One of the worst dry periods in Vietnam's recent history has dried up riverbeds and aggravated saline water intrusion into coastal areas, threatening Vietnam's southern Mekong Delta, the country's rice bowl, said an U.N. expert here on Thursday.

Vietnam, world second largest rice exporter, will face a drop in spring-summer crop production this year, said Koos Neefjes, a climate change policy advisor of the United Nations Development Programme in Vietnam.

The Mekong River, connecting six countries in Southeast Asia, flows into the sea in southern Vietnam. A total of 12 provinces constitute the Mekong Delta, with 17 million people living and farming. The area's rice output stood at 20 million tons last year, about a half of the country's total production. It is one of world 's richest agricultural regions.

In the country's Mekong Delta, the temperature rose to above 35 degree Celsius at day time in the last three consecutive months. Water at rivers ran extremely low. Hot weather killed rice paddy and livestock, and made it difficult for people to access to clean water.

What was worse, water in some rivers in the Delta now contained a higher degree of salt as the sea water pushes inland farther at this dry season, threatening crops.

The Vietnamese Ministry of Agriculture and Rural Development recently forecast that at least 100,000 hectares of rice in the Mekong Delta are under threat. Meanwhile, Neefjes also said that if the weather does not change very soon, 500,000 hectares or even 800,000 hectares of rice would be affected in a few more weeks.

The Vietnam Institute of Hydro Meteorology and Environment said that the country's ongoing drought may last till May.

Neefjes said the worst scenario is that one fourth of the country's total rice output would be affected this year.

Some meteorologists said that this year's return of El Nino weather phenomenon is the mean reason of the drought. The Vietnam Institute of Hydro Meteorology and Environment said current drought is an aftermath of the El Nino.

The return of El Nino brought an unusually warm and dry winter last year, said the institute. The early end to the wet season last year and little rainfall in the first months of this year resulted the severe drought.

Neefjes also held that the cyclical El Nino is the primary reason for the drought. "Last summer, we predicted the drought for 2009 winter and 2010 spring through measurements and data. It is not a surprise."

Neefjes said this year is a weak El Nino year as the drought to some extent is not as worse as in 1997 and 1998.

Neefjes added that building dams and reservoirs could actually help mitigate the effects of drought. With reservoirs, people save water in wet season and release it in dry season. Given the fact that China's Yunnan province on the upper stream of the Mekong River is also suffering from drought and China has no water to release from the reservoirs, it is understandable that the meteorological reason causes the drought, Neefjes said.

Besides, a lot of the dams in Yunnan are built for hydropower purpose. That means water is stored and very soon released to generate power. In this way, water goes back to the river again, said Neefjes.

But the expert also said continuous deforestation and expansion of agriculture along the Mekong River have hampered the capacity of the river basin to store water.

Following the drought, the Vietnamese government has been working hard to cope with it. Vietnamese Prime Minister Nguyen Tan Dung asked ministries and localities to make plans to operate each irrigation system and adopt measures to save water and prevent the wasteful use of water.

But efforts seem not to be enough to deal with the drought, said the Prime Minister and experts.

Mekong Region Commission, a regional inter-government agency, said that countries of the region are not as familiar with drought management as with flood preparedness.

The commission said that climate change effects are likely to intensify both flooding and drought over coming years. Governments should be pay more attention for policy and practical level in drought management.