Hydrologic/Watershed Modeling Glenn Tootle, P.E. Department of Civil and Environmental Engineering University of Nevada, Las Vegas Ph.D. Department of Civil and Architectural Engineering University of Wyoming

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

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

Steps to Hydrologic Modeling 1.Delineate watershed 2.Obtain hydrologic and geographic data 3.Select modeling approach 4.Calibrate/Verify model 5.Use model for assessment/prediction/design

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

Urban Watershed

USGS Quad Map

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

Geographic Data  Land cover  Land use

Geographic Data  Soil type/classification

Hydrologic Data  Meteorological Data –Temperature –Precipitation –Wind speed –Humidity  Extrapolation of point measurements –Theissen Polygons –Inverse distance weighting

Hydrologic Data  Hydrologic Data –Streamflow  Peak discharge  Daily flow volume  Annual flow volume –Soil moisture –Groundwater level Streamflow

Modeling Approaches (examples) TIME SCALE Event-based (minute to day) Continuous Simulation (days – years) Empirical Regression equ’s Transfer Functions Simple models Rational Method SCS Unit Hydrograph Simple Model Physically-based Based on physical processes Complicated Many parameters KINEROS Stanford Watershed Model TOPMODELSWATVIC-3LTOPMODEL

Basis for Many Hydrologic Models  Hydrologic Budget (In – Out = ΔStorage) Watershed Precipitation (P) Groundwater in (GW in ) Evaporation (E) Transpiration (T) Streamflow (Q) Groundwater out (GW out ) Reservoir Infiltration (I) (P + GW in ) – (E + T + I + GW out + Q) = ΔStorage reservoir

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?

Spatial Scaling of Models Lumped Parameters assigned to each subbasin A1A1 A2A2 A3A3 Fully-Distributed Parameters assigned to each grid cell Semi-Distributed Parameters assigned to each grid cell, but cells with same parameters are grouped

Stanford Watershed Model (HSPF)  Physically-based and continuous simulation

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

VIC-3L Example

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: –Q peak –Q volume –t peak Q peak Q t t peak Q volume

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

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

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?

QUESTIONS