Thailand's prime minister on Wednesday told farmers to cultivate less rice to help the country manage its intensifying water crisis, as experts called this year's drought the worst in decades.

Prayut Chan-O-Cha, the junta leader who grabbed power in a military coup two years ago, said his administration was working on a 20-year strategy to diversify the country's agricultural sector, which has long relied on irrigation-intensive rice production.

Thailand is one of the world's top rice exporters, but four consecutive years of below-average rainfall has drained water reserves and left irrigation channels in the heart of the country dry.

Many rice farmers are currently unable to muster enough water to plant second crops, shaving their incomes and plunging many into debt.

"We have to find measures to motivate rice farmers to change to other crops (than rice)," Prayut told reporters on Wednesday, adding that management of Thailand's "limited" water resources must be at the core of agricultural planning.

Thailand is expected to produce around 25 million tonnes of rice this year, he added, without revealing how much less rice will be grown in the coming years.

Water reserves across the country have dipped below last year's levels, which were already considered a record low, according to the irrigation department.

Anond Sanidvongs, a Thailand-based climate expert, said 2016 is shaping up to be the driest in decades.

"The drought problem this year is probably the worst in 40 to 50 years," he told AFP.

Agricultural policies are often divisive in Thailand, with rice and rubber farmers pushing hard for subsidies.

The country's vast network of rice farmers in the north and northeast form a political bloc that has voted for governments led by, or aligned to, billionaire ex-premier Thaksin Shinawatra for more than a decade.

His sister Yingluck was ousted by the 2014 coup but is still wildly popular among rice farmers.

She faces a decade in prison over a rice subsidy scheme derided by critics as a populist handout to her support base.

Yingluck insists her subsidy was an effort to assist poor farmers long-ignored by Bangkok's arch-royalist elite.

They draw support from southern rubber farmers who have taken a hit as global commodity prices plunge.