Assess the Risk for Each Field before an Application
These maps are based on NWS grids and flood forecast modeling. Cropped fields are much smaller than the grids and can vary considerably within a grid. Local characteristics can significantly affect the model’s applicability to a given field. Always apply your own knowledge of the landscape when assessing the risk of runoff for your fields. Considerations include:
Risk increases with soil moisture
The model assumes grid-wide average conditions for soil moisture. If you know that your fields are particularly wet (for example, you experienced a recent rainfall event that was greater than in other parts of the grid) you should know that the risk of runoff from your fields will be higher than what is shown on the risk map. The opposite may hold true if you estimate that your soil moisture values are lower than the grid-wide average.
Even if the map shows low risk of runoff, your fields may not be dry enough to spread
Applying liquid manure (typically equivalent to 1/3 to 1 inch or more of rainfall) to wet fields can lead to direct manure runoff, even if the field is otherwise a low risk site due to low slope or low P-index. Make sure your fields are dry enough to accept additional moisture. If liquid manure should runoff or pond in a field or discharge into tile lines, application must stop until the runoff can be checked. Additionally, operating field equipment on wet fields can lead to soil compaction which can greatly reduce crop yields.
Liquid manure applications increase soil moisture
An application of 27,000 gallons per acre of liquid manure is the equivalent of adding approximately 1 inch of water to your fields. A liquid manure application effectively increases your soil moisture, and therefore the risk of runoff from fields receiving liquid manure will be higher than what is shown on the risk map.
Some fields are always higher risk areas
These are areas of concern on your farm, and might include fields with higher slopes, tighter soils, poor drainage, or close to sensitive features such as ponds, streams, lakes, wetlands, sinkholes, etc. Many of these areas may be identified in your nutrient (or manure) management plan. You should use caution when applying manure in these areas, regardless of what the risk map indicates.