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Is there a good chance of rain?

Is There A Good Chance of Rain?

If it is going to rain tomorrow, you really don't need to irrigate today. And if the ground is already wet and it is going to rain tomorrow, you may need to drain your fields today.

Obvious enough - but how do you figure your odds of the weather forecast being right in advance of tomorrow's rain against a crop's need for just the right amount of water?

Integrating all the data and coming up with a "drain, irrigate, or hold everything" decision is the job the RPI computer model was designed for by agricultural engineer James L. Fouss.

RPI, which stands for Rainfall Probability Index, works from a statistical average of daily rainfall predictions for the forthcoming 36 hours - predictions for that day, that night, and the next day.

It is designed to provide an alternative to managing water tables in the soil based solely on the current depth of the water. Decisions that are based on water table depth alone could lead to draining water that would be needed the next day or irrigating when water will be surplus the next day, according to Fouss.

"The purpose of RPI is twofold: to maximize water storage in the soil by not draining unnecessarily and minimize the time there is too much water or too little water in the root zone," says Fouss.

"If a farmer drains a field today and it doesn't rain, or if a farmer doesn't drain and it does rain and this happens just a few times during the growing season, it can significantly cut crop yields," he adds.

In the future, the RPI concept might have an even more important role. It may help a farmer decide when to apply pesticides and fertilizers in order to maximize their effectiveness and minimize their losses in runoff or through leaching.

Fouss and his colleagues are currently working on an integrated model system with companion field experiments for controlling water tables and applying pesticides and fertilizer.

RPI needs to be run by agencies such as the National Weather Service Regional Centers.

"Individual farmers usually don't have the type of computer or the necessary data to use the model at home," says Fouss. "But it would be fairly easy for an organization like the National Weather Service to add the RPI figure to the regional agricultural weather forecasts."

An RPI of 80 percent or above should trigger draining. An RPI of 50 to 80 percent means delay irrigation, and an RPI of less than 40 percent means irrigation would be worthwhile to increase moisture in the field.

He has discussed the possibility of the Weather Service trying out RPI; they would like to see more field testing first.

Fouss will be using the RPI to guide irrigation and drainage decisions on the fields managed by the ARS Soil and Water Research unit in Baton Rouge, Louisiana, where he is located.

The data will come from the Southern Regional Climate Center of the National Weather Service, so they will be able to see how RPI could fit in with their regional forecasts.

In order to be accurate enough, RPI has to have separate regional versions to take into account rainfall patterns and soil types. Fouss has already formulated RPI models for southern Louisiana, North Carolina, central Ohio, and northern Ohio.

About the southern Louisiana model, Fouss says, "In 3 years out of 6 that we looked at, letting the RPI guide irrigation and drainage would have increased the yield of corn."

The yield increases predicted by the model would have been 14, 82, and 25 percent in 1980, 1982, and 1984, respectively. Possibly even more important, the irrigation water required would have been reduced by 6, 32, 24, and 27 percent in 1981, 1982, 1984, and 1985.

The RPI would be particularly convenient for farmers who use a computer to control their drainage/irrigation systems. It can be integrated with their programs so that the computer automatically overrides responses to the water table depth if RPI has given a counter-indication. - by J. Kim Kaplan, ARS.
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Title Annotation:computer model Rainfall Probability Index helps farmers decide when to irrigate and when to drain fields
Author:Kaplan, J. Kim
Publication:Agricultural Research
Date:Jan 1, 1991
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