1. Hydrologic/Watershed Modeling Glenn Tootle, P.E. Department of Civil and Environmental Engineering University of Nevada, Las Vegas tootleg@unlv.nevada.edu , Ph.D. Department of Civil and Architectural Engineering University of Wyoming tootleg@uwyo.edu

2. Questions? • Who has used / developed a hydrologic model? • What model(s) did you use? • Examples

3. Conceptual Model of Watershed Modeling • Typical Input • Topography • Soil Characteristics • Land cover • Land use • Meteorological data • Typical Output • Streamflow • Subsurface Flow • Depth to water table

4. Steps to Hydrologic Modeling • Delineate watershed • Obtain hydrologic and geographic data • Select modeling approach • Calibrate/Verify model • Use model for assessment/prediction/design

5. What is a Watershed? • Area that topographically contributes to the drainage to a point of interest Natural Watershed • Points of Interest • Road crossing • Stream gage • Reservoir inlet • Wastewater treatment plant • Location of stream restoration

6. Urban Watershed

7. USGS Quad Map

8. Digital Elevation Model (DEM) • Digital file that stores the elevation of the land surface a specified grid cell size (e.g., 30 meters)

9. Geographic Data • Land use • Land cover

10. Geographic Data • Soil type/classification

11. Hydrologic Data • Meteorological Data • Temperature • Precipitation • Wind speed • Humidity • Extrapolation of point measurements • Theissen Polygons • Inverse distance weighting

12. Hydrologic Data Streamflow • Hydrologic Data • Streamflow • Peak discharge • Daily flow volume • Annual flow volume • Soil moisture • Groundwater level

13. Modeling Approaches (examples)

14. Basis for Many Hydrologic Models • Hydrologic Budget (In – Out = ΔStorage) Transpiration (T) Streamflow (Q) Evaporation (E) Groundwater out (GWout) Reservoir Precipitation (P) Groundwater in (GWin) Infiltration (I) Watershed (P + GWin) – (E + T + I + GWout + Q) = ΔStoragereservoir

15. Which Model Should be Used? • It Depends on: • What time scale are you working at? • What hydrologic quantity are you trying to obtain? • What data do you have for your watershed? • How fast of a computer do you have?

16. Spatial Scaling of Models Semi-Distributed Parameters assigned to each grid cell, but cells with same parameters are grouped Fully-Distributed Parameters assigned to each grid cell Lumped Parameters assigned to each subbasin A3 A1 A2

17. Stanford Watershed Model(HSPF) • Physically-based and continuous simulation

18. Variable Infiltration Capacity (VIC-3L) • Continuous simulation and physically-based • Macroscale hydrologic model that solves full water and energy balances

19. VIC-3L Example

20. Calibrating a Model • Typically the model is calibrated against observed streamflow data • Depending on the model complexity, parameters are adjusted until observed streamflow equals model streamflow • Which observed value to use: • Qpeak • Qvolume • tpeak Qpeak Q • tpeak • Qvolume t

21. Sensitive Parameters • Precipitation • Soil parameters • Hydraulic conductivity • Soil water holding capacity • Evaporation (for continuous simulation) • Flow routing parameters (for event-based)

22. Uncertainties • Precipitation • Extrapolation of point to other areas • Temporal resolution of data • Soils information • Surveys are based on site visits and then extrapolated • Routing parameters • Usually assigned based on empirical studies

23. Use of Models • Assessment • What happens if land use/land cover is changed? • Prediction • Flood forecasting • Design • How much flow will occur in a 100 year storm?

24. QUESTIONS