Abstract: To clarify the effects of alternation of drying and wetting on the formation of soil preferential flow in arid valley, taking the wasteland in the arid valley of Honghe River as the research object, we analyzed the soil preferential flow characteristics before and after the simulation of drying and wetting alternation based on dyeing tracer method, water breakthrough curve, and image processing technology. The results showed that, under the simulated alternation of drying and wetting, the matrix flow occurred in the 0-10 cm soil layer, the dyeing depth reached 35 cm, the horizontal width of the preferred path was only 3-10 cm, and the dyeing area curve fluctuated little. Simulated alternation of drying and wetting led to significant increases in the steady effluent, macropores number, and macroporosity. In the 0-20 cm soil layer, the steady effluent after alternation of drying and wetting was about 0.27 cm³·s^-1 higher than that non-alternation of drying and wetting, macropores number in dyeing area was about 1.4 times higher, and the macroporosity was 13.4% higher. The macropores number was positively correlated with stable flow rate. After simulated alternation of drying and wetting, the number of macropores from large to small was 0.6-0.8 mm＞0.8-1.0 mm＞1.0-1.5 mm＞1.5-2.0 mm＞2.0-3.7 mm, while under non-alternation of drying and wetting, it was 0.8-1.0 mm＞0.6-0.8 mm＞1.0-1.5 mm＞2.0-3.7 mm＞1.5-2.0 mm. The macropores number in each pore size range was significantly correlated with the dyeing area ratio. After simulated alternation of drying and wetting, the correlation increased, and the dominant factor affecting the occurrence of preferential flow changed from the macropores number in the pore size range of 1.5-2.0 mm to that of 0.8-1.0 mm. Therefore, the alternation of drying and wetting would affect the characteristics of macropores, which caused the soil to be more prone to preferential flow and with higher magnitude.

Key words: alternate drying and wetting condition, preferential flow, water breakthrough curve, macropore, dyeing tracer method