Soil mineralogy and slope effects on infiltration, interrill erosion, and slope factor

[1] Interactions between the effects of soil mineralogy and of slope gradient on seal formation, infiltration rate (IR), runoff, and soil loss were evaluated, and the slope factor (S-f) functions for various soils with differing mineralogy were examined. Three different soils, on slopes of 9, 15, 20, and 25%, were subjected to 80 mm of simulated rainfall. The final IR was greater than or equal to 20.5 mm h(-1) in a clayey kaolinitic soil and less than or equal to 15 mm h(-1) in smectitic soils. The total runoff, as a percentage of rainfall, ranged from 24 to 18%, 65 to 43%, and 50 to 35% from the kaolinitic, clayey smectitic, and sandy loam smectitic soils, respectively. The total soil loss ranged from 0.32 to 0.45 and from 1.14 to 3.93 kg m(-2) for the kaolinitic and smectitic soils, respectively. In all three soils, increasing the slope gradient increased the detachment of coarse particles (> 0.1 mm) more sharply than that of small particles (< 0.1 mm), especially in the smectitic soils. Combining these results with previous findings for six soils indicated that soils could be divided into two groups according to their Sf values: ( 1) soils that contained smectite and were therefore dispersive and ( 2) soils that did not contain smectite and were therefore stable. For the former soils the regression S-f = 0.47exp(7.7 sin theta) defined the S-f values significantly, whereas for the latter group a linear regression S-f = 0.81 + 1.77 (sin theta) was required. It was suggested that the difference in the S-f functions was mainly due to seal formation enhancement by the smectite.