U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior U.S. Geological Survey Scenario generation for long-term water budget analysis using the TOPMODEL rainfall-runoff model 2007 Alabama Water Resources Conference Perdido Beach Resort, Orange Beach, Alabama

Q out Direct Infiltration Precipitation Evaporation ET Baseflow Saturated Areas Water Cycle

TOPMODEL Overview  TOPography-based hydrological MODEL  Developed by Beven and Kirkby, 1979  “Physically-based watershed model that simulates the variable-source-area concept of streamflow generation.” (Wolock, 1993)  Many variations/improvements to the original model since 1979  Three fundamental assumptions Beven, K.J. and M.J. Kirkby A physically based, variable contributing area model of basin hydrology. Hydrological Sciences Bulletin, v. 24, pp Wolock, David M Simulating the variable-source-area concept of streamflow generation with the watershed model TOPMODEL. USGS WRI

TOPMODEL assumptions  Steady-state recharge to the groundwater  Hydraulic gradient of the water table is approximately equal to the surface slope  Transmissivity profile is exponential with depth

TOPMODEL topographic wetness index (TWI) High values of TWI High potential for saturation Low values of TWI Low potential for saturation Grid cells with the same TWI are hydrologically similar TWI = ln(a/tan b)

TOPMODEL topographic wetness index (TWI) Calculations need not be performed on every single grid cell. Grid cells with approximately the same TWI have similar hydrologic response

Mean TWI = 14 Mean TWI = 11 Mean TWI = 15 Mean TWI = 12 General concept of TWI 10-m cell

TOPMODEL Code  FORTRAN Code used by Leon Kaufmann and Dave Wolock (TOPMODEL executable)  Java GUI, Pre- and Post-processing, provides a Java wrapper for the TOPMODEL executable  ArcMap used to create topographic wetness index ASCII grid file

TOPMODEL Features  Hydrographs on a daily time step  Monthly and annual water balances  Breakdown or flow components  Flow duration curves  Flow statistics and TNC IHA analysis  Hydrologic conditions can be mapped back using the topographic indices  Future “What-if” scenarios

Partial list of Basin Characteristics  Total area  Lake area  Stream length  Soil depth  Permeability  Water holding capacity  Field capacity  Porosity  Percent impervious (S)  Centroid latitude (ET)  Groundwater withdrawal (S)  Surface water withdrawal (S)  Surface discharge (S)  Area upstream of lakes  Depth of root zone

Basin information 12-digit HUCs Topographic Index Basin Characteristics

TOPMODEL User Interface Levels Future Scenario Hydrographs Calibration Single (S) or Nested (N) Basin Scenario-1 Scenario-n Basin Characteristics

Program Flow Watershed Linkage Wetness Index Builder User Request S or N? Basin Characteristics TOPMODEL OUTPUT OPTIONS ___________ Hydrographs Statistics Flow Duration Tabular Graphical Climate Data Edits for Scenarios

TOPMODEL Basin Characteristics + Edit basin characteristics + Save and Retrieve different basin characteristics files + Access the scenario builder

Future Scenarios (Forecasting and What-ifs)  Climate variability  Long-term trends in precipitation  Long-term trends in temperatures  Population and Land use  Impervious area  Surface water withdrawals  Groundwater withdrawals  Surface water discharges

TOPMODEL Scenario Builder Tab access to scenarios Static Trend Seasonal Step

Historical climate record Historical record used to build a future scenario or use past extreme events directly

Rainfall Scenario Using a historical record and assuming a stationary mean, Adjust the annual totals to some trend.

TOPMODEL Main Menu + User friendly + Choose State + Choose gage + Message log reports status and errors

Hydrograph overlay

Flow duration overlay

Work-in-progress  Calibration statistics  Correlation coefficient  Mean absolute error  Bias  Std Err of estimate  Nash-Sutcliffe model-fit efficiency  Flow statistics  Low-flow  Peak-flow  Graphics  Monthly water balance  Annual water balance  Custom plotting menu  Karst component