Improving nitrate load simulation of the SWAT model in an extensively tile-drained watershed

Area of Science:

• Environmental Science
• Agricultural Engineering
• Hydrology

Background:

• Subsurface drainage enhances crop productivity but increases nitrate leaching.
• Current Soil and Water Assessment Tool (SWAT) models simplify nitrate transport, affecting accuracy.
• Accurate nitrate load prediction is crucial for managing agricultural watershed water quality.

Purpose of the Study:

• To improve nitrate load prediction accuracy by revising the SWAT subsurface drainage routine.
• To enhance the simulation of nitrate transport processes in tile-drained agricultural systems.
• To provide a more reliable tool for evaluating the water quality impacts of subsurface drainage.

Main Methods:

• Revised the subsurface drainage routine within the SWAT model.
• Validated the enhanced SWAT model using field measurements of flow and nitrate loads.
• Conducted sensitivity analysis on key parameters influencing nitrate transport.
• Modeled various tile depth scenarios to assess impacts on runoff and nitrate loads.

Main Results:

• The revised SWAT nitrate routine significantly improved nitrate transport simulation accuracy at field and watershed scales.
• Nitrate load prediction accuracy improved from 'unacceptable' to 'satisfactory' or 'good' at the field scale.
• Sensitivity analysis identified transpiration, groundwater discharge, tile flow lag time, and channel hydraulics as key parameters.
• Tile configuration scenarios aligned with established understanding of tile flow processes.

Conclusions:

• The revised SWAT nitrate module provides a more accurate tool for predicting nitrate loads from tile-drained agricultural lands.
• The enhanced model can better assess the water quality effects of subsurface drainage systems.
• This improved modeling capability supports better management practices for agricultural watersheds.