Zhengguo Canal (246 BC): The Spy Who Watered Qin's Fields

The Zhengguo Canal was a large irrigation canal begun in 246 BC by the state of Qin on the Guanzhong Plain, in what is now Shaanxi Province, China. According to the historian Sima Qian, it began as a plot by the rival state of Han to exhaust Qin with a costly building project, but the finished canal instead made the plain fertile and helped Qin grow rich enough to conquer its rivals. The story of its builder, the engineer Zheng Guo, is one of the best-known episodes of the late Warring States period.

A plot to tire Qin

By the 240s BC Qin was the most powerful of the seven warring kingdoms, and its small eastern neighbour Han lived in constant fear of invasion. Sima Qian records that the Han court devised an unusual defence. It sent a skilled water engineer named Zheng Guo to Qin to propose an ambitious canal.

The idea was that such a vast project would tie up Qin’s labour and resources for years, keeping its armies at home. Qin’s rulers, eager to improve farmland, accepted the proposal. Work began in 246 BC, the year the 13-year-old Ying Zheng, the future First Emperor, became king.

The canal and how it worked

The canal drew water from the Jing River, a tributary of the Wei, at a point where it leaves the hills. From there it ran east along the foot of the northern mountains to the Luo River. Sima Qian gives its length as more than 300 li, roughly 120 to 150 kilometres in modern terms.

Following the contour of the land, the canal could deliver water by gravity to fields on its southern side. The Jing River carried a heavy load of fine silt. Spread over the fields, this muddy water both fertilised the soil and helped wash away salts that had made much of the plain poor for farming.

The spy unmasked

At some point during construction, the plot was discovered. The Qin court wanted to put Zheng Guo to death. Sima Qian reports that Zheng Guo defended himself frankly: he admitted that he had first come as a spy, but argued that once the canal was finished it would still be of great benefit to Qin.

The court was persuaded, and he was allowed to complete the work. The canal was finished some years later, probably about a decade after it was begun, although the exact date is not recorded.

The affair had wider consequences. The biography of Li Si in Sima Qian’s history says that the exposure of Zheng Guo encouraged members of the Qin royal clan to demand the expulsion of all foreign-born officials. An expulsion order was issued around 237 BC. Li Si, himself a native of Chu, wrote a famous memorial arguing that Qin’s greatness had always depended on talented outsiders, and the order was withdrawn.

Results: a richer Qin

According to Sima Qian, the canal irrigated more than 40,000 qing of salty land, with harvests of about one zhong, a large grain measure, for every mu of field. Modern conversions of these ancient units vary, but the irrigated area was clearly very large; modern estimates range from several hundred to around two thousand square kilometres.

The historian concludes that, as a result, the land within the passes became fertile, there were no more bad years, and Qin grew rich and strong enough to conquer the other states. The canal was named after its builder. Together with the Dujiangyan Irrigation System in Sichuan, begun about a decade earlier, it gave Qin two great granaries to supply its armies during the final wars of unification.

Later history

The heavy silt that made the water so useful also clogged the channels, and the canal’s intake had to be moved and rebuilt many times as the Jing River cut its bed deeper. In 95 BC, under the Han emperor Wu, the Bai Canal was dug to improve irrigation in the same region, and the two systems were often mentioned together.

Successive dynasties repaired and rebuilt the network, and in the 1930s the engineer Li Yizhi built the modern Jinghui Canal, which follows the same idea of drawing water from the Jing. Remains of the ancient canal and its intake works survive and are protected, and in 2016 the site was listed among the World Heritage Irrigation Structures of the International Commission on Irrigation and Drainage.

Timeline

• 246 BC: Ying Zheng becomes King of Qin; work begins on the canal proposed by Zheng Guo.
• c. 240s–230s BC: Zheng Guo’s role as an agent of Han is discovered; he is allowed to finish the canal.
• c. 237 BC: Qin briefly orders the expulsion of foreign officials; Li Si’s memorial persuades the king to cancel it.
• c. 230s BC: The canal is completed and irrigates the plain north of the Wei River.
• 221 BC: Qin completes the unification of China.
• 95 BC: The Han-dynasty Bai Canal is built in the same region.
• 1930s: The Jinghui Canal revives irrigation from the Jing River.

Why it matters

The Zhengguo Canal shows how infrastructure could decide the fate of states in ancient China. A scheme meant to weaken Qin instead strengthened its farming base, and its story has long been cited as an example of a plan that backfired on its makers.