Hydraulic resistance of soil surface seals in irrigated furrows

Soil surface seals resulting from overland flow in irrigation furrows reduce infiltration rates. A method was developed to quantify the hydraulic resistance of furrow seals. Infiltration was measured with a recirculating infiltrometer on two southern Idaho soils, Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthid) and Bahem loam (coarse-silty, mixed, mesic Xerollic Calciorthids). Surface sealing was prevented on half the furrow test sections with cheesecloth laid on the furrow perimeter. Subseal soil-water potential was measured with a recording tensiometer. A two-dimensional finite-difference infiltration model with measured flux as the wetted perimeter boundary condition calculated matric potential directly beneath the seal. Seal resistance was then calculated by Darcy's law from the measured flux and calculated potential. Surface sealing on the bare furrows decreased infiltration by an average of 46% compared with the cloth-covered furrows. The seal conductivity values, based on a constant seal thickness, decreased rapidly during the initial 100 min and reached 0.1 to 8% of the conductivity of the soil underlying the seal after 300 min. Potential gradients across the seal were inversely related to infiltration rate and the conductivity of the seal. The procedure successfully calculated seal conductivity changes with time and can be used to evaluate the effects of management practices on seal formation