Transcript Structure of Component P Index

The Canadian Approach to P
Indexes (or, at least, my approach)
D. Keith Reid
Presentation to Soil Test P Stratification Working Group
24 July, 2013
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
Particulate Soil P
Dissolved Fertilizer P
Manure P
Other organic P
P Transport
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
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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
Component
P Index
Process-based
Model
Inputs
Generalized soil and landscape
data
Annual average weather
Crop and nutrient management
Generalized soil and landscape
data
Annual average weather
Crop and nutrient management
Assumptions
Risk is proportional
to a number of
independent factors
Source X transport
Complex processes
risk estimated for
can be modeled to
multiple components predict P losses
Scale
Field
Field
Plot, or
Watershed
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
Specific soil and landscape data
Daily weather (long term)
Generalized Crop and nutrient
management data
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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?
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P Pathways to Tile
Soil Surface
Preferential Flow
Water Table
Matrix 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
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surface runoff.
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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