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Estimating N leaching from agricultural fields is complicated. There are many factors that influence N leaching losses, including:

• Climate (precipitation, temperature, evapotranspiration)
• Soils (texture, organic matter, depth)
• Crop rotation and management
• Water management (irrigation, tile-drainage)
• Nitrogen management (source, rate, timing, placement, method of application)
• Crop growth (planting date and nitrogen uptake)
• N movement over or within the soil (leaching, tile-drainage, runoff, groundwater discharge)

Developing N leaching reduction strategies based solely on field research can be difficult since research is often limited to specific locations, management practices, environmental conditions, and soil properties that may not accurately represent all DWSMAs. However, Minnesota specific field research data is used to inform models by helping to set input constraints and output expectations.

In anticipation of the Minnesota Groundwater Protection Rule the MDA signed a contract with the U of M in 2015 (continues into 2023) to provide extensive modeling expertise and assist the MDA’s efforts of modeling N leaching losses and develop N leaching reduction strategies in DWSMAs. Activities have included:

• The U of M conducted an extensive analysis to determine the most suitable models to use considering the varying landscapes, environmental conditions, and management practices across Minnesota. Based on a rigorous analysis, of computer models available, the U of M determined the Soil and Water Assessment Tool (SWAT) was the most appropriate for use in tile-drained landscapes, while the Environmental Policy Integrated Climate (EPIC) model was the most suitable for regions with coarse textured soil and irrigation.
   
• The University also used existing Minnesota specific field research to develop model input information specific for various agricultural management, environmental, and soil conditions for regions throughout the state.
   
• Due to the extensive level of effort required to develop DWSMA specific N leaching model scenarios, the U of M provided initial training to MDA staff. This process enabled staff from the MDA to lead the modeling efforts.
   
• For continued capacity building and quality control, the MDA consults weekly with the U of M to refine model input to best reflect DWSMA specific conditions. This iterative process of continuously reviewing the calculations ensures a high accuracy for the N leaching estimates and N leaching reduction strategies developed for DWSMAs.
   
• Field monitoring data and research into new management techniques or products underway at U of M research farms is continuously being reviewed and used to ensure high accuracy in the calculations. 

Information specific to each DWSMA is used as input to the models.

• Soil information is obtained from the USDA Soil Survey
• Gridmet weather data is obtained from Climate Engine
• Agricultural management information is obtained from multiple sources
  • USDA National Agricultural Statistics Service
  • USGS National Land Cover Database
  • Input from the Local Advisory Team
  • Local farmer and agronomist surveys conducted by MDA staff
• Nitrogen fertilizer BMPs from the U of M Extension.

For some DWSMAs, the MDA will not be doing computer modeling. The need for modeling is based on an analysis on current and previous crop production practices. The use of modeling is not needed if the MDA determines that best management practices (BMPs) and alternative management tools (AMTs) have already been implemented at a level sufficient to expect an adequate reduction N leaching that may not yet be captured in current well monitoring data of nitrate-N concentration due to lag time.

For the analysis of whether modeling is needed, the MDA compares the previous land use and management to current practices over an extended period. Information for the analysis is based on historical records and surveys of local agronomist and farmers.

The MDA will proceed to use modeling to develop N leaching reductions strategies in DWSMA where it is determined that cropland use and management has not substantially changed over that period, or if U of M BMPs and AMTs are not currently being used.