1. Hydrological Modeling for Upper Chao Phraya Basin Using HEC-HMS UNDP/ADAPT Asia-Pacific First Regional Training Workshop Assessing Costs and Benefits of Adaptation: Methods and Data March 11-14, 2013 Dr. Dilip K. Gautam RIMES, AIT Campus, Bangkok

2. Hydrological Model A model is a simplified representation of reality. A mathematical model consists of series of equations defining the system we are dealing with. The function of model is to convert the given input into an output. A hydrological model is the mathematical representation of the response of a catchment system to hydrologic events during the time period under consideration. Hydrological phenomena are extremely complex, highly non-linear and highly variable in space and time. A model is needed to predict the watershed runoff for the design and management of water resources utilization and flood control projects.

3. Classification of Hydrological Models

4. Hydrological Model Outputs for Climate Change Impact Assessment Simulated flow peaks, volumes and hydrographs at the outlets of subbasins and the points of special interest such as reservoirs, weirs or other hydraulic structures Simulated long flow sequences for water budget and drought analyses Simulated extent of flooded areas for different precipitation events and various antecedent basin conditions

5. Hydrologic processes that need to be captured by the model Single-event precipitation-runoff transformation Continuous precipitation-runoff transformation Snow accumulation and melt Interception, infiltration, soil moisture accounting Evapotranspiration Regulated reservoir operation

6. HEC-HMS US Army Corps of Engineers, Hydrologic Engineering Center’s Hydrologic Modeling System software Designed to simulate both single event and continuous rainfall- runoff process Simulates precipitation-runoff and routing processes, both natural and controlled HEC-HMS uses a separate model to represent each component of the runoff process including:  runoff volume;  direct runoff (overland flow and interflow);  baseflow;  channel routing.

7. HEC-HMS representation of watershed runoff

8. Key Components of Model Runoff Volume models: separate infiltration from pervious surface, runoff from impervious surface, compute the direct runoff volume Direct Runoff models: transform direct runoff volume from excess precipitation into fast component of flow Base Flow models: compute slow subsurface drainage component Routing models: compute flow attenuation and translation over channel Reservoir models: flow regulation

9. Data Required Digital Elevation Model (DEM), land use, soil types and other physiographic data Precipitation, temperature data Evaporation/evapotranspiration data Discharge, Water level and Rating curve data Channel and reservoir hydraulic data Generated sequence of meteorological data representing various scenarios of future climate

10. Upper Chao Phraya Basin, Thailand Catchment Area = 105553 sq. km.

11. Dams and Reservoirs Bhumibol dam in the Ping River (Storage 13462 MCM) Sirikit dam in the Nan River (Storage 9510 MCM) Kwae Noi dam in Kwae Noi River (Storage 766 MCM) Kiew Kor Mha dam in Wang River (Storage 171 MCM) Kiew Lom dam in Wang River (Storage 112 MCM)

12. Data preparation using HEC-GeoHMS Delineate catchment and river network Obtain catchment characteristics data (area, slope etc) Make Thiessen polygon Obtain Thiessen weights Prepare basin file

14. Data preparation using HEC-DSSVue Time series data (rainfall, discharge etc.) Pair data (elevation-storage)

15. Model Setup Basin model Meteorological model Time series data Pair data Control specification

16. Meteorologic model Precipitation Evapotranspiration Snowmelt : not applicable for upper Chao Phraya

17. Precipitation methods Gauge weights : selected for upper Chao Phraya Inverse distance Gridded precipitation Frequency storm SCS storm Specified Hyetograph Standard project storm

18. Evapotranspiration methods Monthly Average : selected for upper Chao Phraya Priestley-Taylor Gridded Priestley-Taylor

19. Snowmelt methods Gridded temperature index Temperature index

20. Control Specifications Simulation start date/time Simulation end date/time Time interval

21. Model Calibration Finding optimal parameter values Minimizing difference between simulated flow and observed flow Objective functions Peak weighted RMS error Percent error peak Percent error volume RMS log error Sum of absolute residuals Sum of squared residuals Time weighted error

22. Search Algorithms Nelder Mead Univariate Gradient

23. The Basin Model

24. Simulated Hydrograph at Basin Outlet R2 = 0.71BIAS = 6.7 %NS = 0.71

25. Conclusions Semi-distributed physically based deterministic hydrological models are powerful tools for assessing climate change impact on water resources. Continuous modeling approach could be taken to assess the impact on flow volume. Care should be taken to interpret the results as there are lots of uncertainties in the model inputs, parameters and structure of the model. Uncertainties associated with climate models will also be carried over.

26. Thank You ! Dr. Dilip K. Gautam, Senior Hydrologist, RIMES E-mail: dilip.gautam@rimes.intdilip.gautam@rimes.int Website: www.rimes.int