A Limited Well and a Fixed Canal Turn: Allocating Water Between Rice and Maize
Anil has 0.7 ha of transplanted rice (90-day variety) at panicle initiation and 0.3 ha of maize at tasseling. His source is a dug well that recharges about 55 m3/day, plus a canal turn of 4 hours every 7 days; the pump delivers 20 m3/h. The rice field is puddled sandy loam with percolation of 6 mm/day; current crop evapotranspiration is 5 mm/day for rice and 5.5 mm/day for maize. The farmer keeps 4 cm of standing water in rice and irrigates maize every 10 days with 40 mm. He wants to continue both crops without buying water. His pump can run only 3 hours per day because of electricity supply. The next 10 days are forecast to be hot and dry. The well has shown no salt problem, but the canal turn is fixed and cannot be increased. Anil has no mulch and no storage tank.
1. Calculate the daily water demand of both crops at the stated rates and compare it with the sustainable supply. Show the deficit. Which crop is at greater risk in the next 10 days, and why?
Read the worked answer
Rice: 0.7 ha = 7,000 m2; 5 mm ET + 6 mm percolation = 11 mm/day = 0.011 m × 7,000 = 77 m3/day. Maize: 0.3 ha = 3,000 m2; 5.5 mm/day = 0.0055 × 3,000 = 16.5 m3/day. Total = 93.5 m3/day. Sustainable supply = 55 m3/day well + (80 m3 per 7 days)/7 = 11.4 m3/day canal = 66.4 m3/day. Deficit = 27.1 m3/day, or 271 m3 over 10 days. The pump's 3 h/day at 20 m3/h = 60 m3/day is not the limiting factor; the well recharge is. Rice is at greater absolute risk because it is at panicle initiation and uses 77 of the 93.5 m3/day; maize is also at a critical tasseling stage, but its demand is smaller. Neither crop can be kept at the current allocation.
2. Prepare a 10-day allocation plan. Include pump hours, canal water use, AWD or other water-saving measures, and how you would protect the most sensitive stages. State assumptions.
Read the worked answer
10-day plan. Target rice at 7.5 mm/day (5 mm ET + 2.5 mm percolation) = 52.5 m3/day, and maize at 4.5 mm/day = 13.5 m3/day; total 66 m3/day, matching supply. Practice alternate wetting and drying (AWD) on rice: allow the surface to dry to saturation but not crack; keep a thin layer only at panicle initiation. Use the canal turn (80 m3) on the rice field and spread it over the most sensitive block. Run the well pump about 2.5-2.75 h/day (50-55 m3), prioritizing maize every 3-4 days (about 40 m3 per application) and the rest to rice. Mulch maize between rows to cut ET by about 10-15%. If the pump cannot run on a day, skip maize, not rice at panicle initiation. Measure soil moisture by hand before each irrigation; do not irrigate by calendar. This plan assumes no rainfall and no seepage loss; if rain occurs, store the saved water for the next turn.
3. What long-term changes would reduce the risk of a repeat? Consider crop choice, land leveling, mulching, and irrigation method.
Read the worked answer
Laser-level the fields and reduce the rice area or switch part of the land to a shorter-duration/drought-tolerant crop; the current 0.7 ha rice is too large for the well. Convert maize to furrow or sprinkler irrigation and mulch; install a farm pond to capture canal and monsoon runoff; adopt AWD and direct-seeded rice where suitable; keep a drought-tolerant contingency crop; and maintain a 10-day water budget before sowing. Do not sink another shallow well unless a recharge test shows at least 55 m3/day.