce maize area or install a water-harvesting structure; the well cannot support 2 ha of maize plus 1.5 ha of other crops in a dry spell.