Drought, an old enemy of Africa, is now knocking loudly on Egypt's door and threatening the country that the fifth-century Greek historian Herodotus described as the gift of the Nile.

Years of meager rainfall at the headwaters of the Nile point to an impending crisis that some say will be more serious than the seven lean years of biblical times.At this critical moment, the water level of Lake Nasser, the largest manmade reservoir on the African continent, is edging below 495 feet.

Experts predict that in less than 70 days, the turbines of the Aswan Dam will stop turning when the water level in the lake drops another 10 feet. The Aswan Dam provides 30 percent of the country's electricity and is a source of light and life for the burgeoning population of 52 million Egyptians.

The water of the Nile, which the dam has helped to conserve, irrigates 6 million acres of precious farmland that is now in danger of being reclaimed by the desert. Egypt today is about to reap the consequences of a climatic disaster that has its origins further south in Africa and is now spreading its tentacles up the Nile valley.

From the seven bridges that span Cairo, the reduced flow of water is evident to the casual visitor. Even the tourist boats that ply the Nile between Luxor and Aswan have been obliged to curtail their operations. At one vital stretch of the river, where the water is low, the boats off-load their passengers onto coaches. Further down the river, they return to their boats for the rest of the journey.

For the past 10 days, preachers in the mosques have been pleading with their congregations to save water, but their prayers and exhortations have come very late.

Last November a team of British consultants warned the Egyptian government that the country could be badly affected by the African drought. The report by consultants Sir Murray Macdonald and Partners said years of poor rainfall in Ethiopia had depleted Lake Nasser to its lowest level since 1960 and urgent conservation measures were required.

The British report was endorsed by Zaki Kenawi, one of Egypt's most distinguished irrigation experts. He said consumption had increased so considerably that even in good years Egypt would be hard-put to find the water it needed. In the absence of more rain in Africa, Egypt would face disaster unless consumption came down.

The waters of the Nile flow through eight countries before reaching Egypt _ Sudan, Ethiopia, Uganda, Tanzania, Kenya, Rwanda, Burundi and Zaire as one goes upstream. Ethiopia, the source of the Blue Nile, is a key country, and plans in Addis Ababa to build a series of dams have alarmed Egypt.

Since time immemorial the Nile has been the lifeline of the Egyptian farmer. As the river begins to dry up, the farmers in the countryside have still to come to grips with the prospect of water scarcity. They use water as liberally as they have done for decades. Earlier this month President Hosni Mubarak addressed the problem for the first time and warned the nation of the challenge represented by the declining Nile. "If the water level this year is less than last year, this will affect agricultural production," he said in a televised speech. "There will be less electricity, less light and less power for the factories."

Since Kenawi sounded the alarm, other experts have weighed in with their own ideas for conservation.

One group of United Nations-sponsored irrigation engineers has called for drastic cutbacks in new projects that depend on Nile water. They'd like an immediate planning freeze on new luxury hotels and tourist resorts.

Even desert reclamation schemes, in their opinion, should be suspended. The response of the authorities has been to concentrate less on water conservation and more on avoiding power breakdowns. Cairo above all, which Egyptians affectionately refer to as the "Mother of the World," must be spared.