By combining old and new aerial images, archaeologists are getting a new look at the complexities of supplying water to an ancient desert city.

In northern Jordan, along a wadi once called the Golden River, the ancient city of Jerash sits uneasily alongside its modern namesake. With its columned temples, Roman forum, and other monuments, it's one of Jordan's biggest tourist attractions and a key to the region’s long, complex history. But the expansion of modern Jerash is slowly destroying what remains of the old city.

Recently, a team of archaeologists from Aarhus University and Münster University combined historical aerial images with modern, laser-scanning surveys to map those changes and get a big-picture view of the ancient city. Previous archaeological work at Jerash has mostly focused on individual sites or on standing monumental architecture rather than more subtle or mundane aspects of city life, even though the latter is what kept its residents alive. Comparing modern laser scans with aerial photos spanning the last century let the researchers identify what has been lost, discover which sites are most at risk, and make connections between features that might have been much harder to recognize from the ground.

An ancient city

Jerash was an important settlement by the time Rome conquered the land around it in 63 BCE. Under the Roman Empire, it grew into a thriving urban center, with nearly 2.5 miles of walls enclosing a city of colonnaded streets and large public monuments. Under the Byzantine Empire and then the Umayyad Empire, the city’s population continued to grow.

Jerash was at its peak in January of 749 CE when a massive earthquake devastated the region. Most of the survivors left Jerash, and only a few buildings were rebuilt during the medieval period. By the early 1800s, travelers passing through the ruined city reported that a few nomadic Bedouins had camped among the ruins, but few people still lived there permanently.

But in the late 1800s, the Ottoman Empire began a new settlement at Jerash, mostly on the eastern bank of the wadi, on top of the ancient remains of that half of the city. Its population began to swell again in the late 20th century, when two Palestinian refugee camps took shape close to the city, growing into small towns in their own right.

Today, the city is home to over 50,000 people, with almost another 50,000 living nearby in the camps. As in many cities in the Middle East, growing modern urban populations live shoulder-to-shoulder with the ancient past—and when people start vying for space, the living tend to win. Archaeologists often find themselves racing against urban growth to preserve the region's cultural heritage.

History from above

Archaeologist David and his colleagues (Søren Munch Kristiansen of Aarhus University's Center for Urban Network Evolutions, Achim Lichtenberger of Münster University, and Rubina Raja of Aarhus University's Center for Urban Network Evolutions) gathered aerial photos from the past century and examined them side by side with modern laser scans. This process let the researchers look for archaeological features and assess what had been damaged by the last 100 years of urban growth. It also showed where to focus future excavations and conservation efforts.

The survey revealed just how complex the city's long history had been and that people have been burying the old under the new for centuries here. A wall-like structure cuts across the Roman hippodrome, where the city’s ancient residents once gathered to watch horse and chariot races. Archaeologists have debated for years about the purpose of the division. But when Stott and his colleagues zoomed out to look at the aerial images, they were able to link the structure to a channel supplying water to fields to the southwest of the city.

And the channel, according to the archaeologists, was built long after the hippodrome had been repurposed as an industrial building under the Byzantine Empire and then a burial ground under the Umayyads. It seems to be built atop the whole history.

“It shows just how deep and complex the history of the city is,” Stott told Ars.

The water must flow

The study also revealed the complexities of the ancient city’s water system. The only perennial spring within the ancient city’s walls, Ain Kerawan, is situated down in the wadi-bed, where its water couldn’t reach more elevated parts of the city in the days before mechanical pumps. This meant the city’s survival relied on water brought in from further up the wadi or from springs in the surrounding hills, which required a complex network of cisterns, aqueducts, siphons, and channels carved by hand from the local rock.

“People should understand just how complex and sophisticated the engineering behind the Roman water management systems was in order to support a city of this size, and what an achievement of ingenuity and hard work it represents,” Stott told Ars.

Archaeologists here had already studied several key pieces of ancient urban infrastructure, like the two large rock cisterns that once supplied the densely populated northwest quarter of the city. One of them was cut from the rock in the 2nd century CE and repaired sometime in the 5th or 6th century CE, according to earlier excavations. And some of that ancient infrastructure is still in use today.

“A number of the Ottoman period channels are still in use for irrigating the fields in the valley below the city,” Stott told Ars. “The Roman reservoir at Birketein to the north of the city is used by the locals for swimming when water is available.”

Mapping the water system

The aerial images let Stott and his colleagues make connections between the cisterns and several fragments of ancient aqueducts running into the city from higher in the hills, so they can better understand how the whole system worked together. "An important aspect of this work is collating all of the mapping information in a GIS [geographic information system] so that we can analyze different aspects of the water management system and urban fabric together,” said Stott.

The spring that filled the Birketein reservoir, where modern residents swim, would have supplied about a quarter of the ancient city, along with water from upstream in the wadi. This water flowed into town along a network of channels cut into the western slope of the valley. But those channels reached only the lower-lying neighborhoods of the western city. The higher areas, which were also more densely populated, relied on aqueducts to carry water from springs much higher in the hills.

Looking at the aerial images, Stott and his colleagues discovered fragments of another aqueduct flowing into the city's southwest quarter, which would probably have supplied a rock cistern there. From above, it seems to have some unusual features that might be the remains of a mechanism called an "inverted siphon," a pipe for carrying water underneath an obstruction like a valley or riverbed.

“The discovery of an inverted siphon at Jerash would be exciting, and it would contribute to our understanding of the city’s water supply, demonstrating that it was just as sophisticated as at other Roman cities across the empire,” Stott told Ars.

Lessons for a changing climate

The really interesting part of the story, of course, is how the city's water supply changed over time as urban populations grew or as the region's climate fluctuated from arid to relatively wet. How past societies adapted—or failed to adapt—could offer practical lessons for modern cities facing the reality of climate change.

“Jerash was a thriving urban center, but at some point it stopped thriving and could no longer function as a city. Damage to the infrastructure, for example, caused by the devastating earthquake in 749 CE, and long-term processes such as a drier climate, disease, and changing political and economic circumstances all likely contributed to this,” Stott told Ars. “It is interesting to consider through such historical perspectives whether our modern cities can withstand the same pressures.”

Piecing together the details of that story, however, is going to require archaeological work on the ground. It’s hard to pin dates, or even an order of construction, on ancient structures based on aerial imagery alone (the aqueduct cutting across the hippodrome is a notable exception). But Stott and his colleagues say their study can help identify the best targets for future surveys, remote sensing, and excavations in Jerash, and help provide a big-picture context for finds on the ground.

PNAS, 2018. DOI: 10.1073/pnas.1721509115 (About DOIs).