Variations in soil detachment by rill flow during crop growth stages in sloping farmlands on the Loess Plateau

• Soil detachment capacity and rill erodibility decreased during growing season. • Rill erodibility ( K r ) decreased as an exponential function of root density. • Soil detachment capacity was simulated by shear stress, soil cohesion, bulk density, and root length density. Near soil-surface characteristics change significantly with crop growth in sloping farmlands and thereby induce the temporal variations in soil detachment by rill flow. In this study, variations in soil detachment capacity by rill flow ( Dc ) and soil erosion resistance to rill flow as reflected by rill erodibility ( K r ) and critical shear stress ( τ c ) during crop growth stages were quantified in sloping farmlands of the Loess Plateau, China. The soil samples were collected from four different plots (one control and three crop plots—soybean, corn, and millet) and subjected to flume experiments under different shear stress ( τ ) levels (5.16–15.61 Pa). The near soil-surface characteristics including soil moisture content (SMC), soil bulk density (BD), soil cohesion (Coh), mean weight diameter (MWD), soil organic matter content (SOM), root length density (RLD), root surface area density (RSAD), and root mass density (RMD), were measured to explain the variations in soil detachment during crop growth stages. The results showed that SMC fluctuated greatly, whereas BD, Coh, MWD, SOM, RLD, RSAD, and RMD increased gradually with crop growth. Measured Dc and fitted K r significantly decreased with crop growth during one growing season. The mean values of Dc for soybean, corn, and millet decreased by 61.19–67.23%, K r decreased by 55.53–61.36%, and τ c increased by 16.0–18.31% compared to those for the bare soil control. The decrease in Dc and K r were significantly affected by BD, Coh, SOM, MWD and RLD, RSAD, and RMD ( p < 0.01). Seasonal variations in Dc were well simulated by τ , BD, Coh, and RLD ( R 2 = 0.96, NSE = 0.96). The findings demonstrate that it is essential to consider the seasonal variations in soil detachment for optimizing soil and water conservation measures in sloping farmlands.