The Hydraulic City

Buckley et al, 2010 present a good argument for the role of climate in the demise of the Khmer empire. Using Vietnamese tree rings that span the past 750 years, they show how severe inter-annual and inter-decadal drought periods intersected by intense monsoonal rainfall acted as a stressor leading to this societies’ collapse.

Angkor, the capital of the Khmer empire, if often referred to as the ‘hydraulic city’. This is because of the extensive canal, embankment and reservoir networks that supplied the city and its sprawling suburbs with water and irrigation for agriculture. These networks once supported a complex agricultural system.

A diagram of the water network in Angkor. Source: Blogspot

As the city expanded however, these networks became increasingly complex, convoluted and more difficult to manage, making them more vulnerable to large scale climatic events. Buckley et al argue that the tree rings show a severe drought at the time associated with the collapse of Angkor, which coincides with the transition from Medieval Climate Anomaly to the Little Ice Age. Drought periods were then disrupted by period of intense rainfall. This combination of drought and intense rainfall exacerbated erosion in the northern catchment. This sediment was then washed into the water system in the south which resulted in blocked water networks and the in-filling of canals. Buckley et al suggest that a single severe flood event could fill an entire canal section. In Angkor today, evidence of modifications in attempt to solve incurable failures in the water network can be found. 

Many academics have developed these arguments more recently and the argument remains very much the same. There is a vast amount of evidence from paleo-climate proxies that supports this theory and this argument has become a key explanation for the collapse of the Khmer empire. Although only the abstract is available at the moment, Afrifa et al, 2014 demonstrate the ongoing development of this theory. Fang et al, 2013 explain how drought, caused by a reduced Asian summer monsoon and dampened Westerlies, was a key factor in decline.

Angkor Wat, surrounded by a major water work. Source: Angkor Guide

In one case, radiocarbon dates taken from sediment found in a canal showed that it was filled in during the 14^th century. This shows that the canal was filled by flood debris from the time of Angkor and was never cleared out. This specific canal was one of the primary links between Angkor and Lake Tonle Sap which supplied water to the empire and Buckley et al conclude that this is evidence of the devastating impacts climate change had on the empire.

On top of devastating impacts of climate on water supply, other economic, social and geopolitical factors were also at play. Buckley et al acknowledge these, which include growing conflict with the Siamese Kingdom of Ayuddhaya (which is often blamed as the cause of collapse) and the increasing importance of maritime trade which caused the Khmer empire to turn away from inland agriculture and increased integration with regional trade.

I will look into these other factors in my next few posts.