With an economy already in tatters, Egypt's farmers fear the building of an upstream Nile dam in Ethiopia could lead to water shortages and crop failures with catastrophic effects on their livelihood.

"We don't want this dam," says Saeed al-Simari, standing on his modest land in Egypt's fertile Nile Delta region.

"We want to plant our land, we need water. It's hard enough with the water we have, imagine when we don't have anymore," said Simari.

"We are very worried about our crops," he told AFP.

Ethiopia is pressing ahead with construction of a $4.2 billion (3.2 billion euro) Grand Renaissance Dam, set to become Africa's biggest hydroelectric dam when completed.

The announcement of the project caused a national outcry in Egypt, with politicians, media and farmers warning that the dam could pose a national security threat.

Water experts in Egypt say there is already a water deficit in the country due to the exploding population.

"The average person uses 620 to 640 cubic metres (21,000 to 22,600 cubic feet) per year. With the water poverty level defined at 1,000 cubic metres, we are already below the water poverty level," says Alaa al-Zawahry, a dam expert and member of a government commission tasked with studying the downstream impact of Ethiopia's dam.

Egypt, which fears the project may diminish its water supply, says its "historic rights" to the Nile are guaranteed by two treaties from 1929 and 1959 that allow it 87 percent of the Nile's flow and give it veto power over upstream projects.

But a new deal was signed in 2010 by other Nile Basin countries, including Ethiopia, allowing them to work on river projects without Cairo's prior agreement.

In May, Ethiopia began diverting the Blue Nile a short distance from its natural course for the construction of the dam, but has assured its neighbours downstream that water levels would not be affected.

But Egyptians fear a doomsday scenario in which water shortages would lead to crop failures and electricity cuts.

A study by international experts on the dam's impact on the river has been submitted to Egypt and Sudan, which also relies on Nile resources and supports Ethiopia's hydro-electric project.

Egypt has dismissed the study's findings, which minimise the dam's impact, and has called for further assessments.

The first phase of the Grand Renaissance Dam is expected to be complete in 2016 and will generate 700 megawatts of electricity. When the entire project is complete it will have a capacity of 6,000 megawatts.

Five-year filling period most taxing for Egypt

The filling of the dam is expected to take around five years and this according to experts will be the most taxing phase for Egypt.

Egypt's Aswan Dam -- which controls annual floods and provides water for irrigation -- has a strategic reserve of 70 billion cubic metres, which will drop by 15 billion each year of the filling phase of the Renaissance Dam, says Zawahry.

After five years, "there will be an electricity shortage and the strategic reserve will be used up," he told AFP.

Ethiopia, for whom the dam promises a much-needed source of energy, has pledged to maintain dialogue with Egypt to resolve any problem.

Zawahry says constant coordination between both countries is crucial.

"There will always be a conflict between Ethiopia wanting to produce more electricity and Egypt receiving the water it needs," he said.

But it is difficult to accurately predict the exact impact of the Renaissance Dam.

"It's all a question of probability," said Zawahry, with many variables playing a part.

"On the Nile, from Ethiopia to Aswan, there are several dams but they are small and their effects are small. But if there will now be a 74 billion cubic metre dam, the management of both dams has to be very well coordinated," he said.

The water ministers of Egypt, Sudan and Ethiopia are to hold talks soon to discuss the progress of the dam, Egyptian officials have said.

"We have heard many encouraging statements from the Ethiopian side saying that the dam will not affect Egypt. The mood is positive," said Khaled Wassef, spokesman for the ministry of water resources and irrigation in Egypt.

"We need the full information on issues like how long exactly will it take to fill the dam, the way it will be managed," Wassef told AFP.

But on the fields, the farmers are less optimistic.

They say water shortages will force them to use underground wells rather than Nile water, which is richer in nutrients thanks to the silt deposits.