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Water efficiency vital

27 July 2011

AHDB Potatoes is urging growers to maximise water use efficiency after one of the driest springs for over a century (Met Office). Parts of the Anglian region are officially in a state of drought recently declared by Defra and the Environment Agency.

East Anglia had its driest spring on record, with just 21% of average rainfall, while the south east fared little better with 30%.

Areas in the south west, south east, the Midlands and Wales are also experiencing near-drought conditions. Met Office figures show spring rainfall across England and Wales averaged 86.9 mm, just 45% of the long-term average and only 2mm more than 1990, the driest spring for 101 years.

Although some welcome rain has fallen recently in those areas, it was mostly as showers. While this helped river flows increase, these receded quickly in the second week of June as dry weather returned.

The Environment Agency declared drought orders in Cambridgeshire, Lincolnshire, parts of Bedfordshire and Northamptonshire and west Norfolk on June 10. Parts of the south-west and south-east of England, the Midlands and Wales are said to be in a near-drought state.

The agency is now working with water companies in the drought areas to prioritise water abstraction, with industrial and farming users all competing for supplies.

This follows voluntary restrictions on watercourses through the central Fens, affecting about 140 licences. Farmers have been irrigating during the night to reduce water lost to evaporation.

Unless prolonged rainfall occurs soon restrictions could spread.

The agency is monitoring water resources around Romney Marsh in Kent, where irrigation got off to an exceptionally early start. And some licence holders across parts of England and Wales have had to stop abstracting four to six weeks earlier than normal as a result of conditions on their licences, which are linked to river flows.

“People who planted late might run short of water for scab control,” says AHDB Potatoes water specialist Chris Steele. “But the bigger concern is how irrigation will be affected in July and August when growers need to bulk up crops.”

He advises growers to check their abstraction licenses to understand their entitlement and assess the balance of water that they have available to use for irrigating their crops. They need to keep in close contact with their local Environment Agency representative to establish the local situation.

With the declaration of drought the following best practice guidelines may help eke out potentially dwindling abstraction supplies this summer. 

These include:

• Monitoring soil moisture to assess crop water requirements
• Irrigating overnight to minimise water loss through evaporation
• Ensuring irrigation pressure is correct – too low a pressure creates big droplets which are more prone to run off (and increase soils’ erosion risk); too high causes small droplets, which leads to misting and drift
• Checking rain guns are at the correct angle to avoid drift and ensure even application – no under- or over-lapping
• Maintaining boom nozzles to ensure correct water pattern and even application
• Measuring water abstraction rates and recording them
• Irrigating overnight to minimise water loss through evaporation.

 “In places where water becomes really short, sticking to these guidelines could make the difference between a local restriction on abstraction and an outright ban,” says Chris.

Irrigation schedules
The amount of irrigation that should be applied at any one time is limited both by the current soil moisture deficit (SMD) and the infiltration rate of the soil.

For example, if irrigation is to be applied at an SMD of 25mm there is no benefit from applying more than 25mm of water. It is usually sensible to take rainfall into account and apply say 20mm so that there is spare capacity in the soil should rainfall occur. Typical irrigation scheduling for three scenarios is shown below.

Estima variety for pre-packing grown on a silt loam with a history of common scab

  10% crop cover to 6 weeks after Tuber initiation (TI) 6 weeks after TI to mid-August Mid-August to desiccation
Typical schedule 10mm @ 12mm SMD 25mm @40mm SMD 20mm @50mm SMD

Lady Rosetta grown on a loamy sand for crisping off-the-field in late August

  20% Crop cover to 4-5 weeks after TI 4-5 weeks after TI to late harvest  
Typical Schedule 20mm @ 25mm SMD 25mm @ 35 mm SMD  

Maris Piper for chips grown on sandy loam destined for storage

  15% crop cover to 4 – 5 weeks after TI 4-5 weeks after TI to late August Late August to desiccation
Typical Schedule 15mm @ 15-18mm SMD 25-30mm @35-40mm SMD 18mm @35mm SMD

More information on best practice for irrigation click here.

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