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• Rill morphological parameters were changed with slope gradients. • Rill morphological parameters showed general increasing trend when slope position moved downward. • Most rill morphological parameters showed significant correlations with rill erosion amount. • Best morphological indicators are recommended for empirical rill erosion estimation. In the present study, the rill morphological parameters and their spatial distributions were investigated using indoor rainfall simulation experiments in combination with 3D terrestrial laser scanning technique. Moreover, the optimal rill morphological indicators for rill erosion evaluation were selected based on the relationship of rill morphological parameters and erosion amount. The results showed that total rill length (L), rill density (ρ), maximum width (W max), the degree of contour line departure (μ) and shape ratio (η) increased with the increase of slope gradient, whilst the mean rill depth (D mean), mean rill width (W mean) and width/depth ratio (W/D) decreased with the increase of slope gradient. The spatial distribution pattern of each rill morphology parameter was also changed with slope gradient, showing differences in upper slope, central slope and lower slope. Most of rill morphological parameters were highly correlated with rill erosion amount, except the mean rill width (W mean), degree of contour line departure (μ), and rill density (ρ). The plane area of rill (P area) or the degree of rill dissection (δ) was selected as the optimal morphological parameter to predict rill erosion amount, followed by the maximum area of cross section (CS max), the maximum rill depth (D max), and the mean rill depth (D mean). In particular, the parameters of CS max and D max are recommended for rill erosion estimation based on the convenience and ease of data acquisition. [ABSTRACT FROM AUTHOR]