The science of water harvesting
In the semi-arid Loess Plateau of Gansu province, where Dingxi is located, annual precipitation can be as low as 300-400 mm. This region's soil is highly susceptible to erosion, with some of the highest rates in the world. To cultivate crops in this water-scarce environment, farmers use a method of micro-catchment water harvesting known as the ridge-furrow system.
This technique reshapes the land into a series of parallel ridges and furrows. The ridges, which are compacted, are miniature watersheds. Because the soil is compressed, its ability to absorb water is low, which generates high runoff from even light precipitation. This runoff is channeled directly into the adjacent furrows where crops like maize, potatoes, or sunflowers are planted. This process concentrates the limited rainfall precisely in the root zone of the plants.
The efficiency of this system is quantified by its runoff coefficient, the fraction of rainfall that becomes runoff. Studies in the region show that specially prepared catchment surfaces, often using compacted earth or even plastic film, can achieve runoff efficiencies of 85% to over 90%. This means that for every 10 mm of rain, 9 mm is collected and directed to the crops. To further maximize water availability, farmers often cover the ridges with plastic film, a technique that increases runoff and reduces soil evaporation and warms the soil, which can accelerate crop growth.
A living laboratory for arid agriculture
The practice of terracing on the Loess Plateau has ancient roots, with various forms used for over 2,700 years to combat soil erosion. The modern ridge-furrow rainwater harvesting system is a scientific refinement of these traditional principles, developed and promoted in Gansu since the 1990s to address both water scarcity and food security.
This agricultural area functions as a large-scale experiment, continuously studied by institutions like the Gansu Provincial Key Laboratory of Aridland Crop Science. Research focuses on optimizing the geometry of the ridges and furrows, developing new mulching materials, and measuring the precise impact on crop yields. Compared to conventional flat planting in the same region, the ridge-furrow system dramatically increases crop production. Studies have documented yield increases for wheat and corn ranging from 20% to over 88%, depending on the specific technique and rainfall for the year.
The system's success is tied to its ability to improve soil moisture and water use efficiency (WUE), a measure of crop biomass produced per unit of water consumed. By concentrating runoff and reducing evaporation, the terraces ensure that a larger proportion of the scarce rainfall is used for plant transpiration, translating directly into greater yields in one of the world's most challenging farming environments.