Cropland fallow reduces agricultural water consumption by 303 million tons annually in Gansu Province, China

The high-intensity utilization of global cropland causes water shortage and food crisis, which seriously affects the realization of SDG 2 (zero hunger), SDG 6 (clean water and sanitation) and SDG 15 (life on land), and threatens the sustainable social, economic and ecological development. Cropland fallow can not only improve the quality of cropland and maintain ecosystem balance, but also have a significant water-saving effect. However, in most developing countries, such as China, cropland fallow has not yet been widely promoted, and there are few reliable fallow cropland fallow identification methods, making it even more challenging to assess the water-saving effect. To remedy this deficit, we propose a framework for mapping cropland fallow and evaluating its water savings. First, we used the Landsat series of data to interpret the annual land use/cover changes in Gansu Province, China from 1991 to 2020. Subsequently, the spatial-temporal variation of cropland fallow in Gansu province (giving up farming for one to two years) was mapped. Finally, we evaluated the water-saving effect of cropland fallow using evapotranspiration, precipitation, irrigation maps, and crop-related data, instead of actual water consumption. The results showed that the mapping accuracy of fallow land in Gansu Province was 79.50 %, which was higher than that of most known fallow mapping studies. From 1993 to 2018, the average annual fallow rate in Gansu Province, China, was 10.86 %, which was at a low level in arid/semi-arid regions worldwide. More importantly, from 2003 to 2018, cropland fallow reduces annual water consumption of 303.26 million tons in Gansu Province, accounting for 3.44 % of agricultural water use in Gansu Province and equivalent to the annual water demand of 655,000 people in Gansu Province. Based on our research, we speculate that the increasing pilot projects of cropland fallow in China can bring significant water-saving effects and help achieve China's Sustainable Development Goals.