Angkor — Medieval ‘Hydraulic City’ — Unwittingly Engineered Its Environmental Collapse
The architects of Cambodia’s famed Angkor – the world’s most extensive medieval “hydraulic city” – unwittingly engineered its environmental collapse, says research by UNSW scientists and a team of international scholars.
Angkor at sunset. (Credit: Image courtesy of University Of New South Wales)
This revelation, published in the Proceedings of the National Academy of Science, supports a disputed hypothesis by French archaeologist Bernard-Philippe Groslier, who 50 years ago suggested that the vast medieval settlement of Angkor was defined, sustained, and ultimately overwhelmed by over-exploitation and the environmental impacts of a complex water-management network.
A succession of monarchs ruled the Angkor area from about 800 AD, producing the architectural masterpieces and sculpture now preserved as a World Heritage site. By the 13th century the civilisation was in decline, and most of Angkor was abandoned by the early 15th century, apart from Angkor Wat, the main temple, which remained a Buddhist shrine.
Groslier surmised that a network of roads, canals and irrigation ponds established between the 9th and 16th centuries proved too vast to manage. He argued that extensive land clearing for rice fields supporting up to a million people living beyond Angkor’s walled city produced serious ecological problems, including deforestation, topsoil degradation and erosion.
Latter-day archaeologists disputed Groslier’s view because he was unable to support his hypothesis with empirical data about the landscape beyond Angkor’s central temple complex.
Using modern day aerial photography and high-resolution ground-sensing radar, the international research team, including UNSW’s Professor Tony Milne, studied an area of nearly 3000 square kilometres, confirming Groslier’s hypothesis by correlating their images to existing maps, topographic data sets and supporting information from extensive ground-based archaeological investigations.
The team discovered more than 1000 man-made ponds and at least 74 more temple sites in the Angkor region, revealing ruins covering an area of 1000 square kilometres.
The study’s radar images were acquired from NASA via an airborne imaging radar (AIRSAR) data instrument capable of accurately reconstructing surface structures through cloud cover.
“The instrument can produce high-resolution images detecting surface structures as small as 20 cms in height and distinguish very subtle differences in surface vegetation and soil moisture,” says Professor Milne from the School of Biological, Earth and Environmental Sciences.
“This was of particular use in uncovering the archaeological landscape at Angkor. The distinctive spatial patterning of features manifests itself primarily in slight variations in topographic relief. This also influences the amplitude or ‘brightness’ of the radar signal returned to the sensor.”
“Both the topographic relief and the surface brightness can be helpful in identifying the possible location of former roads, canals and rice fields,” says Professor Milne. “When excavations were carried out, they prove to be the site of a canal or temple moat”.
PNAS | September 4, 2007 | vol. 104 | no. 36 | 14277-14282
A comprehensive archaeological map of the world’s largest preindustrial settlement complex at Angkor, Cambodia Damian Evans; Christophe Pottier; Roland Fletcher; Scott Hensley; Ian Tapley; Anthony Milne; Michael Barbetti
Edited by Michael D. Coe, Yale University, New Haven, CT, and approved June 29, 2007 (received for review March 17, 2007)
The great medieval settlement of Angkor in Cambodia [9th–16th centuries Common Era (CE)] has for many years been understood as a “hydraulic city,” an urban complex defined, sustained, and ultimately overwhelmed by a complex water management network. Since the 1980s that view has been disputed, but the debate has remained unresolved because of insufficient data on the landscape beyond the great temples: the broader context of the monumental remains was only partially understood and had not been adequately mapped. Since the 1990s, French, Australian, and Cambodian teams have sought to address this empirical deficit through archaeological mapping projects by using traditional methods such as ground survey in conjunction with advanced radar remote-sensing applications in partnership with the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL). Here we present a major outcome of that research: a comprehensive archaeological map of greater Angkor, covering nearly 3,000 km2, prepared by the Greater Angkor Project (GAP). The map reveals a vast, low-density settlement landscape integrated by an elaborate water management network covering >1,000 km2, the most extensive urban complex of the preindustrial world. It is now clear that anthropogenic changes to the landscape were both extensive and substantial enough to have created grave challenges to the long-term viability of the settlement.