The Canadian Approach to P Indexes (or, at least, my approach)

D. Keith ReidPresentation to Soil Test P Stratification Working Group

24 July, 2013

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What is a P-Index?

• It is a RISK MANAGEMENT TOOL to identify parts of the landscape with the greatest potential for P loss to surface water.

• This should guide management decisions by farmers and land managers– It is not intended to predict P losses from individual storm

events• It should be relatively simple and easy to use.

– This necessitates compromises between accuracy, flexibility and ease of use

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Types of P Indexes

• Additive– Components of risk for P loss (erosion, soil test, runoff, P

applications) are assigned factors, and factors added together. The current Ontario P Index is this type

• Multiplicative– P source factors are multiplied by a transport factor– Critical Source Areas (CSA) make up a small area

• Component– Source X Transport calculations are done for each

component separately, then these are added together to give a total P Index

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Component P Index

P form P Source P Transport

ParticulateDissolved

Soil PFertilizer PManure POther organic P

Surface runoff Preferential flow to tiles Matrix flow to tiles Matrix flow to surface drains Deep percolation to

groundwater Wind erosion

• If all combinations were considered, there would by 2X4X6 = 48 possible components

• Not all sources or transport factors will be valid in all circumstances, and some (but not all) transport factors will be common across different forms and sources

Structure of Component P Index

Source Factor Transport Factor

Particulate P (Soil Erosion x Bioavailable P concentration in sediment)

X Sediment delivery through surface runoff

Dissolved P release from soil (soil P concentration) X Runoff

Particulate P (Soil Erosion x Bioavailable P concentration in sediment)

X Sediment delivery through tile

Dissolved P release from soil (soil P concentration) X Macropore flow through tile

Subtotal = Inherent risk of P loss

Dissolved P release from fertilizer X Runoff

Dissolved P release from fertilizer X Macropore flow through tile

Dissolved P release from manure or other materials X Runoff

Dissolved P release from manure or other materials X Macropore flow through tile

Subtotal = Applied risk of P loss

Sum = Total risk of P loss

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P Indexes versus Process-based Models

“Traditional”P Index

ComponentP Index

Process-basedModel

Inputs

Assumptions Risk is proportional to a number of independent factors

Source X transport risk estimated for multiple components

Complex processes can be modeled to predict P losses

Scale Field Field Plot, orWatershed

Time Step Annual Annual or Seasonal Daily

Outputs Risk Rating Risk Rating based on estimates of P losses

P loss estimates for individual storm events

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Generalized soil and landscape data Annual average weatherCrop and nutrient management

Generalized soil and landscape data Annual average weatherCrop and nutrient management

Specific soil and landscape data Daily weather (long term)Generalized Crop and nutrient management data

Questions and data gaps encountered

• Forms of P to consider– Total P?– Dissolved P – All bioavailable.– Particulate P – Partly bioavailable, but how much?– Different forms have subtly different transport pathways

• P concentration in sediment– How much and how bioavailable?– Is it related to soil test? (Quebec – yes; Ontario – no)

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More Questions

• End point for P Index predictions:– Edge of field – easier, but could target fields with no impact– Edge of stream – more relevant to water quality

• Contributing areas– Distance extending from streambank into fields– Can it be estimated without LIDAR data?– How does width vary seasonally?

• Setback distances– Is the impact from mitigation of P concentration in runoff, or

simply that P isn’t applied in contributing areas?

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And even more questions…

• Impact of incorporation– Several studies report on contrast between no incorporation

and full incorporation.– None address normal situation, with partial incorporation

• Impact of Tile Drains– Need to account for both positive (mitigation of runoff and

erosion) and negative (increased contributing area) effects– P movement from surface to tiles in Great Lakes basin will be

by macropore flow (cracks and earthworm channels)– Risk will vary among soil types– Need to determine proportion of tile flow coming through

macropores from surface – cation concentration?9

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Soil Surface

Water Table

Matrix Flow

Preferential Flow

Pathways for P movement to sub-surface drains. Water moves to tiles both through the soil matrix, and preferentially in cracks and bio-pores. This preferential flow by-passes the mechanisms that would bind the P, so carries similar concentrations of dissolved and particulate P as surface runoff.

P Pathways to Tile

Summary

• P Indexes will not mitigate P losses on their own, but are a tool to target mitigation to where it will be most effective

• As we learn more about P sources and transport, the P Indexes will continue to change

• P transport pathways will vary with soils, climate and cropping systems, and P Indexes must reflect that

• Tile drainage means that conditions in the Great Lakes basin are different from much of North America

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