China’s Hu Line Maps Severe Soil Erosion

China’s maps repeatedly reveal the same pattern, even when that is not their purpose. Its crowded cities and productive farmland are concentrated in the south-eastern third, while the enormous north-west is sparsely inhabited. The contrast has proved so consistent that geographers captured it with one diagonal line in 1935.

Scientists charting the areas where China loses soil have now encountered that line again. They were not specifically seeking it, yet the long-established population boundary precisely follows the areas where the country’s most serious erosion is concentrated.

A gradual loss

Soil is displaced whenever water flows. Rainfall breaks it apart, surface runoff picks it up, and rivers transport it away in a process known as water erosion. It is scarcely noticeable on level land, but on hillsides it removes the topsoil on which crops depend.

A research group at Yunnan University aimed to quantify this loss across the whole country. Soil scientist Xingwu Duan and colleagues used a soil-loss model incorporating rainfall, topography and land use to map erosion between 2000 and 2020.

The consequences are clear. Erosion removes fertile surface soil more quickly than natural processes can replace it, while a global study links the most severe cases to the ways people clear and cultivate land. When that layer disappears, crop yields decline.

The famous Hu Line

This diagonal is not a national or regional boundary. Known as the Hu Line, it was created by geographer Hu Huanyong in 1935 to illustrate China’s highly uneven population distribution, stretching from Heihe in the north-east to Tengchong in the far south-west.

The Hu Line is renowned for its persistence. Roughly 94% of China’s population lives on its wetter, flatter south-eastern side, despite this area accounting for less than half of the country’s territory. The dry plateaus of the north-west remain largely unpopulated.

Climate and topography closely mirror that division, explaining why it appears in this study. The boundary between the two Chinas lies where intense rainfall, steep terrain and heavily worked farmland coincide. It is also where soil loss is fastest.

Mapping the most severe erosion

Before this research, there had been no nationwide identification of the distribution of China’s severe erosion. Researchers understood that the country was heavily affected, but not where the damage was concentrated or how those concentrations changed over time.

The researchers therefore divided China into six erosion zones according to severity: four severe zones and two comparatively mild ones. The four severe zones were not randomly dispersed. Instead, they formed a sequence along the Hu Line.

Such a clear diagonal pattern required an explanation. It did not correspond with provincial borders or river basins. Rather, it aligned with a line designed to represent population rather than soil, suggesting that human activity was intertwined with the issue.

Creeping towards the south-west

The most striking finding was not the location of erosion, but its direction of movement. The team monitored the centre of all lost soil each year, and it did not remain fixed. It shifted consistently towards the south-west.

Across the two decades, this centre moved a substantial distance across the map, drawn towards the rugged landscape around Yunnan. In this region, deep valleys and intense downpours erode steep slopes where soils are already shallow.

No previous work had mapped this nationwide shift across China. This one pattern-the gradual movement of erosion’s centre into more challenging terrain-could alter where the country directs its protective measures.

What drives it

No single factor explains erosion along the line. Four pressures combine: steep ground increases the speed of water; concentrated rainfall provides the force needed to transport soil; intensive cultivation exposes soil at vulnerable times; and certain soils are more easily washed away than others.

On their own, these conditions may cause little damage. Light rain falling on a flat, planted field removes almost no soil. When they occur together, however, losses generally appear to be greater than any one factor would produce alone, at least according to the pattern in the data.

The team remains cautious about causation. Their findings show that these conditions occur alongside the greatest losses, but establishing the contribution of each factor in individual fields is more difficult and remains under investigation.

Keeping soil in place

Established methods can reduce soil loss, and the study highlights those appropriate to China’s terrain. Terracing hillsides is the oldest of them, while a recent paper found that existing terraces reduce cropland erosion in China by about half.

Restoring trees and grass to bare slopes is another major measure. China has applied this approach on a vast scale, returning sloping farmland to forest. Roots and vegetation cover help retain soil in place.

Restoration has already demonstrated results. A separate study recorded substantial reductions in sediment and runoff where China restored vegetation on its most eroded land. Changing which crops are grown, and when they are planted, provides a third level of protection.

The wider picture

Before this research, severe erosion in China was recognised as a problem but lacked a precise national map. It now has one. Soil losses are concentrated along the Hu Line, while their centre is moving south-west into steep country with thin soils that can least afford further loss.

For planners, this map changes the challenge. Instead of addressing erosion uniformly across an immense country, China can focus terraces, replanting and improved cropping practices on the narrow belt where soil is disappearing most rapidly.

In a country that feeds nearly a fifth of the world’s population from limited farmland, identifying the land at greatest risk is itself a valuable gain. Soil retained today is the soil in which future crops will grow.