Once upon a time, farming was a blissfully low-tech business on Colombia's northern plains.

The lush tropical climate allowed rice farmers to harvest two or three times a year and protect their crops using little more than scarecrows and slingshots to fend off hungry birds.

But recently, climate change has been sowing chaos in their fields.

"The weather has gone completely crazy," said 46-year-old farmer Oscar Perez.

"We live in a constant state of uncertainty with these nerve-wracking questions: what to do, what to plant, what direction to take?"

In less than a decade, the average minimum temperature for the region has risen three degrees Celsius, average relative humidity has shot up to a steamy 85 percent and rains have become increasingly erratic, alternating between deluge and drought.

All that has taken a heavy toll on farmers.

Rice yields in Colombia have fallen from six tonnes per hectare to five over the past five years because of climate fluctuations, according to national rice-growers' federation Fedearroz, which has 12,000 members.

- 'Race against clock' -

Decreasing yields is a country-wide problem for Colombia, which has 450,000 hectares of rice fields and more than 200 towns and villages where growing rice is the main economic activity.

The slide in harvests comes at a delicate time.

Under a trade deal with the United States, rice tariffs will be removed in five years in Colombia, a net rice importer.

That means the country is likely to be flooded by cheap rice grown in the United States, where production costs are nearly half as much as they are in Colombia.

"We have this race against the clock, and climate change is accelerating it," said Patricia Guzman, technical director at Fedearroz.

Alarmed, two years ago the local office of the Consultative Group on International Agricultural Research, which sponsors research to improve farming practices worldwide, began analyzing crop-related data in Colombia on an unprecedented scale.

From weather forecasts to soil studies to solar radiation measurements, the group churned through numbers with cutting-edge software in a project dubbed "Big Data."

It came up with a list of highly localized recommendations for farmers, particularly on the ideal windows for planting.

"Farmers can be reluctant to change the way they traditionally do things, especially when someone from the city comes and tells them what to do. But with climate change they've lost their bearings so they're in distress," said Sylvian Delerce, a French researcher who developed the project.

- One harvest a year -

One key recommendation that has changed farmers' lives is cutting back to just one harvest a year to avoid over-working the soil.

Another is avoiding planting rice during the first half of the year, when a bacteria that decimates rice paddies has begun to fester because of high temperatures and the prevalence of single-crop farming.

"For a lot of farmers it's been really hard to accept harvesting just once a year," said Alfonso Blanco, a strapping 55-year-old who runs a 600-hectare farm outside the northern city of Monteria and planted maize instead of rice in the first half of the year under the group's advice.

Fedearroz agronomist Cristo Perez has begun bringing skeptical farmers to the organization's research fields to see the atrophied, brown-stained shoots of first-semester rice for themselves.

"Look at the damage, I tell them. You're going to lose your harvests if you plant too early," he said.

The federation also trains farmers to spread the word in the countryside, where mobile weather stations have replaced scarecrows.

The meetings often open with a group prayer in deeply Catholic Colombia.

"We understood the situation pretty quickly," said Rosario Ganem, a 55-year-old farmer who has traded her ancestors' methods for the new techniques.

The results, she said, have been "fantastic."