A group of ethnic villagers who oppose a plan to divert water from the Yuam River to Bhumibol dam is planning to submit a protest letter to Prime Minister Prayut Chan-o-cha.
They say the project's Environmental Impact Assessment (EIA) report lacks transparency and the project will have detrimental effects on their livelihood.
Responding to the National Environment Board's decision to approve the controversial EIA, Satan Chivavichaipong, coordinator of a civil society network of ethnic villagers living along the Yuam, Ngao, Mei, Salawin River basin, said he was disappointed with the decision.
Mr Satan said the EIA report lacked transparency, local participation and credibility.
He said villagers will reject the project and monitor its progress, adding they also will send a protest letter to the prime minister.
"We will submit the letter with our demands that the government revoke the problematic EIA study and call for the cancellation of the project," Mr Satan said.
"This project is no different from the act of 'stealing' a water resource from our communities along the Yuam River, which is crucial for feeding our families."
He added the planned Mae Yuam reservoir, part of the water diversion project, would push at least 100 families out of their homeland and inundate some farms.
The Department of Royal Irrigation has planned to divert water from Yuam River in Mae Hong Son province to a dam in Tak province via a 60-kilometre tunnel.
The Mae Yuam reservoir would be built to support the project.
It is believed that 3,641 rai of forest zone will be demolished by the project, including 800 rai of a protected forest soon to be designated as a national park.
The 70.675 billion-baht project aims to divert 1.795 billion cubic metres of water per year to the dam.
Decharut Sukkamnoed, an independent economist, said on Facebook the cost of water in the area is calculated at 6.04 baht per cubic metre but the value of water being used in rice plantations is 2.11 baht per cubic metre.
"It means the project is not worth the investment," he said.
"To make it worth it, the backers need to change two-thirds of rice plantations into expensive economic plants that will enhance the value of water to 6.84 baht per cubic metre."
Among other critics of the scheme is Panudech Kerdmali, secretary-general of the Seub Nakhasathien Foundation, who said he was disappointed by the board's decision to give a green light to the controversial project.
The foundation and its allies are calling for the project to stop due to its negative impact on sensitive protected forest areas.
Mr Panudech said the project is one of 77 water management projects in the protected forest zone which the government wants to develop.
They had been suspended for many years, he said, adding the government's action was contrary to its target of preserving the protected forest.