This has caused massive drought on hundreds of thousands of farmland, blocked waterway transport and seriously polluted the rivers, a conference heard on Tuesday.

At the conference on management of the Hong-Thai Binh river system, PhD Rodolfo Soncini-Sessa of the Technical University of Milan said that the flow of Hong River must reach about 1,200 cubic metres per second to supply enough water for the Red River Delta region.

However, in the past several years, the flow was strongly reduced to 900 cubic metres per second.

"It is vital to keep the water level of 2.3 metres high in Hanoi's Long Bien District to ensure the supply of water for the city," he said.

The Red River covered Hanoi and 25 other provinces in the north with a population of 26 million people and total agricultural area of nearly 1.1 million hectares.

In terms of waterway transport, he said the minimum of the water level in the area around Long Bien Bridge must be 1.1 metres, but many vehicles were banned to operate as the water level was down to under 0.4 metres sometimes.

According to Rodolfo Soncini Sessa, the decrease of water level at pumping stations in lower section of the river and the construction of hydro-power plants has affected the flow.

Since 1960, Vietnam have spent more than US$500 million to curb the changes of flow in the Red River, but the effectiveness remains limited.

A $2.2 million project, which has been implemented in Vietnam since 2012, will help improve study on the flow allocation of the two rivers, maintain essential ecological services and improve the economic benefits of hydro-power production and agriculture, he said.

The project has managed to specify 24 indexes on exploiting the river water in five sectors and the shortage of electrical energy of hydro-power plants in dry seasons.

The project is being funded by the Italian Ministry of Foreign Affairs, the Technical University of Milan and the Institute of Water Resources Planning under the Ministry of Agriculture and Rural Development.

Bui Quang Tuan, representative of the Institute of Water Resources Planning, said the irrigation system was downgraded as many pumping machines were built since 1960 and reached about 70 per cent of their performance.

Moreover, the encroachment of the river, and the discharge of waste water from industrial zones and residential areas have made it seriously polluted, he said.

Deputy Minister of Agriculture and Rural Development Hoang Van Thang said the ministry has carried out a programme on improving the irrigation system with the estimated investment capital of VND20 trillion ($940 million) until 2020.

The programme will supply more water from reservoirs in Hoa Binh, Thac Ba, Tuyen Quang and Son La. A series of irrigation works, such as Trung Ha, Bach Hac, Dai Dinh, Thanh Diem, Dan Hoai, Xuan Quan and Long Tuu, and a system of dykes will be built and upgraded.