SPATIAL DISTRIBUTION OF DRAINAGE FROM UNIFORM SOIL COLUMNS.

Zero-suction lysimeters and gravity-drained columns have been used routinely to measure water and solute transport through soils, both in the field and in the laboratory. Such procedures are particularly influenced by the bottom boundary condition imposed by a typical lysimeter and can greatly increase the spatial variability of drainage. This study was designed to demonstrate experimentally the spatial distribution of water and solute in the drainage from a uniform moist soil column. A column of soil (305 x 305 x 305 mm) sieved to a uniform size (0.17-0.50-mm diameter) was constructed over a drainage collection apparatus. This allowed the effluent to be separated into 100 equal size cells while suction between 0 and -2.66 kPa was maintained at the lower boundary in combination with 1 cm h-1 and 3 cm h-1 of water application rates to the surface. When the column was drained under gravity, 90% of the water drained into just four cells. The degree of variability in spatial distribution of the effluent decreased significantly once suction was applied at the bottom boundary. Similarly, under zero-suction, 80% of the applied chloride concentration was recovered from just 4% of the drainage area. Under gravity, spatial distribution of drainage was influenced both by unstable flow within the matrix and by the bottom boundary. Gravity lysimeters devoid of any potential to drive the movement of water and solute through an artificial bottom boundary may result in overestimation of preferential flow pattern and may not adequately represent the magnitude of matrix flow in a uniform field soil draining at natural field potentials.