‘Al-Shisr,’ said our driver.

Al-Shisr is the sheikh’s ancestral home.

We drove up the track to the village. Perhaps a hundred tower houses stood on top of a cliff, with another cliff above the village soaring up into the mist. It made me think of some forgotten, hidden world from a childhood story. We drove through a gate in the walls surrounding the village, and along narrow lanes of sand and gravel. It was as if we had travelled back in time hundreds of years. The streets were empty, but occasionally a child would peer at us from a darkened doorway. A few chickens scattered before the wheels of our Land Cruiser. We turned uphill up another lane and came to a set of beautiful carved wooden gates set in a high wall, which opened inwards as we approached.

Inside the whitewashed walls was a garden of paradise, cool and mysterious. Water rippled from a fountain and splashed over the edge of a basin, cascading into marble channels that formed a grid of running water going backwards and forwards across the garden. Palms and almond trees provided shade, and a spiky grass grew everywhere, with bougainvillea climbing the white walls, and oleander and euphorbia and other shrubs, the names of which I do not know, planted here and there alongside the channels of flowing water.

It is a magical place.

Beyond the garden an arched colonnade led to the interior of the house, and along it came white-robed men, to greet us and collect our luggage. Beyond the colonnade we entered a marble hall of infinite coolness and grace, clad in tiles of intricate geometrical designs, where the sheikh awaited us.

In the afternoon, when the midday heat had passed and the sun was sinking in the sky, I left Harriet behind at the sheikh’s villa, and set off with Ibrahim down the hill from the village and into the Wadi Aleyn. There was another way into the wadi than the perilous tracks along which we had come. A graded track, the red sand scraped smooth by earth movers, ran alongside the wadi, and along it rumbled huge Tata lorries and dumper trucks, churning up clouds of dust which coated our vehicle in grit. Soon we could see the construction site where the holding pens for the salmon are being built. Gangs of Indian labourers were spread all over the site, where three large basins have been excavated in the side of the mountain and are being lined with concrete. Two tanks will hold freshwater. The third will hold saltwater.

From the first freshwater basin a spillway has already been built down to the edge of the wadi. When the summer rains come, the gates of the holding tank will open, and the salmon will swim down the spillway, and run the waters of the wadi. At least, that’s the plan, anyway.

Ibrahim drove up to a line of Portakabins and stopped. I got out and was greeted by a large man wearing orange overalls and a hard hat. ‘Hi,’ he said, extending a hand and speaking with a Texan accent, ‘Dr Jones? I’m Tom Roper, and I’m the project engineer here. You want a look around?’

We went into the Portakabin and Tom showed me a huge wallchart with the project plan mapped out on it. He went through the timetable. It looked to me as if we were on schedule.

‘Sixteen weeks to completion of the holding tanks. Then four weeks to plumb them into the aquifer and start filling them with water, to test the integrity of the lining and the sluice gates and check our oxygenation kit is working. Then we wait for the salmon to arrive, and the summer rains to come.’

We went through everything in detail, and then I looked out of the window at the activity across the site. There must have been several hundred people spread about the hillside, digging, laying concrete onto wire mesh or unrolling huge coils of Alkathene pipe.

‘The guys are working well,’ said Tom. ‘We haven’t had any major problems on site. It’s just a very hot and dusty job. I’m working a month on, a week off.’

‘Where do you go on your week off?’

‘If I can get up to Dubai, I go there, but the flight connections aren’t great. Otherwise I just sit in the Sheraton in Sana’a, drink a few beers and lie around the pool. There’s nothing to do here; there’s nothing to see except rocks and sand.’

I thought of the beautiful village of Al-Shisr, the ancient mosques and even older pre-Islamic buildings and tombs we had seen on our drive through the mountains, and wondered at his lack of curiosity, but said nothing.

I told him I wanted to walk down to the bed of the wadi for a closer look at what the salmon would have to cope with. ‘Yes, do that,’ said Tom. He laughed and said, ‘I guess those fish will just fry and die. You know that, don’t you?’

‘Well, maybe they will. We’ll try and avoid that if possible.’

Tom Roper shook his head and laughed again. ‘It’s not my business what y’all do with your money. I’m a project engineer; 1 do what I’m paid to do. I’ve built stuff in oilfields. I’ve built dams. I’ve built airstrips. I tell ya, I’ve never built fish tanks in the desert before now. You might as well take a heap of dollar bills and burn them as build all this. Your fish will just fry and die. But, hey, I’ll do what you pay me to do.’

I left Tom in the cabin. He might be an excellent engineer, but I am not especially interested in his views on salmon. I am the fisheries scientist, and it is my considered opinion that we will achieve something here. He should stick to digging holes and lining them with concrete.

I walked the few hundred yards downhill to the bed of the wadi. By the time I got there, even though it was dry heat and late afternoon, I was dripping with sweat.

The wadi bed was a mass of boulders, small and large. A trickle of water ran through it, and as I scrambled along I saw that in some places stone channels had been cut to ease the flow of water. There was just about enough flow in the wadi at the moment for a couple of minnows to swim along. Upstream, the wadi ran through a date palm plantation where I knew that the water would flow through irrigation gutters hewn out of the stone. Beyond the plantation I could see where the wadi came down from the hills. The gradient was not as steep as I had feared, and I could see no obvious obstacles to salmon running up when the wadi filled with water.

Turning the other way I could see a few blue pools lying under cliffs so steep and tall the water was in shade all day long. The permanent shadow prevented complete evaporation of the water coming down the wadi. There had been no rain here for twelve weeks, so this water was likely to be coming from the aquifer. It dried up altogether in the heat of spring and early summer, and then filled again in the heavy summer rains.

I leaned back against a boulder, closed my eyes, and tried to shut out the noise of lorries and bulldozers, and men’s voices from the hillside above. I tried to imagine darkening skies and the rain falling. I tried to imagine the first heavy drops sputtering in the dust, leaving minute impact craters wherever they fell. I tried to imagine the rain falling faster, little rivulets forming, running down into the wadi. I tried to imagine streams of water running down the surrounding ravines, and the trickle in the wadi turning to a stream, then to a river, then to a brown and boiling torrent.

I could half picture this in my mind if I tried hard enough, and forgot about the sun that was now reddening my face and neck and burning my forearms. Even in November the heat here is more than I am used to.

Then I tried to imagine the gates of the holding tanks opening, and a bow wave of water coming down the new concrete spillway a few hundred yards away, and waves slapping together where it met the water of the wadi. I tried to imagine the salmon slipping down the spillway, finding the waters of the stream and, following the instincts of tens of thousands of years, heading upstream to spawn. I could not imagine it.