Gansu, Yuzhong: This type of field is called pebble mulch, which is covered with a top layer of pebbles to reduce moisture evaporation. I am not sure where people get the pebbles from.
Picture source: Land Resources in the Loess Plateau of China, edited by The Northwest Institute of Soil and Water Conservation, Academia Sinica, published by Shaanxi Science and Technique Press, 1986.