Session 9: The GWLF Model

1. Session 9: The GWLF Model Virginia Tech Biological Systems Engineering Department DEQ TMDL Modeling Workshop; Richmond November 19-20, 2013

2. To provide an overview of the GWLF model To describe when and why calibration might be performed To describe the TMDL development process using GWLF and the origins of parameter values used to populate the model To introduce participants to the GWLF tools used to develop inputs and assess outputs from the model Objectives

3. Overview of GWLF

4. 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 –Schneiderman et al. Vensim GWLF. • 2003-2010 – Yagow, GWLF2010. Brief History of GWLF

5. UAL – Unit Area Load (simple) Forest 0.3 kg N/ha Agriculture 8.4 kg N/ha • Loading functions Example of Nitrogen from No-till corn • dissolved in water – 3.4 mg/L • attached to sediment –1.5 mg/kg sed • Physical process oriented (complex) solubility, adhesion/dispersion, oxidation/reduction Broad Model Types GWLF

6. 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 GWLF Characteristics

7. Runoff and the Daily Water Balance Infiltration R M E SCS Curve Number Method U Q PC S G D

8. Daily precipitation and temperature Curve numbers by land use and soil type Initial storage amounts Partitioning coefficients between storages Monthly ET coefficients GWLF Water Balance Parameters

9. Pervious areas • Detachment - sheet and rill erosion • Transport – power function of monthly runoff • Sediment delivery • Impervious areas • Daily buildup • Exponential washoff during runoff Sediment

10. Daily (t) Erosion by Land Use (k) Xkt = 0.132 * REt * (KLSCP)k * Ak Monthly (j) Sediment and Transport SXj = DR * ΣXkt TRj = Σ Qt5/3 Annual sediment load is distributed in proportion to monthly transport capacity Sediment from Pervious Areas

11. Monthly rainfall erosivity coefficients • Surface erosion parameters by land use • Area • Soil erodibility (K-factor) • Percent slope and slope length (L and S-factors) • Vegetative cover (C-factor) and practices (P-factor) • Urban dry deposition rates on impervious areas • Sediment delivery ratio GWLF Sediment Parameters

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

13. runoff depth area concentration Dissolved Nutrient Loading Functions • Calculated on a monthly basis, m, and • Based on runoffamount • Aggregated at time step, t • For each land use or source area, k

14. sediment yield concentration in the soil Solid-Phase Nutrient Loading Functions • Based on sediment load • Calculated on a monthly basis, m • For each land use or source area, k

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

16. Sediment and groundwater N and P concentrations Dissolved N and P concentrations by land use Impervious area N and P build-up associated with sediment Manure applications by land use and spreading period Monthly point source loads Monthly populations with each of the 4 types of septic systems Septic system N and P daily loads Plant uptake N and P daily loads GWLF Nutrient Parameters

17. 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 * bank height * bulk density Channel and Streambank Erosion Parameters

18. Point sources • Monthly loads of N and P • Post processed accounting of sediment • Nonpoint sources • Watershed variables • Monthly variables • Land Use / Pollutant Source variables • Post-processed accounting of most BMPs Source Characterization

19. 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 Why Do We Use GWLF?

20. Based on Penn State Visual Basic code • Modified for batch processing • Monthly output matrix by land use and pollutant • Includes Evan’schannel erosion component and Schneiderman’sleakage coefficient • Several constants added as variables • Impervious sediment build-up rate • channel depth based on regional curves • Includes various corrections to code The GWLF2010 Model

21. Hydrologic Calibration (when and why)

22. 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 Calibrating GWLF Hydrology

23. If simulating loads in comparison between the impaired and one or more reference watersheds • Either calibrate all watersheds or none • If observed data are available for a neighboring comparable size watershed • Calibrate as a surrogate watershed and • Apply the same calibration adjustments to all watersheds. Guidelines for Calibration

24. Hydrologic Calibration

25. TMDL Model Development with GWLF

26. Obtain local GIS data and attribute files from the web Obtain monitored and permit data from DEQ Perform a stressor analysis to identify major stressor(s) Take a tour of the watershed with local hosts Select an approach for setting the TMDL endpoint Evaluate parameter values for impaired and reference watersheds in a comparable manner Calibrate, if possible Request anecdotal information and feedback from local TAC members Constantly review the reasonableness of model input and output values to detect inadvertent errors. GWLF Model Development Essentials

27. GWLF Watershed Parameters • Derived by GIS: • Watershed/sub-watershed boundaries, areas, and centroids • Stream lengths • Beginning and ending frost-free months • Septic system populations • Derived from literature: • Average hours/day of daylight • Regional erosivity coefficients • Sediment N and P loading factors • Calculated: • Average ET for dormant and growing seasons • Sediment delivery ratio • Average stream channel depth • Recession coefficient • Channel erosion “a-factor” coefficient • Groundwater N and P loading factors

28. Derived from GIS: • Land use distribution by sub-watershed (HRU) • Soils characteristics – soil erodibility, permeability, moisture content • Percent slope • Derived from literature: • Runoff curve numbers • Vegetative cover C-factors • N and P loading rates from various sources • ET for dormant and growing seasons GWLF Land Use Parameters

29. Data preparation spreadsheets • Landuse (landuse_xxx.xlsm) • Watershed (watershed_xxx.xlsm) • 3 input files • Transport (tranxxx.dat) • Nutrient (nutxxx.dat) • Weather (weather44xxxx.dat) • 1 output file (xxxxxx_lu.csv) • Program executable file (GWLF2010.exe) • Response file (xxxxx.rsp) • Post-processing spreadsheet (assessment_xxx.xlsm) GWLF Basic Setup

30. GWLF Exercise

31. GWLF-VT File Locations Data Preparation Files Executable Response Files

32. transport.dat GWLF InputFiles weather.dat nutrient.dat

33. GWLF2010 Output Monthly Totals by Watershed Parameters Monthly Totals by Land Use and Output Parameters

34. Questions?