Estimating Preferential Flow to a Subsurface Drain With Tracers

Potassium bromide and calcium nitrate were used as tracers in sprinkler irrigation water and applied to a field plot drained with a single subsurface drain line during two irrigations. Irrigations were centered above a drain conduit installed 1.1m below the soil surface. Drain flow was measured, and water samples were collected from drain discharge and analyzed for NO3 and Br content. A hydrograph separation technique, using a mass balance and the assumption of a dual porosity model, was applied to tracer concentrations and flow rate of drainage water to estimate the preferential flow and matrix flow components of subsurface drainage. Individual hydrographs of both matrix and preferential flow were constructed. Preferential flow was found to contribute less than 2% of the total drain outflow but, nonetheless, transported on a mass basis: 24% and 12% of the bromide and 20% and 9% of the nitrate reaching the drain, respectively, during two sprinkler irrigations.