Earth Shots

Campbell, Robert Wellman, ed. 1998. “Phnom Penh, Cambodia: 1973, 1985.” Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. This article was released 1 January 1998.


Phnom Penh, Cambodia
1973, 1985

From 1975 to 1978, Cambodia was ruled by the brutal 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, appearing tan in the 1973 image, show a half-kilometer gridwork in 1985.

(A note on terms: Phnom Penh is pronounced p-NOM PEN. Phnom means “hill” or “mountain” in Khmer; Penh is a woman’s name.1 More than 90% of Cambodians are ethnic Khmer, and Khmer is the national language.2 Cambodia has also been known as Kampuchea.)
The city at the rivers

Phnom Penh is just west of the four-way river intersection, which is called the Chattomukh (“Four Faces”). From the northwest and northeast, respectively, flow the Tonle Sab and Mekong Rivers. These waters merge and split into the Basak River and the Mekong, which flow southeast to the South China Sea.4

The Mekong River is the 12th longest in the world, flowing 2,600 mi from western China to the Mekong Delta in southern Vietnam.5 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.6

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. While the images were both acquired after the monsoon season, the 1985 image clearly shows more surface water than the 1973 image.

Phnom Penh is the Mekong River’s largest city. Its population fluctuated wildly during the 1970s and 1980s; from an estimated 1.2 million in 1971 it swelled with war refugees to 2 million or more by 1975, when it was forcibly evacuated to almost nothing by the victorious Khmer Rouge communists.7 From 1978 (the last year of the Khmer Rouge regime) to 1987, Phnom Penh’s population grew from about 50,000 to 700,000. 8Because of the extreme instability in these decades, data on Cambodia are often fragmentary and contradictory.9
Khmer Rouge irrigation

From 1975 to 1978, Cambodia was governed by ideological pro-Chinese communists known as the Khmer Rouge (“Red Khmer”), who gained a reputation for extreme brutality. Even Pol Pot, the Khmer Rouge ruler, claimed that 10% of the population died (800,000 of 7-8 million), and other estimates were higher.11 Many of these deaths were from hunger, disease, war, and forced work, but there were also mass executions.

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 utilized irrigation. 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.12 Historical and archeological documents also indicate a local irrigation system in the twelfth-century Khmer state, possibly storing and distributing water so that rice could be grown year-round, two or more crops per year.13

The Khmer Rouge set out to build a system of canals, ditches and dikes. 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, slavelike conditions.14

There is disagreement whether this sacrifice and coercion even succeeded in irrigating Cambodia.15 Many projects were headed by loyal party leaders with no technical skills.16 Teachers, technicians, and other skilled (usually urban) professionals were hated by the Khmer Rouge as corrupting urban influences, and many were executed.17 There were reports of many ditches collapsing when it rained.18 It is likely that by the end of the Khmer Rouge regime, expertise had improved, but 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.19


These irrigation works are visible in the 1985 image, especially in the northeast quadrant of the zoom-in. Do you notice anything odd about them which could cause problems? (See one answer below.)

Satellite images

LM1135052007300390 (Landsat 1 MSS, 3 January 1973)

LM5126052008534890 (Landsat 5 MSS, 14 January 1985)

Defense Mapping Agency, 1973 (compiled 1968, revised 1973), Operational Navigation Chart K-10: edition 7, scale 1:1,000,000.

Answer to the Question Above

The ditches and canals are in straight lines, regularly spaced, at right angles. One might expect gravity-dependent canals to curve, like the creeks visible in the 1973 image. The Khmer Rouge built irrigation works along the 1-km gridlines of their military maps, ignoring hills, villages, and other topography. It is claimed that some canals actually did more harm than good, disrupting natural water supplies and encouraging erosion.20

It appears that each district had to dig a certain amount of ditches, whether needed or not.21 Workers had rigid daily quotas, so that some finished early and some could never finish.22 There were rigid decisions about which varieties of rice were acceptable, diminishing the diversity of varieties which had adapted to local conditions.23

Finally, the Khmer Rouge have been criticized for applying inappropriate models from the beginning. In emulating the Chinese system, for example, they ignored the amount of human labor needed to lift the water up to the fields. Where one square kilometer of Yangtze River lowlands may support 1500 laborers, the Mekong uplands may support only 300.24

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