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American Water Resources Association

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  • Rain Garden Evapotranspiration Accounting Methods

Rain Garden Evapotranspiration Accounting Methods

  • 10 Oct 2018
  • 12:00 PM - 1:00 PM
  • One Parkway Building, 1515 Arch Street, 18th Floor Planning Commission Room, Philadelphia, PA
  • 30


(depends on selected options)

Base fee:

Registration is closed

Rain Garden Evapotranspiration Accounting Methods


Amanda Hess, PhD

Water Resources Engineer at OptiRTC and Post Doctoral Researcher at Villanova University


Current rain garden design in Pennsylvania (PA) is based upon a static snapshot of rain garden performance under infrequent conditions. This approach does not consider dynamic processes such as infiltration and evapotranspiration (ET). Current design allows for credit of 20% void space for soil and 40% for gravel thus incentivizing the use of imported gravel over native soils. These practices encourage overdesign of rain gardens, which is not sustainable and results in fewer rain gardens being implemented.

Rain garden weighing lysimeters located on Villanova’s campus are configured with two soil depths (i.e. 66 cm and 46 cm) and soil media (i.e. sand, sandy loam, loamy sand, loam, silt loam, and clay loam) to determine the water balance. A proposed accounting method used a period of 6 days, the statistical dry time between events of 0.25 mm, and 12 days, conservatively half the statistical dry time between events of 2.5 mm, to accredit ET volume reduction into the current design regime. Data from all lysimeters types provided calibration and verification of an unsaturated flow model called Soil Water Atmosphere Plant (SWAP). The program is validated for all soil types and depths, producing the same ET removal as the data suggest. The SWAP program is used to estimate ET removal for different soil types, depths, and outflow configurations outside the physical study. The resulting void space credit range is 31-40% for sand soils of varying depth, which is higher than 20% and 30% recommended by PADEP and PWD, respectively. These results can be easily incorporated into current rain garden design in PA to provide more accurate and higher void space credits.

Speaker Bio: 

Amanda Hess is a water resource engineer at OptiRTC where she focuses on implementing smart stormwater controls and a post doctoral researcher at Villanova Center for Resilient Water Systems where she focuses on the rewrite of the PA DEP Stormwater Manual. Amanda received her Masters and PhD from Villanova University in 2014 and 2017, respectively, where she focused on quantifying evapotranspiration from rain garden plants.



This seminar does qualify for 1.0 Professional Development Hour (PDH). A Certificate of Attendance will be available on site for AWRA-PMAS members only. The meeting price for non-members who wish to receive a Certificate of Attendance for the PDH is $10.00 ($3.00 for meeting + $7.00 for certificate). 


Please note: all lunch orders will close by noon on the day before the presentation. In addition, all lunch orders will need to be paid for online by this time. We are unable to refund the cost of lunch or meeting fees because they are paid ahead of time based on number of registrations.

Thank you!

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