Simulating nitrate leaching under dryland conditions: model validation for water and solute movement
There is an increased awareness of the possible leakage of nitrate out of agricultural systems and its potential to contaminate underground and surface water. This has led to the use of simulation mode s for studying the dynamics and movement of nitrate-nitrogen within and beyond the root zone of crops. Before these models can be applied to field situations they need to be tested and validated against realistic field conditions. We tested the water flow and solute movement portion of LEACHM (Leaching Estimation And CHemistry Model), and validated it with respect to moisture and chloride changes of a prairie soil under fallow conditions during the growing season. The retentivity and conductivity functions proposed by Campbell (1974), as used in the original version of the model behaved poorly for our soil. We modified the model using the Van Genuchten function and used this to simulate water content changes during the 1975 and 1976 growing seasons. The "pan coefficient" was used as an optimization factor to calibrate the 1976 moisture data. A pan coefficient of 0.4-0.5 was found to be appropriate under fallow while a value of 0.7 was used for wheat crop. Moisture distribution with depth, and changes with time, were well simulated by the model. The distribution of 171 kg/ha of chloride applied as KCl at a depth of 10 em was simulated using LEACHM. There was good agreement between the predicted and measured chloride distribution in 1975 and 1976.
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