Long-Term Water Budget of Two Rain Gardens in Madison, WI

1. Long-Term Water Budget of Two Rain Gardens in Madison, WI Bill Selbig U.S. Geological Survey Wisconsin Water Science Center Madison, WI

2. Birth of a Monitoring Program • WPDES requires MS4s have permit to discharge storm water into waters of the state • Storm water monitoring plan is part of permit requirements • Permit application part of joint effort among 19 jurisdictions surrounding Madison

3. Madison Area Group • City of Fitchburg • City of Madison • City of Middleton • City of Monona • City of Sun Prairie • City of Verona • Village of DeForest • Village of Maple Bluff • Village of McFarland • Village of Shorewood Hills • Village of Waunakee • Town of Burke • Town of Blooming Grove • Town of Madison • Town of Middleton • Town of Westport • Town of Windsor • Dane County • UW Madison

4. Why Study Rain Gardens? • Hot “BMP of the day” • Small land requirements (big bang for buck) • Good option for municipalities to retrofit existing open space • Homeowners generally accepting of concept • Few studies documenting performance under varying native soil conditions or vegetation types • Fewer studies examine rain gardens in undisturbed substrate

5. What is a Rain Garden? • Shallow depressions planted with native wildflowers and other plants that soak up rainwater or melted snow from impervious surfaces • Sized based on percentage of impervious surface area contributing to garden

7. Form and Function

8. Madison Rain Garden Study • Primary objectives • Evaluate effectiveness of rain gardens at infiltrating storm water with different soils • Sandy soil • Clay soil • Evaluate effectiveness of rain gardens at infiltrating storm water with different vegetation • Turf grass • Native prairie species

9. Madison Rain Garden Study • Secondary objectives • Measure potential evapotranspiration • Estimate vertical volumetric flux • Document maturation of vegetation over time • Perform volumetric mass balance • No disruption to underlying soil structure

10. Site Selection • Soil type should be mostly sand or silt/clay • Rooftop drains to open space with room for 2 rain gardens • Rooftop can be divided to equally drain runoff into downspouts leading to each rain garden • Logistics: slope, access, power, etc.

11. Two Locations Selected Silt/Clay Soils Sandy Soils

12. Verification of Infiltration Rates Preconstruction Infiltration Rate = 4.0 in/hr SAND SOILS Preconstruction Infiltration Rate = 0.15 in/hr CLAY SOILS

13. Construction • All excavation, construction and maintenance performed by City of Madison personnel • Plant selection and installation performed by City of Madison • Two rain gardens constructed side-by-side at each location • Each rain garden planted with turf grass or native vegetation • 5:1 ratio with 6 inch depth

14. Breaking Ground

15. Adding Compost

16. Planting Vegetation

17. Two Rain Gardens in Silt/Clay Soil Native Species Turf Grass

18. Two Rain Gardens in a Sandy Soil Native Species Turf Grass

19. Project Milestones

20. Instrumentation to Measure Volumetric Mass balance

21. Evapotranspiration Volume In Datalogger Soil Moisture Pond Depth Volume Out

22. Volume In Pond Depth Soil Moisture Volume Out

23. Soil Cores and Neutron Logging

24. Sand rain garden soil cores reveal clay loam down to approximately 4 feet in one garden and sand in the other

25. Silt/Clay rain garden soil core reveals sand down to approximately 3 feet then turns to clay

26. Rain Garden Performance

27. Rain Garden Performance

28. Infiltration Rates for Native Vegetation in a Sand Soil

29. Performance Summary Summary period: 2004 – April to December 2005 – April to September • Storage • Evapotranspiration • Recharge

30. 1540 ft2 1540 ft2 1.1 inches 1.1 inches Rain Garden Storage

31. Evapotranspiration • Using modified Penman-Monteith equation • Parameters: • Solar radiation • Wind speed • Precipitation depth • Humidity • Air Temperature • Applies correction factor for vegetation type

33. Precipitation and Evapotranspiration

34. Percent Moisture at Three Depths in a Native Rain Garden with Clay Soils 2.0 ft 1.25 ft 0.5 ft ETc dependent on vegetation type

35. Percent Moisture at Three Depths in a Grass Rain Garden with Clay Soils

36. Estimate of Recharge Using Soil Moisture • We can measure the volume of infiltrated water using a mass balance equation: • Vground = Vin – Vout + P - ET • We can estimate also estimate the vertical flux of water using soil moisture sensors and the following equation: • dQ/dt = ( - FK)*z where, • Q = discharge • = soil moisture content FK = field capacity of soil z = thickness of soil horizon t = time

37. Recharge Through Mass Balance Vground = Vin – Vout + P – ET Vin = 166 ft3 Vout = 0 ft3 P = 31.5 ft3 ET0 = 3.9 ft3 Vground = 193.6 ft3

38. Is it Really Recharge?Soil Moisture as a Function of Soil Structure 4.0 feet 3.25 feet 2.5 feet 0.5 feet 1.25 feet 2.0 feet

39. Winter Recharge 0.20 in/hr 0.50 in/hr

40. Summary • All rain gardens have so far performed quite well • Sand soil infiltrates faster than clay soil • Native vegetation performs better than turf grass • 5:1 ratio captured nearly 100% of runoff • Infiltration rates improved after rain gardens constructed • Infiltration occurs in frozen soils

41. Questions Yet to be Answered… • What ratio of rooftop to rain garden can we get away with? • Where does the water go once it percolates below the surface? • How much more ET can we expect from native plants versus turf grass? • Will infiltration rates improve over time? • What happens if we switch?

42. Rain Gardens are not Weeds!!