7 February 1997

Get the scheduling spot on with the right model

This weeks Water Watch article considers irrigation scheduling. Recent research shows forecasting can be just as accurate as in-field soil measurements. Charles Abel reports

NEW work conducted at ADAS Gleadthorpe confirms the accuracy of the computerised Irriguide irrigation scheduling service. But farmers still need to supply the right information for best results.

"Put garbage into the system and youll get just the same out," says Simon Groves, research consultant at ADAS Gleadthorpe.

Trials in 1992/93/94 compared advice from Irriguide with actual measurements of soil moisture content in trial plots of both potatoes and sugar beet. "The results show that in dry conditions Irriguide was extremely accurate and within less than 10% of the true soil moisture content," explains Roger Bailey, who led the work.

The trials used neutron probes to check the true moisture content of the soil. However, these were no ordinary neutron probes, Mr Groves stresses. "We calibrated them very carefully, to suit the local soil type and we used them with scientific precision."

With potatoes the objective was to apply water when Irriguide indicated a 35mm soil moisture deficit. Sugar beet irrigation followed a typical limiting deficit regime. Just before each application the true soil moisture deficit was checked.

The results (see graph) show how closely the forecast soil moisture deficit from Irriguide matched the true soil moisture deficits measured by the neutron probe. "It was a very good match," says Mr Groves.

"Only in wet years were we getting some inaccuracy, because the modelling assumes all soils are free draining and any excess water drains away within 24 hours," Dr Bailey comments. "In reality there are many factors which can keep water in the profile for longer."

But that does not mean Irriguide advice should be considered over-cautious, Dr Bailey stresses. "In dry conditions it is virtually spot on. It is certainly as good as any other irrigation model.

Assessing how much irrigation water has gone onto the crop is also important. Rain gauges sited in the field are ideal. It is also important to ensure equipment is set up and used properly so that water is applied uniformly.

If that is not done the desired amount of water may not be applied, disrupting Irriguides modelling. "It is something farmers are reluctant to do, but the time would be more than justified," Mr Groves asserts.

Inaccurate application can be particularly troublesome where probes are being used, Mr Groves points out. Variable application can mean probe data does not represent the entire crop.

For 1997 Irriguide gets a facelift. The program now runs on individual consultants PCs, producing user-friendly reports with graphic representations of SMD. It is also in a format where new scientific findings can easily be added to update the system. &#42

Accurate soil moisture measurements (yellow dots) mirror Irriguides forecast soil moisture deficits (line), confirming its value as an irrigation scheduling tool, says Simon Groves of ADAS Gleadthorpe (below left).


IRRIGUIDE


&#8226 25mm of lost transpiration cuts potato yield 6t/ha.

&#8226 Model v close to true soil conditions in dry seasons.

&#8226 Add canopy development and rainfall for more accuracy.

&#8226 Check field application rates to avoid errors.


What is irriguide?

Irriguide uses detailed Met Office data to model the loss of water from crops. All the farmer need provide is:

&#8226 field location

&#8226 crop type

&#8226 soil type

&#8226 planting and emergence dates

&#8226 amount and date of irrigation applied

&#8226 local rainfall if possible.

The model predicts canopy development, and root growth, explains Mr Groves. "This is important, because we only need to know the dryness of the soil in the rooting zone, not the entire profile."

The rate of water loss from the crop is assessed using Met Office data, including temperature, rainfall, humidity and sunlight. Adding crop cover details during the season improves accuracy, by accounting for any growth checks due to frost, spray application or more rapid crop growth.

The computer calculates changes in the soil moisture deficit and at a trigger deficit advises application. It can also predict the date of irrigation using forecasts of imminent rainfall or dry conditions. That can avoid watering before rain, which not only wastes a valuable resource, but can also leach crop nutrients and incur a crop growth check.