Computer Modeling Process

The MDA’s development of N leaching reduction strategies with modeling in Drinking Water Supply Management Areas (DWSMAs) is a multi-step process. The generalized process is outlined below using simple management and N leaching values as an example. Deviations from this process may occur based on unique circumstances in DWSMAs.

Step 1

Determine the cropland area in the DWSMA and survey agronomists and farmers within the DWSMA to obtain current management practices on the cropland. The collected information is summarized into categories with the associated acreage as shown in Table 1.

*Total cropland acres=700

Step 2

Using the computer model, the MDA will estimate the annual N leaching from each crop production system. A weighted average based on the acreage distribution is used to estimate the average annual N leaching for the entire DWSMA, Table 2.

**Average annual N leaching=17.6 lbs N/acre

Step 3

The MDA consults with the DWSMA’s Local Advisory Team of agronomists, farmers, and U of M Extension to determine viable alternative management practices (not presently used) that could be adopted to reduce N leaching (e.g., reduced N rates, alternative cropping systems) and protect groundwater. The MDA then enters these alternative management practices into the model to estimate their N leaching losses. The alternative management practices in Table 3 are identical to crop production systems 1-4 in Table 2, except they include a cover crop.

Step 4

The first N leaching reduction strategy is a best management practice (BMP) only option, in this case an option for reducing N application rate for corn to the U of M recommended rate in some of the fields (140 lbs-N/acre with dryland corn following soybean and 180 lbs-N/acre with irrigated corn following soybean). BMP requirements for split applications are already being followed. For this example, 200 acres in Crop Production System 2 are shifted into Crop Production System 1, and 200 acres in Crop Production System 4 are shifted into Crop Production System 3. These potential N management changes are estimated to reduce annual N leaching by 11% DWSMA-wide, Table 4b.

**Average annual N leaching=17.6 lbs N/acre

^{^}Average annual N leaching=15.5 lbs N/acre (11% Reduction)

Step 5

The MDA also calculates N leaching reduction strategy options that combine BMPs with AMTs (such as cover crops and/or perennial crops) or substitutions in BMPs (e.g. to a higher N rate) through the adoption of AMTs. Both Table 5b and 5c are examples of AMTs (cover crops and perennials) substituting for a high N rate than the BMP. The N rates remain the same as the current management in the DWSMA, but cover crops or perennials were added. Table 5b estimates the DWSMA-wide N leaching reduction if 200 acres in crop production system 4 were to be changed to 4a (same management but with a cover crop). Table 5c estimates the DWSMA-wide N leaching reduction if 100 acres of crop production system 1 (dryland, corn-soybean, 140 lbs-N/acre) were to be converted to 5 (hay system).

**Average annual N leaching=17.6 lbs N/acre

^♦ Average annual N leaching=16.1 lbs N/acre (8% Reduction)

  ^♦ Average annual N leaching=16.1 lbs N/acre (8% Reduction)

Step 6

The options for N leaching loss reduction presented above are discussed with the LAT. Additional N leaching reductions may be made based on LAT feedback. It is important that the options are meaningful and relevant and fits within what the farmer can reasonably do. The N leaching reduction options provided above would require a collective approach among the farmers in the DWSMA to meet the management and land use change needs. Additionally, it may be possible to calculate and provide N leaching reduction options at an individual farm operation level if requested by the LAT.

Step 7

After N leaching reduction strategies are reviewed by the LAT, the information will be summarized in a report, published, and made available to the public. The report will include tables seen in previous steps.