Hydrological Modeling for Upper Chao Phraya Basin Using HEC-HMS

UNDP/ADAPT Asia-Pacific First Regional Training WorkshopAssessing Costs and Benefits of Adaptation: Methods and Data

March 11-14, 2013

Dr. Dilip K. GautamRIMES, AIT Campus, Bangkok

Hydrological ModelA 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.

Classification of Hydrological Models

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

Hydrologic processes that need to be captured by the model

Single-event precipitation-runoff transformation

Continuous precipitation-runoff transformation

Snow accumulation and meltInterception, infiltration, soil moisture

accountingEvapotranspirationRegulated reservoir operation

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.

HEC-HMS representation of watershed runoff

Key Components of ModelRunoff 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

Data RequiredDigital Elevation Model (DEM), land use, soil

types and other physiographic dataPrecipitation, temperature dataEvaporation/evapotranspiration data Discharge, Water level and Rating curve dataChannel and reservoir hydraulic dataGenerated sequence of meteorological data

representing various scenarios of future climate

Upper Chao Phraya Basin,

ThailandCatchment Area = 105553 sq. km.

Dams and ReservoirsBhumibol 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)

Data preparation using HEC-GeoHMSDelineate catchment and river networkObtain catchment characteristics data (area,

slope etc)Make Thiessen polygonObtain Thiessen weightsPrepare basin file

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

Model SetupBasin modelMeteorological modelTime series dataPair dataControl specification

Meteorologic modelPrecipitationEvapotranspirationSnowmelt : not applicable for upper Chao

Phraya

Precipitation methodsGauge weights : selected for upper Chao

PhrayaInverse distanceGridded precipitationFrequency stormSCS stormSpecified HyetographStandard project storm

Evapotranspiration methodsMonthly Average : selected for upper Chao

PhrayaPriestley-TaylorGridded Priestley-Taylor

Snowmelt methodsGridded temperature indexTemperature index

Control SpecificationsSimulation start date/timeSimulation end date/timeTime interval

Model CalibrationFinding optimal parameter valuesMinimizing difference between

simulated flow and observed flowObjective functions

Peak weighted RMS errorPercent error peakPercent error volumeRMS log errorSum of absolute residualsSum of squared residualsTime weighted error

Search AlgorithmsNelder MeadUnivariate Gradient

The Basin Model

Simulated Hydrograph at Basin OutletR2 = 0.71 BIAS = 6.7 % NS = 0.71

ConclusionsSemi-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.

Thank You !

Dr. Dilip K. Gautam, Senior Hydrologist, RIMESE-mail: dilip.gautam@rimes.int

Website: www.rimes.int