Earthshots: Satellite Images of Environmental Change

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» Phnom Penh, Cambodia

From 1975 to 1978, Cambodia was ruled by the Khmer Rouge regime, which sought among other things to build a vast system of irrigation canals. These images show an area around Cambodia's capital city of Phnom Penh where such waterworks were built. Many areas east of the Mekong River show a gridwork of canals by 1985.

Map of the featured area.

Phnom Penh is the largest city along the Mekong River. Its population fluctuated wildly during the 1970s and 1980s; the population of 1.2 million in 1971 swelled with war refugees to 2 million or more by 1975, when it was evacuated to almost nothing by the victorious Khmer Rouge communists. From 1978 (the last year of the Khmer Rouge regime) to 1987, Phnom Penh's population is estimated to have grown from about 50,000 to 700,000 people. The 2019 population was well over 2 million.

A note on terms: Phnom Penh is pronounced p-NOM PEN. Phnom means "hill" or "mountain" in Khmer; Penh is a woman's name. More than 90% of Cambodians are ethnic Khmer, and Khmer is the national language. Cambodia has also been known as Kampuchea.

Phnom Penh lies just west of the four-river intersection called the Chattomukh ("Four Faces"). From the northwest and northeast, respectively, flow the Tonle Sap and Mekong Rivers. These waters merge and split into the Basak River and the Mekong, which flow southeast to the South China Sea.

The Mekong River is the 12th longest in the world, flowing 4,200 km from western China to the Mekong Delta in southern Vietnam. Every autumn, monsoon rains are too great for the Mekong to carry, and it floods a large area of Cambodia. This flood even reverses the flow of the Tonle Sab River, northward to the Tonle Sap ("Great Lake"), which can expand to ten times its normal size.

This area receives 152 to 203 cm of rain annually, most of which falls during the southeast monsoons from mid-May to early October. Landsat images are effective for quantifying changes in surface water. The pair of images from 1995 shows the dramatic effect the annual flooding can have.

From 1975 to 1978, Cambodia was governed by Pol Pot. His regime was known as the Khmer Rouge ("Red Khmer").

In their desire to radically transform Cambodia, the Khmer Rouge emulated both contemporary Communist China and the Khmer "golden age" of the 11th–13th centuries, both of which used irrigation. The historic canals around China's Yangtze River delta harnessed rainy-season floodwaters, carrying them out to the surrounding lowlands where in the dry season people lifted the water up into their rice fields. Historical and archeological documents also indicate a local irrigation system in the 12th-century Khmer state, possibly storing and distributing water so that rice could be grown year-round, two or more crops per year.

But the Khmer Rouge irrigation system followed the Chinese plan more closely than the historic Khmer state plan. And in doing so, they made many miscalculations including ignoring the amount of human labor needed to lift the water up to the fields. Where one square kilometer of Yangtze River lowlands in China needs the support of 1,500 laborers, only 300 laborers were available to work the Mekong uplands in Cambodia.

Many projects were headed by loyal party leaders with no technical skills, which is another reason the canals were not more effective. Teachers, technicians, and other skilled (usually urban) professionals—hated by the Khmer Rouge as corrupting urban influences—were executed. Thus, primarily inexperienced and unskilled citizens, including the evacuated city-dwellers, were forced to work in the countryside growing rice and building these irrigation works, with rigid work quotas and hard, slave-like conditions. The lack of experience led to inefficient canals that occasionally collapsed in the rain.

Not only were the canals poorly constructed, but they also were built in straight lines, regularly spaced, at right angles along the 1-km gridlines of their military maps, ignoring hills, villages, and other topography. Some claim that many of the canals actually did more harm than good, disrupting natural water supplies and encouraging erosion. This pattern can be seen in most of these images. In more recent years, the irrigation system built during the Khmer Rouge regime is much less defined in satellite imagery.

It appears in the images that each district had to dig a certain amount of ditches, whether needed or not. Indeed, workers had rigid daily quotas, so that some finished early and some could never finish. There were strict decisions about which varieties of rice were acceptable, diminishing the diversity of varieties which had adapted to local conditions.

Though it is likely that by the end of the Khmer Rouge regime canal construction expertise had improved, the post-Khmer Rouge government had to devote considerable resources to repairing irrigation works. One official said 80% of the projects had been poorly constructed, though it varied by region.

Although many of the canals were poorly built, some of them can still be used. The Cambodian Prime Minister Hun Sen started supporting the re-digging of the canals in 2005, and now that the people who are rebuilding them are no longer under the Khmer Rouge, they get paid a small amount, and the crops that are harvested aren't taken away by the government.


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