Drip System in a Potted Chrysanthemum Unit: Head-to-Tail Pressure Gap
An entrepreneur in Kolhapur has 1,200 potted chrysanthemums in 20 rows, each 30 m long, irrigated by a drip system. The pump is 2 hp, the mainline is 25 mm, and each row has a 16 mm lateral with 40 button emitters rated at 2 L/h. The head pressure gauge reads 1.8 bar; a gauge at the far end of the longest lateral reads 0.6 bar. Plants near the head are dark green and in bud; plants at the far end are shorter, wilt by 11 a.m., and some have aborted flower buds. The tank is open and algae-coated, the sand filter is backwashed once a month, and the laterals have not been flushed for two months. The entrepreneur runs the system for 20 minutes every morning and believes more pressure means more water.
1. Explain why the plants at the far end are stressed. Use the pressure readings and describe a simple field test to confirm distribution uniformity.
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The far-end plants are under-irrigated because pressure is lost along the line and emitters are probably partly clogged. The drop from 1.8 bar at the head to 0.6 bar at the far end is far greater than the 10–20% variation normally accepted for uniform drip. Button emitters are non-pressure-compensating, so flow falls roughly with the square root of pressure. At 0.6 bar compared with 1.8 bar, flow is roughly 40% lower. Head plants receive more water; far-end plants receive less, wilt by 11 a.m. and abort buds. Algae and infrequent filter cleaning point to clogging, which increases friction loss and reduces flow further.
Field test. Run the system and place 12–16 identical catch cans under emitters at the head, middle and tail of three laterals. Run for 10 minutes and measure the volume in each can. Calculate distribution uniformity as the average of the lowest quarter of cans divided by the overall average, multiplied by 100. Below about 80% is poor. Measure pressure at the same points. This separates a hydraulic design problem from emitter clogging.
2. Propose a prioritised repair and redesign plan, including filtration, flushing and emitter choice. Give reasons for the order.
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- Clean the water source. Cover and clean the tank to stop algae entering the system. Add a 120-mesh screen filter after the sand filter. Backwash the sand filter when the pressure difference rises by about 0.3–0.5 bar, not on a fixed monthly schedule.
- Flush the system. Remove end caps and flush the mainline and laterals at high velocity until the water runs clear. Do this monthly and after any repair. For clogged emitters, soak in a dilute acid or replace them; do not poke them with wire, which damages the orifice.
- Fix the hydraulics. Shorten laterals or split each 30 m row into two shorter sections, increase the longest laterals from 16 mm to 20 mm, and add a pressure regulator. These steps reduce friction loss before changing emitters.
- Choose pressure-compensating emitters. Pressure-compensating emitters give uniform flow only within their stated pressure range, often about 0.5–4 bar. The far end at 0.6 bar is at the edge of that range, so hydraulic fixes must come first.
- Monitor. Install gauges at the head and far end, and keep a log of filter pressure and flush dates.
The order matters because filtration and flushing remove the cause of clogging, while hydraulic changes reduce the pressure gap. New emitters alone will not solve a system that is dirty and badly balanced.
3. Recommend a scheduling approach based on crop demand rather than a fixed 20 minutes. State what you would measure and how you would adjust for weather.
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Schedule by crop demand. Estimate daily water use from reference evapotranspiration (ETo) and a crop coefficient. For actively growing pot chrysanthemums, use a crop coefficient of about 0.8–1.0. Multiply ETo by the crop coefficient to get daily water use in mm, then convert to litres for the planted area or pot count.
Check with simple measurements. Weigh five representative pots before and after irrigation, or use a tensiometer or moisture meter at 10 cm depth. Measure leachate EC weekly to detect salt buildup. Use catch cans to check how much water actually reaches the far-end pots.
Example calculation. If 1,200 pots use about 0.5 L per pot per day, the crop needs about 600 L/day. Nominal system output is 20 rows × 40 emitters × 2 L/h = 1,600 L/h, but poor uniformity and pressure loss reduce the effective amount. Run longer and split irrigation into two morning cycles rather than one short cycle.
Weather adjustment. Increase run time on hot, windy days and reduce it on cloudy or rainy days. Irrigate early morning to reduce evaporation and foliar disease. Add mulch or group pots to reduce evaporation. Recheck the schedule after fixing the pressure problem so the calculation reflects the repaired system.