The Egyptian farmer stood in his dust-blown field, lamenting his fortune. A few years ago, wheat and tomato-filled greenhouses carpeted the land. Now the desert was creeping in. “Look,” he said, gesturing at the sandy soil and abandoned greenhouses. “Barren.” The farmer, Hamed Jarallah, attributed his woes to dwindling irrigation from the overtaxed Nile, the fabled river at the heart of Egypt’s very identity. Already, the Nile is under assault from pollution, climate change and Egypt’s growing population, which officially hits 100 million people this month. And now, Mr. Jarallah added, a fresh calamity loomed. A colossal hydroelectric dam being built on the Nile 2,000 miles upriver, in the lowlands of Ethiopia, threatens to further constrict Egypt’s water supply — and is scheduled to start filling this summer. “We’re worried,” he said. “Egypt wouldn’t exist without the Nile. Our livelihood is being destroyed, God help us.” […]