Simulating nitrate drainage losses from a Walnut Creek watershed field

Area of Science:

• Agricultural hydrology
• Environmental modeling
• Soil science

Background:

• Agricultural practices significantly impact water quality, particularly nitrate (NO3-N) losses through subsurface drainage.
• Accurate modeling is crucial for assessing the environmental effects of nitrogen fertilizer management in crop production systems.
• The Walnut Creek watershed in central Iowa faces challenges related to nutrient runoff from agricultural fields.

Purpose of the Study:

• To evaluate an enhanced version of the Root Zone Water Quality Model (RZWQM).
• To validate the model's performance using field-measured data from a central Iowa agricultural site.
• To assess the model's capability in simulating crop yields and nitrogen losses under various application scenarios.

Main Methods:

• Utilized 6 years (1992-1997) of field data, including subsurface drainage, NO3-N concentrations/loads, and crop yields (corn and soybean).
• Calibrated the RZWQM with 1992 data and validated it with data from 1993 to 1997.
• Performed nitrogen-scenario simulations to analyze corn yield response and nitrate loss fractions.

Main Results:

• The RZWQM demonstrated high accuracy in simulating subsurface drainage flow (EF = 0.99, D = 1%).
• Simulated NO3-N losses showed good agreement with measured data (EF = 0.8, D = 13%).
• Corn grain yields were predicted with less than 10% difference from observed values.

Conclusions:

• The validated RZWQM is a reliable tool for simulating agricultural field conditions in central Iowa.
• Model simulations indicate that corn yield response plateaus at nitrogen application rates above 90 kg ha(-1).
• The RZWQM can effectively predict the impact of nitrogen application rates on crop yields and nitrate losses via subsurface drainage.