Calculating Credits and Debits for Compensatory Mitigation in
Wetlands of Western Washington
Patricia JohnsonWashington State Department of Ecology
Debits and Credits
Debits = area-based units representing the loss of wetland functions at an impact site
Credits = area-based units representing the gain of wetland functions at a mitigation site
Background
Creates functional currency to compare functions and values lost to those proposed
Based on the Washington State Wetland Rating System for Western Washington
HGM based
Function Groups
Improving water quality – Water Quality
Flood storage and flow reductions – Hydrologic
Habitat for plants and animals – Habitat
Photo courtesy of King County, WA
Score for each function based on a qualitative rating of:
The potential of the site to provide the function
The potential of the landscape to maintain each function at the site scale
The values each function may have for society
What is the potential of the site to provide the function?
Based on indicators of structure.
Site Potential:
What is the potential of the landscape to affect the function at the site scale?
Landscape Potential:
What is the value of the function to society?
Value:
Scoring for each function
Site potential
+Landscape potential
+Value
High = 3Medium = 2Low = 1
H,H,H = 9H,H,M = 8
H,H,L = 7 H,M,M = 7
M,M,M = 6H,M,L = 6H,L,L = 5
M,M,L = 5M,L,L = 4L,L,L = 3
Example Wetland Impact (Debit):
Before After
Example Wetland Impact (Debits)Rating of Wetland Unit BEFORE impact
Function Improving Water Quality Hydrologic Habitat
Site Potential (H, M, L)
M = 2 L L
Landscape Potential (H, M, L)
H = 3 H M
Value (H, M, L)
L = 1 L L
Score for Wetland Unit 2 + 3 + 1 = 6 5 4
Losses (Debits) and Gains (Credits)
are estimated by multiplying the score
by the area and
by the appropriate Modifier
Photo courtesy of King County, WA
Calculating DebitsModifier: Temporal Loss Factor
Calculating Debits
Score (Water Q.) x Area x TLF = Debits (Water Q.)
Score (Hydrologic) x Area x TLF = Debits (Hydrologic)
Score (Habitat) x Area x TLF = Debits (Habitat)
Example: DEBITS from ImpactDELAYED Mitigation
Improving Water Quality Hydrologic Habitat
Score for Wetland Unit 6 5 4
Acres of impact (non-forested)
1
Basic mitigation requirement (BMR) 6 x 1 = 6 5 4
Temporal loss factor (Delayed) 3
DEBITS 6 x 3 = 18 15 12
Total Debits: 18 + 15 + 12 = 45
Permittee purchases 45 credits from ILF Program
Calculating Credits
Basic Credits = Increase in Score [Score at “maturity” of site – Score before]
Creation : Score before = 0 Re-establishment: Score before = 0 Rehabilitation: Score before = (from form)Enhancement: Score before = (from form)
Example ILF Mitigation Site (Credits)Before After
Example ILF Mitigation Site (Credits)
Rating of Unit BEFORE mitigation
Improving Water Quality Hydrologic Habitat
Site Potential (H, M, L) M = 2 M M
Landscape Potential (H, M, L) H = 3 H L
Value (H, M, L) H = 3 H H
Score 2 + 3 + 3 = 8 8 6
Improving Water Quality Hydrologic Habitat
Site Potential (H,M,L) H = 3 H H
Landscape Potential (H,M,L) H = 3 H L
Value (H,M,L) H = 3 H H
Score 3 + 3 + 3 = 9 9 7
Example ILF Mitigation Site (Credits)
Rating of Unit AFTER mitigation
Calculating CreditsModifier:
Risk of Failure
Type of Mitigation Risk Factor
Advance mitigation The site meets criteria in Charts 1 and 3 of the site selection guidance [i.e. identified in a local plan and is sustainable] AND meets the criteria in Charts 4-11 for the appropriate functions. (Ecology publication #09-06-032) Advance means that at least two years has passed since plantings were completed or one
year since “as-built” plans were submitted to regulatory agencies.
1.0
Advance mitigation without meeting criteria in Ecology publication #09-06-032 0.83
Concurrent Mitigation Mitigation site meets criteria in Charts 1 and 3 of the site selection guidance [i.e. identified in a local plan and is sustainable] AND meets the criteria in Charts 4-11 for the appropriate functions. (All worksheets for Chart 3 and in Appendix D of Ecology publication #09-06-032 are submitted)
Risk factor applies to all types of mitigation.
0.9
Mitigation site chosen meets the criteria in Charts 2 and 3 of the site selection guidance [i.e. identified as a site with potential and that is sustainable] ; AND meets criteria in Charts 4-11 for the appropriate functions. (All worksheets for Chart 3 and in Appendix D of Ecology publication #09-06-032 are submitted)
Risk factor applies to all types of mitigation.
0.80
Site does not meet criteria in site selection guide, or guide was not used. Re-establishment, rehabilitation, or enhancement of an aquatic bed,
shrub, or forest community Re-establishment, rehabilitation, or enhancement of an emergent
community Creation of an aquatic bed, shrub, or forest community with data showing
there is adequate water to maintain wetland conditions 5 years out of every 10.
Creation of an emergent community with data showing there is adequate water to maintain wetland conditions 5 years out of every 10.
Creation of an aquatic bed, shrub, or forest community without adequate hydrologic data.
Creation of an emergent community without adequate hydrologic data.
0.67
0.5
0.67
0.5
0.5
0.4
Calculating Credits
Increase in Water Q. Score x Area x RF = Credits (Water Q.)
Increase in Hydrologic Score x Area x RF = Credits (Hydrologic)
Increase in Habitat Score x Area x RF = Credits (Habitat)
Improving Water Quality Hydrologic Habitat
Rating of Unit AFTER mitigation 9 9 7
Rating BEFORE mitigation
0
Increase in Score (A-B)
9 - 0 = 9 9 7
Acres CREATED 1
Basic mitigation Credit
9 x 1 = 9 9 7
Risk Factor 0.9
CREDITS 9 x 0.9 = 8.1 8.1 6.3
Example ILF Mitigation Site (Credits)
Proposed Credits for CREATED Wetland Areas
Example ILF Mitigation Site (Credits)
Proposed Credits for Enhanced Wetland Areas
Improving Water Quality Hydrologic Habitat
Rating AFTER mitigation 9 9 7
Rating BEFORE mitigation
8 8 6
Increase in Score (A-B)
9 – 8 = 1 1 1
Acres ENHANCED 12
Basic mitigation Credit
1 x 12 = 12 12 12
Risk Factor 0.9
CREDITS 12 x 0.9 = 10.8 10.8 10.8
Improving Water Quality Hydrologic Habitat
CREATED Credits 8.1 8.1 6.3
ENHANCEDCredits 10.8 10.8 10.8
TOTALCREDITS 8.1 + 10.8 = 18.9 18.9 17.1
PROPOSED Total Credits for the Project = 54.9
Example ILF Mitigation Site (Credits)
Proposed TOTAL Credits
Total Credits: 18.9 + 18.9 + 17.1 = 54.9
Comparing Debits to Credits
Wetland Impact Debits with Delayed TLF: 45 total Debits 18 debits for Water Quality 15 debits for Hydrologic 12 debits for Habitat
PROPOSED Credits for ILF Mitigation Site: 54.9 total Credits 18.9 credits for Water Quality 18.9 credits for Hydrologic 17.1 credits for Habitat
An ILF mitigation site must fulfill at least as
many credits (released) as the number
sold to permittees (debits).
Photo courtesy of King County, WA
Credit – Debit Tool is Guidance
SCORES from the Tool provide a STARTING POINT
Tool may not capture site-specific factors on debit or credit end
Do not want Tool to drive the design
Questions?
31
Patricia JohnsonWA Dept. of Ecology(360) [email protected]
http://www.ecy.wa.gov/programs/sea/wetlands/mitigation/creditdebit/index.html