MOVEMENT OF WATER AND NITRATE IN AN UNSATURATED AGGREGATED SOIL DURING NONSTEADY INFILTRATION—A SIMPLIFIED SOLUTION FOR SOLUTE FLOW

Abstract

Dynamics of water and nitrate movement during infiltration under unsaturated conditions is examined in highly aggregated (0–2 mm size) Molokai soil. Experiments involved mainly horizontal infiltration into packed soil columns at three different initial water contents. Ammonium nitrate was mixed in the top 5 cm of soil immediately before or a week preceding the start of water application. After 3 or 4 different depths of wetting, the columns were sectioned to determine water content and nitrate distribution in the soil. Uniqueness of the experimental soil-water content vs. distance/time1/2 relationships and soil-water diffusivities for different initial soil-water contents indicated the validity of the simple diffusion theory for flow of water in this aggregated soil. Measured nitrate profiles also showed that the initial water within the micropores is mobile and miscibly displaced by applied water to form the wetting front. Comparison of the velocity of movement of the nitrate peak with the velocity of movement of water at the peak indicated that nitrate ions moved with water at a medium initial soil-water content. At a higher initial value, the nitrate peak was retarded by a factor of 0.75, while at a very low initial water content it was somewhat enhanced (by a factor of 1.13). The velocity of water at the peak was very nearly equal to the velocity of water at the inlet end of the column. A simplified solution derived from solute flow under the time-dependent velocity, considering the dispersion coefficient as a linear function of velocity, is shown to have useful predictive value.