NYS P Index: Dissolved P Transport Score

The next step in assessing a field with the P Index is to determine the potential for dissolved P loss, shown in the diagram, below.

The Disspolved P Transport Score is determined by adding the factors, as follows.

Dissolved P Transport score = Soil Drainage Factor + Flooding Frequency Factor + Flow Distance to Stream Factor

Note: The Dissolved P Transport Score is a value between 0 and 1, so all raw scores greater than 1 are rounded back to 1.

Soil Drainage Factor

Dissolved P is lost from fields in runoff. Runoff occurs when soils become saturated with water, so that each additional raindrop and/or contribution from upslope drainage causes water to move over the soil surface. The soil drainage class for each soil type has been defined in the soil survey (and also in the P Index Manual). The linkage between soil drainage and runoff can be seen in the table, below.

Soil Drainage Description Infiltration capacity/
permeability 		Runoff
potential
Deep, well-drained sands and gravels
high
low
Moderately drained, moderately fine to moderately coarse texture
moderate
moderate
Impending layer, or moderately fine to fine texture
low
high
Clay soild, soils with high water table
very low
very high

The NYS P Index applies higher factors to soils with poorer drainage, as show in the table below.

Soil Drainage Class Drainage Factor
Well to Excessively well drained
0.1
Moderately well drained
0.3
Somewhat poorly drained
0.7
Poorly to Very Poorly drained
1.0

Flooding Frequency Factor

During flood events, nutrients can be washed away from fields by the expanded stream channel. Flooding frequencies have been defined for each soil type in the soil survey (and also in the P Index Manual). Fields with higher flooding frequencies receive higher factors, as shown in the table, below.

Flooding Frequency Flooding Factor
Rare to Never ( > 100 years)
0
Occasional (10 - 100 years)
0.2
Frequent ( < 10 years)
1.0

Flow Distance to Stream Factor

This is a measurement of the distance that water flows from the field-edge to the edge of the nearest, receiving watercourse. For practical purposes, the measurement is made on the predominate flow exiting the field. The shorter the distance, the higher the risk. Also, if the flow connects with a perennial stream (i.e. a stream that flows all-year long), the field will have a higher risk than if it connects with an intermittent stream (i.e. a stream that does not flow all-year long).

The steps involved in this assessment are:

  1. Identify the field and stream on a topographic map. If on the map, solid blue-line streams represent perennial streams and dashed blue-line streams indicate intermittent streams.
  2. Look for the predominant flow path for the field based on the contour lines.
  3. Measure the flow path, again based on the contour lines, between the point where the predominant flow exits the field and the point where it enters the watercourse.
  4. Go to the field and check your work. Many times the situation isn’t what it seems on the map. Also, flows and watercourses can look very different depending on the time of year.

Intermittent Stream (??) in August

Intermittent Stream (!) in March

Once the stream type is determined and the flow distance is measured, the flow distance factor is calculated by….

Flow Distance to blue line stream as found on topographic map:

If intermitent stream (dashed blue line)
 > 200 feet
0
25-200 feet
1 - ((Distance - 25) / 175)
 < 25 feet
1
 
If perennial stream (solid blue line)
 > 300 feet
0
50-300 feet
1 - ((Distance-50) / 250)
 < 50 feet
1

Now all the factors are complete for the Dissolved P Transport score.

Dissolved P Transport score = Soil Drainage Factor + Flooding Frequency Factor + Flow Distance to Stream Factor

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