Irrigation, Water Management and Watershed Practices
This domain tests irrigation scheduling and method selection, plus watershed treatment from ridge to valley.
Core rules
- Water requirement = evapotranspiration + percolation + conveyance losses. Delta is the depth of water needed over the whole season, expressed in cm. Duty is the area irrigated per unit discharge, expressed in ha per cumec.
- Scheduling: irrigate when the ratio of irrigation water to cumulative pan evaporation (IW/CPE) reaches about 0.8–1.0. Soil moisture between field capacity and permanent wilting point is the available water range.
- Critical stages for irrigation: wheat - crown root initiation; paddy - panicle initiation; cotton - flowering and boll formation; groundnut - flowering and pegging; onion - bulb formation; sugarcane - formative stage.
- Methods: flood, furrow, basin and border (surface); sprinkler (undulating land, light soils, about 60–70 per cent efficient); drip (about 90 per cent efficient, 40–60 per cent water saving, suited to orchards and wide-row crops, allows fertigation).
- Watershed treatment ridge to valley: contour trenches and afforestation on the ridge, contour and graded bunds on the slope, gully plugs and check dams in the valley, plus a farm pond for storage.
Worked example
Cumulative pan evaporation averages 5 mm per day and the irrigation water is 40 mm. With an IW/CPE ratio of 0.8, when is the next irrigation due? Reasoning: irrigate when cumulative evaporation reaches IW ÷ 0.8 = 40 ÷ 0.8 = 50 mm. At 5 mm per day, the interval = 50 ÷ 5 = 10 days. If the crop is at a critical stage, irrigate slightly earlier.
Traps that cost marks
- Swapping the definitions of duty and delta.
- Claiming flood irrigation is most efficient.
- Ignoring drainage; waterlogging destroys aeration and reduces yield.
- Treating the whole watershed with one structure instead of ridge-to-valley planning.