We ask Grok to Summarize TT Volume Credit Calculations

The ET Volume Credit (evapotranspiration credit) in Pennsylvania DEP's Post-Construction Stormwater Management (PCSM) framework helps demonstrate compliance with volume management requirements under 25 Pa. Code § 102.8(g)(2). It addresses the net change in runoff volume for storms up to and including the 2-year/24-hour event by crediting volume reduction in vegetated stormwater control measures (SCMs) through evapotranspiration (and related processes like void space recovery in media).

This credit is primarily implemented via the official DEP PCSM Spreadsheet (Volume Worksheet), which automates calculations for structural SCMs. The methodology draws from Villanova University research (Hess, 2017 dissertation on rain garden evapotranspiration accounting). It evaluates volume reduction potential in vegetated systems, including infiltration during storm events and void space credit in engineered media (often categorized under ET in the spreadsheet).

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Key Eligibility and Inputs for ET Credit

• Vegetated? column must be "Yes" in the spreadsheet's Structural SCM Volume Credits table.
  
  
• Media Depth (ft) ≥ 0.5 ft (approximates rooting depth; soil depth can serve as a proxy in some cases).
  
  
• Vegetation requirements (for credit eligibility): Plug plantings (typically 2–3 ft spacing) with a mix of native woody, herbaceous, and grass species. Seed mixes can supplement but not replace plugs. Grasses alone are insufficient (deeper-rooted plants needed). Mowing limited (e.g., ≤1–2 times/year to ~6" height in many cases).
  
  
• Infiltration Area (ft²): Bottom surface area of the SCM (side slopes generally excluded for vegetated ET credit, as soils are assumed saturated at rooting depth).
  
  
• Volume Routed to SCM (cf): Runoff volume directed to the SCM (from disturbed area + direct precipitation on SCM).
  
  
• Infiltration rate (Ksat, in/hr) influences the ET percentage (higher rates allow more overall volume reduction credit).

ET credit applies only to the portion of routed volume not already credited via infiltration.  The sum of Infiltration Credit + ET Credit cannot exceed the Volume Routed to SCM (prevents double-counting).   ET Credit Calculation Formula (Equation 5 in Spreadsheet Instructions) ET Credit (CF) = ET Volume Reduction (%) × Media Depth (ft) × Infiltration Area (ft²)

• The ET Volume Reduction (%) comes from lookup tables based on media depth and infiltration rate (Ksat).
  
  
• The spreadsheet handles interpolation and balancing with infiltration credit automatically.
  
  
• Assumptions: Sandy loam soil baseline; medium-range crop coefficient; evaluated over a ~6-day period following a 1-inch storm (research basis). The method simplifies site-specific/regional ET factors.

Table 1: ET Volume Reduction (%) for Ksat ≥ 0.25 in/hr (Higher Infiltration, Full Void Space/ET Credit)This table applies when infiltration is not limited. Percentages decrease with greater media depth (reflecting diminishing relative returns).

• 0.5 ft: 34.3%
• 1.0 ft: 30.7%
• 2.0 ft: 26.3%
• 3.0 ft: 25.3%
• 4.0 ft: 24.7%
• 5.0 ft: 24.2%

(Full table in DEP PCSM Spreadsheet Instructions, Rev. Jan 26, 2026, includes values down to 0.1 ft increments.)

Table 2: ET-Specific Credit for Ksat < 0.01 in/hr (Low Infiltration, Limited to Wilting Point–Field Capacity) Uses only the plant-available water volume (wilting point to field capacity) in the media. Much lower percentages.

• 0.5 ft: 6.6%
• 1.0 ft: 7.3%
• 2.0 ft: 3.1%
• 3.0 ft: 2.1%
• 5.0 ft: 1.2%

(Percentages generally peak around 0.7–0.8 ft then decline.)  (Interesting)

For 0.01 ≤ Ksat < 0.25 in/hr: Linear interpolation between Table 1 and Table 2 values. Infiltration Credit (Paired with ET)Infiltration is calculated separately (Equation 4) and added to ET:

• Includes infiltration during the storm (assumed 12 hours at saturated conductivity with 10% safety factor on rate) + post-storm drawdown of storage.
  
  
• Formula (simplified): Accounts for rate × area × time, capped by storage and routed volume.
  
  
• ET credit is "balanced" so total credits do not exceed routed volume. If infiltration credit is high (e.g., 70 CF of 100 CF routed), ET credit may show lower (e.g., 30 CF), even if actual ET potential is higher.

Additional Notes and Limitations

• No ET credit if media depth < 0.5 ft or not vegetated.
  
  
• Side slopes typically not included in ET area (bottom only).
  
  
• The credit is a simplification — site-specific modeling or alternative ET analysis can be attached for potentially higher credit (with supporting documentation).
  
  
• Applies in the context of the 2-year/24-hour storm volume management (net change from pre- to post-construction, using TR-55 Curve Number method on disturbed area only).
  
  
• In prior spreadsheet versions, pervious undetained areas sometimes received separate ET-based credits; current versions focus on structural vegetated SCMs.
  
  
• Related guidance appears in the draft/upcoming Pennsylvania PCSM Manual (Appendix D: Evapotranspiration), which discusses how the spreadsheet accounts for ET, vegetated infiltration SCMs with periodic ponding, and related equations/tables (e.g., media/rooting depth considerations). The manual emphasizes training on the new ET approach and potential overrides for soil types beyond sandy loam.

For full details, including exact tables, equations, and examples:

• Download the latest DEP PCSM Spreadsheet Instructions (revised January 26, 2026) from the PA DEP website (Stormwater Management > Construction Stormwater section).
  
  
• Refer to the PCSM Spreadsheet Volume Worksheet itself (it automates credits once inputs are provided).
  
  
• Consult the Pennsylvania PCSM Manual (when finalized) or draft versions for Appendix D details on ET in vegetated SCMs.

Alternative hydrologic modeling (with justification) or site-specific ET studies can supplement or replace spreadsheet credits if approved. Always verify with the latest DEP resources or your regional office, as methodologies evolve with manual updates. Non-structural or other SCM credits (e.g., Managed Release Concept/MRC, riparian buffers) have separate procedures.

Resources

Soil Educational Booklet - The Soils Guide