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Modeling with GWLF Gene Yagow BSE 5354 NPS Modeling March 15, 2011

Modeling with GWLF Gene Yagow BSE 5354 NPS Modeling March 15, 2011. Brief History of GWLF. 1992 – Haith, Mandel, & Wu. GWLF. Generalized Watershed Loading Functions, Version 2.0, Users Manual. 2000 – Dai et al. BasinSim 1.0, A Windows-Based Watershed Modeling Package, Users Guide. VIMS.

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Modeling with GWLF Gene Yagow BSE 5354 NPS Modeling March 15, 2011

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  1. Modeling with GWLFGene YagowBSE 5354 NPS ModelingMarch 15, 2011

  2. Brief History of GWLF • 1992 – Haith, Mandel, & Wu. GWLF. Generalized Watershed Loading Functions, Version 2.0, Users Manual. • 2000 – Dai et al. BasinSim 1.0, A Windows-Based Watershed Modeling Package, Users Guide. VIMS. • 2001 – Evans et al. AVGWLF, Version 3.2, Users Guide. Penn State University. • 2002/2003 – Yagow modifications for 2002 NPS Assessment and TMDL modeling

  3. GWLF Characteristics • Spatially and physically lumped parameters • Continuous simulation • Daily time step • Includes surface and groundwater runoff • Uses loading functions • Dissolved and particulate phases • Runoff, sediment, nutrients, pesticides • Represents both PS and NPS

  4. Runoff and the Daily Water Balance SCS Curve Number Method

  5. Sediment • Pervious areas • detachment - sheet and rill erosion • transport – power function of monthly runoff • sediment delivery • all annual detached sediment is transported by end of model year (March 31) • Impervious areas • Daily buildup • Exponential washoff during runoff

  6. Sediment from Pervious Areas • Daily (t) Erosion by Land Use (k) Xkt = 0.132 * REt * (KLSCP)k * Ak • Monthly (j) Sediment Supply and Transport Capacity SXj = DR * Σ Xkt TRj = Σ Qt5/3 Sediment distributed in proportion to monthly transport capacity.

  7. Groundwater Rural Sources Dissolved Loads Solid-phase Loads Septic Systems Point Sources Urban Sources Nutrient Loads and Sources

  8. runoff depth area concentration Dissolved Loading Function • Calculated on a monthly basis • Using a daily time step • For each source area

  9. sediment yield concentration in the soil Solid-Phase Loading Function • Calculated on a monthly basis • For each source area

  10. ponded direct discharge short-circuited normal Normal System DS Septic System Nutrient Loads a = monthly population d = days/month e = nutrient load/person u = plant nutrient uptake/person

  11. Channel and Streambank Erosion • AVGWLF (Evans et al., 2001) • Based on the lateral erosion rate concept LER = a * stream flow0.6 a = (0.00147 * PD) + (0.000143 * AD) + (0.000001 * CN) + (0.000425 * KF) + (0.000001 * MS) – 0.00016 • PD = fraction of developed land • AD = animal density (animal equivalent units/acre) • CN = area-weighted CN • KF = area-weighted K-factor • MS = mean topographic slope (%) • CE = LER * stream length * bulk density * bank height

  12. Source Characterization • Point sources • Monthly loads of N and P • Post processed accounting • Nonpoint sources • Watershed variables • Monthly variables • Land Use or Pollutant Source variables

  13. GWLF InputFiles

  14. monthly variables source area variables Transprt.dat initialization and watershed variables Variable number of lines!

  15. Variable number of lines! Nutrient.dat 1,10,12 • Watershed variables • Soil and groundwater N and P • Manure spreading variables • Land Use variables • Pervious area runoff N and P • Manured area runoff N and P • Urban area N and P buildup • Monthly Variables • Point source N and P monthly loads • Septic System Population by System Type • Septic system variables

  16. Transport Input File Nutrient Input File 1,10,12

  17. Weather.dat The weather file starts with the month of April and runs through March for as many years as you want to simulate. Days in month 1 Daily temperature and precipitation Days in month 2 Daily temperature and precipitation Days in month 3 Daily temperature and precipitation . . .

  18. The GWLF-VT Model • Based on Penn State Visual Basic code • Modified for batch processing • Monthly output matrix by land use and pollutant • Includes various corrections to code • Includes Evans channel erosion component • Includes some Schneiderman et al. modifications • Several constants added as variables

  19. GWLF-VT File Locations Response Files Executable

  20. GWLF-VT Transport Input

  21. GWLF-VT Nutrient Input

  22. Exercise 1:Basic GWLF Model Run

  23. GWLF-VT Output Files and Summaries • Monthly Loads • e.g., benthXXX_lu.csv • By watershed parameter for non-land uses, e.g. groundwater, septic systems • By land-use / pollutant parameter • Annual Average Load Summary • e.g., benthXXX-sum.csv

  24. Average Annual Output Parameter Values By Month Average Annual Output Parameter Values by Land Use Category GWLF-VT Output Monthly Totals by Watershed Parameters Monthly Totals by Land Use and Output Parameters

  25. Exercise 2:Explore Model Inputs and Outputs

  26. The Reference Watershed Approach

  27. Reference Watershed Approach • Used in place of a numeric standard • Defines Target TMDL Sediment Load • Existing conditions • Modeled load from the Reference Watershed = TMDL Target Load

  28. TMDL Target Load Reducing load in the impaired watershed to the target TMDL load is expected to restore the benthic community Example Benthic TMDL Impaired Load Non-impaired Toms Brook Watershed Reference Watershed

  29. Why Do We Use GWLF? • Meets basic requirements for TMDL modeling • continuous simulation • representation of major NPS pollutant sources • Has modest input data requirements • Is appropriate for applications where relative loads or percent reductions are the desired endpoint

  30. Reference Watershed Selection • Non-impaired • Same eco- or subeco-region • Comparable land use distribution

  31. Calculation of Target TMDL Load • Unit Area Load: • Calculate UAL for the reference watershed (kg/ha) • Multiply by the area of the impaired watershed (ha) • Area-Adjusted Load: • Scale the reference watershed to the area of the impaired watershed while maintaining its original land use distribution • Model the target TMDL load directly from the area-adjusted reference watershed (kg)

  32. Reference Watershed for Toms Brook Toms Brook (TMB) Hays Creek (HYS)

  33. Area-Adjustment HYSadj 4,252 ha HYS 20,789 ha TMB 4,252 ha

  34. Area-Adjustment Details • Scale reference watershed area to that of the impaired watershed using the Ratio = • Land use area, LUadj = LUref• Ratio • Stream Length, StrLadj = StrLref• Ratio • Sediment delivery ratio, SDRadj = SDRimp • Mean channel depth, Dadj = Dimp

  35. Exercise 3:TMDL Reference Loads • Calculate area-adjustment • Run GWLF for multiple watersheds

  36. Post-Processing of GWLF Model Output

  37. GWLF Post-Processing • Assessment_XXX.xls • Summarizes output from impaired and reference watersheds and sub-watersheds • Provides a vehicle for post-processing

  38. GWLF and Sub-Watersheds Modeling without sub-watersheds provides no spatial distribution of watershed characteristics, except as they relate to different land uses. ErosionA = 1000 kg SDRA = f(area) = 0.10 LoadA = ErosionA*SDRA =100 kg A

  39. 0.15 0.10 0.05 0.10 0.05 0.15 Modeling sub-watersheds with GWLF using common parameter values Erosion3 = 500 kg Load3A = 50 kg Erosion2 = 400 kg Load2A = 40 kg SDR3 = f(area) = 0.12 Load3B = 60 kg SDR2 = f(area) = 0.15 Load2B = 60 kg 2 3 B Erosion1 = 100 kg SDR1 = SDRA = 0.10 Load1A = 10 kg 1 A The main advantage of evaluating parameters by sub-watershed is the ability to include spatial variation that will more effectively help to identify problem areas.

  40. Exercise 4Running GWLF with Sub-watersheds

  41. Representing BMPs in GWLF • Pass-through fractions (post-processed) • Land use changes • Reductions from upslope land uses (post-processed)

  42. Pass-through Fractions

  43. Exercise 5:Representing BMPs as Land Uses Changes in GWLF

  44. Additional Post-Processing • NPDES Loads – Existing Conditions • NPDES Loads – Future Conditions • NPS Permitted Loads • Stormwater runoff permits • MS4 accounting • Sediment detention accounting

  45. Sediment Detention Accounting pond pond • Assume fractional detention (e.g., 50%) of loads upstream of pond. • Pond at outlet: entire sub-watershed • Pond at mid-point: proportional loads from each HRU above the pond.

  46. Calibrating GWLF Hydrology • Although GWLF was developed for application in non-gaged watersheds, calibration has been recommended by other modelers, where observed data is available. • Calibration requires concurrent rainfall and observed flow data for each watershed for some multiple-year period within the last 20 years • Rainfall used for the calibration period should include a full range of wet, normal, and dry conditions • Land use and other hydrologic influences need to reflect conditions for the chosen calibration period • Observed data must be available for both watersheds, otherwise neither should be calibrated.

  47. Hydrologic Calibration

  48. TMDL Allocation Scenarios TMDL = WLA + LA + MOS Modeling Endpoint = TMDL - MOS

  49. Site-Specific BMPsfor Implementation Planning

  50. Identify Appropriate BMPs

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