Spatial variability of surface roughness and hydraulic conductivity after disk tillage: implications for runoff variability

Hydraulic conductivity and surface roughness were measured twice on freshly tilled soil immediately after each of two disk tillages. The objective was to measure the modification in their spatial arrangement induced by tillage. One hundred and thirteen points were measured on a 1600-m2 area, each with a 1-m2 sampling area. The average values for roughness and field saturated hydraulic conductivity, Kfs, differed significantly after each tillage. This was attributed to the different soil conditions at the time of tillage. Both magnitudes showed some degree of spatial autocorrelation, mostly in the tillage direction, but no cross-correlation. Kfs showed a periodic behavior in the direction perpendicular to the tillage rows that might reflect the effect of traffic. The spatial distribution of surface roughness was completely different after the two tillages made with the same equipment. The spatial distribution of Kfs after two tillages made with the same equipment were similar. An analysis with a runoff model suggests that the spatial modification of both Kfs and surface roughness by tillage is not capable, alone, to explain the lack of stability of runoff in replicated plots. Simulations suggest that the lack of stability in runoff among replicated plots might be explained by the spatial modification of surface tillage combined with an infiltration dominated by a bimodal model of surface crusting regulated by microrelief.