Abstract: 【Objective】The aim of this study is to explore the seasonal variations of soil critical shear stress under two typical farmlands in the hilly region of Loess Plateau in order to provide a reference for ensuring food security and establishing a soil erosion process model.【Method】Maize and millet lands in the hilly region of Loess Plateau were used as the study plots. The scouring experiments were carried out in a laboratory flume to simulate soil detachment by concentrated flow under six different shear stresses ranging from 5.71 to 17.18 Pa. The soil critical shear stress (τ_c) was estimated by the soil erosion process WEPP model. The seasonal variation of soil critical shear stress were discussed under maize and millet farmlands during the crop growing season.【Result】Results show that soil critical shear stress under typical farmlands in the hilly region of Loess Plateau displayed significantly seasonal variations (P＜0.05), which increased during the whole growing season. During the growing season, soil critical shear stress in maize land displayed a pattern of the seasonal variation following declined firstly, then increased, declined again, and rose. τ_c values varied from 1.51 Pa to 4.89 Pa, with a mean value of 3.0 Pa. The minimum and maximum values of soil critical shear stress for maize appeared at the five-leaf stage and at the harvesting stage, respectively. Soil critical shear stress in millet land displayed a pattern of seasonal variation following declined firstly, then increased. τc values varied from 1.06 Pa to 6.53 Pa, with a mean value of 2.93 Pa. The minimum and maximum values of soil critical shear stress in millet land appeared at the seedling stage and at the maturing stage, respectively. The seasonal variations of the soil critical shear stress of maize land and millet land were influenced by the seasonal change of soil cohesion and initial water content, and crop root growth. There were positive relationships between soil critical shear stress and soil cohesion, initial moisture content, content of water-stable aggregates, and crop root weight density. Seasonal variations of soil critical shear stress of maize and millet farmlands could be well simulated with soil cohesion, initial moisture content, and root weight density (R²＞0.74, NSE＞0.72). 【Conclusion】The growth of crop root system, and the seasonal change of soil cohesion and initial moisture content are the main factors influencing the seasonal variations of soil critical shear stress in typical farmlands in the hilly Loess Plateau. Soil critical shear stress of the two farmlands shows positive relationships with soil cohesion, initial water content and root weight density, which are important parameters to simulate the seasonal variations of soil critical shear stress in typical farmlands in the hilly region of Loess Plateau.

Key words: hilly region of Loess Plateau, soil critical shear stress, seasonal variations, root growth