Topographic and seasonal variations of unsaturated hydraulic conductivity as measured by tension disc infiltrometers at the field scale

Infiltration and sheet flow depend mainly on hydraulic conductivity under n early saturated conditions which can Vary in space and time. We have studie d the variation in relation to the topography at different stages of a cult ivation year to understand the origin of sheet flow over the land where mai ze is grown. The sampling was done to a complete factorial design with two landscape positions, upslope and downslope, two sites in relation to wheeli ng, no and under wheeling, and three dates in relation to soil surface chan ges and agricultural operations, June and October 1995 and April 1996. Four measurements were randomly distributed on each of the four sites and each of the three dates. The hydraulic conductivity was measured with a tension disc infiltrometer controlling tension at the soil surface. Steady-state in filtration rates were measured at four soil water tensions: 0.1, 0.3, 0.6 a nd 1.5 kPa. We also measured bulk density and soil moisture content. Hydrau lic conductivity decreased during the year at all tensions by a factor of 2 -3 where there was no wheeling, and a factor of 1-6 under wheeling. This de crease was greater downslope than upslope, particularly under traffic. The topographic effect was less than that of time but significant. Whereas the hydraulic conductivity was similar over the field at the beginning of the c rop cycle (June), it differed according to the landscape position at the en d (April), especially under traffic. These topographic variations of the hy draulic conductivity may be due to slight variations of soil components and hydric conditions in the particular case studied. They are important in re lation to the characteristics of rainfall and runoff and help explain the s patial and temporal distributions of sheet flow and erosional processes tha t themselves may contribute to a redistribution of soil particles and induc e feedback effects on sheet flow and infiltration.