Wisconsin Phosphorus Index, What is the PI?

What is the Wisconsin Phosphorus Risk Index (PI)?

The Phosphorus (P) Index is an agricultural land management planning tool for assessing the potential of a cropped or grazed field to contribute P to the nearest stream or lake. Use of the P Index allows growers to evaluate the relative risk of surface water pollution resulting from different management practices on a particular field in each year of their planned crop rotation.

How it works:
Wisconsin's P Index uses a set of equations to estimate P losses from an individual field to nearby surface water for an average weather year. The P Index currently includes two primary components:

a Particulate P Index that estimates annual delivery of sediment-bound P,
a Soluble P Index that estimates annual runoff dissolved P loads, including dissolved P losses from unincorporated manure or fertilizer P applications.

Edge-of-field losses of particulate and soluble P are summed and then multiplied by a P delivery factor that accounts for P deposition or infiltration as runoff travels from the field to the stream. The pathway assumed for P transport from field to a stream or lake is shown in the figure below.

Pathway for P movement from field to surface water assumed for P Index calculations

The component equations in the P Index are based on the most relevant research results available. Soil characteristics, crop management, soil test P, average precipitation, site erosion potential and the runoff flow path from the field to surface water are used in the P Index calculations.

The Particulate P Index equations use the annual erosion rate of a field and the P concentration in the eroded sediment to estimate annual sediment-bound P loads. Erosion rate is calculated with the latest version of the Universal Soil Loss Equation, RUSLE2. Sediment P concentration is estimated using routine soil test P and soil organic matter.

The Soluble P Index equations use annual field runoff volume and the average dissolved P concentration of the runoff to estimate annual dissolved P loads.

Annual non-winter rainfall runoff volumes are estimated using Wisconsin long-term precipitation records and a modification of the widely used runoff curve number method.
Winter runoff volumes are estimated using long-term, frozen-soil-period stream gauge data for agricultural watersheds in Wisconsin.
Runoff dissolved P concentrations are calculated from soil test P using equations based primarily on the results of Wisconsin simulated and natural runoff experiments conducted by Dr. Larry Bundy's research group at the UW-Madison Dept. of Soil Science.

In addition, the Soluble P Index includes an estimate of dissolved P losses from any manure or fertilizers applied to the soil surface based on the water-soluble P content in the applied material and the probability of runoff occurrence following manure application.

How new applications of fertilizer, manure, organic by-products, and other P-containing materials are handled in the P Index
The effects of P additions on subsequent P losses in runoff are accounted for in two ways:

Direct dissolved P losses from materials left on the soil surface are included in the Soluble P Index as described above.
The surface soil P concentrations used in the Particulate and Soluble P Index calculations are increased in proportion to the P applied. This adjustment is made for all new additions of P and is in addition to the calculation of the Surface-Applied P Index for unincorporated P applications.

Surface soil test P is also adjusted within the P Index calculations to account for crop P removal.

Wisconsin P Index Research and Development
The P Index is a work-in-progress with many contributing researchers and specialists. Wisconsin P Index equations are modified as relevant research data becomes available.

The Calculations and Terms (PDF) document gives a more detailed description of the P Index equations and their sources.

Description of some of the relevant research is available here: http://www.soils.wisc.edu/extension/nonpoint/runoff.php

We have found a good relationship between P index values and annual P loads in runoff from cropped fields with year-round runoff monitoring:

Verification of the Wisconsin Phosphorus Index through in-Field Runoff Monitoring (PDF)
Powerpoint presenation version (PDF)

The results of a pilot study for using the P Index as a P delivery assessment tool for stream water quality improvement project are in the Wisconsin Buffer Initiative Final report.