A down-slope grass filter area is currently the only below-field practice that can be evaluated with the P Index in SnapPlus. Selecting a field-edge filter area in SnapPlus turns on a setting in RUSLE2 that adds a thirty-foot cool season grass strip to the bottom of the critical slope. Under most RUSLE2-modeled field conditions, a majority of the sediment that is eroded from the slope is deposited within the filter area. The sediment that is transported through this grass filter is what is used as the field-edge sediment for calculating sediment-bound P losses from the field. A grass filter on the critical slope in RUSLE2 does reduce rainfall runoff estimates, but not estimated frozen soil runoff, so soluble P losses are not decreased by as much as particulate P losses.
Figure 8 is a comparison of P Index values for the example corn grain and soybean field with a down-slope grass filter and without. When this is selected, need to make sure that the central assumption in RUSLE2 that runoff is moving through the grassed area as unchannelized overland flow. If the flow through the grassed area is concentrated in a visible channel as it would be in a grassed waterway, then the filter area is not an appropriate selection. In SnapPlus, the edge-of-field grass filter selection label includes the word “designed” to indicate to planners that the selected grassed area should be known to be acting as a filter in the manner of grassed filter strips following NRCS design specifications and to not have concentrated flow channels.
Figure – Rotational average P Index without and with a grass filter area for a corn soybean rotation on a fall chisel-plowed Loyal silt loam field with 4% slope, 47 ppm soil test P, 2.5% organic matter, and 10,000 gallons of liquid dairy manure incorporated in the fall before corn. The P Index is shown by component: Particulate P I is P in eroded soil, Soluble P I soil is dissolved P from soil, Soluble P I manure is dissolved P from manure.