Rill erosion is under the influence of several parameters which their recognitions and optimal combination will be necessary in modeling. One of the soil degradation reasons on hills of Mashhad vicinity is rill erosion that makes recognition of the behavior and control essential. So in this research, by establishing 50-m transects in each different slope length, effective parameters in shaping rills, including canopy cover, ground cover, gravel, sand, silt, clay, slope, and mutual effects between length and degree of slope have been measured. Surface area of rills has been calculated by measuring width and depth of each rill and their geometric sections. Gamma test has been applied in order to find optimal combination of input parameters. Whereas the statistical tests including gamma, VRatio, gradient, and standard error were different, combined statistics index of Modified Ideal Point Error (MIPE) has been employed. This statistic criterion shows that parameters composition including surface gravel, silt, slope and mutual effects between length and degree of slope is the optimal model. Ground cover, amount of silt, and gravel surface, respectively, play more important role when the priority of their effect is being considered.

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