The Office of the National Water Resources (ONWR) has warned of heavy rainfall from August to September due to the impact of the La Nina phenomenon, but the total amount of rainfall this year is expected to be 26% below average.
Deputy secretary-general Paitoon Kengkarnchang said the weather will shift to a typical La Nina pattern in July until the end of this year.
The country recently entered the rainy season, which officially runs from May 20 till mid-October, according to the Meteorological Department.
Rainfall is expected to decline from June to mid-July. Heavy rainfall of 60-80% is expected from August to September, causing flash floods in many areas, Mr Paitoon said. At least two storms are expected to hit the country.
"Although we have faced a La Nina pattern this year, the amount of water is believed to be 26% less than normal," he said.
"Some reservoirs might have a lower volume of water while some will have more water, so we do need to have a good water-management plan for different areas."
He added that state agencies have been working together to strengthen the early warning systems for floods with an effort to mitigate damage to local communities hit by rain.
Since the recent rains began, he said, 1,078.1 billion cubic metres of water have gone to 35 major reservoirs nationwide.
Of those, 28.9% are in the North, 15.9% in the West, 15.6% in the South, 20.9% in the Northeast, 12% in the East and 6.8% in the Central region.
Generally, the current volume of water storage in reservoirs this year is 41.185 billion cubic metres or 51%, compared with 43.370 billion cubic metres or 55% last year.
Regarding the rice-planting plan, he said the Ministry of Agriculture and Cooperatives will ask farmers to speed this up during the rainy season.
Based on the plan, there are 62.44 million rai available for planting rice.
This covers both irrigation and non-irrigation zones, of which 1.29 million rai have already been planted, the office said.