A Survey of Major Watershed (Rainfall-Runoff) ModelsKnowing what is out there in the WorkplaceCapabilities of each modelRequirements of each modelWhat is the best model?ExamplesUngaged area needing WRE management (expansion)Balancing $$, needs and requirements (Consulting)Flood-frequency analyses, forecasting, extrapolating for future scenarios

Consider this Scenario:1. A major expansion of urbanization is planned in forested land malls, apts, parks, industry

2. How will the planned urbanization change the response of the land to rainfall? Increase chances of floods? Flashier? Bring down GWT? What kind of Water Resources Management Strategy/Measures would be needed to ensure long-term water needs?Watershed Models are used to answer such questions

GOALSBe aware of the Major Models out there and their basic workingsUnderstand Data needs and requirements Merits/Demerits of Models (situational)Ponder over: What is the Best Model? (open question)Be able to make an informed decision on the choice of a Model (for Consulting, Industry, Public Works)

The Hydrologic CycleAll watershed Models (or Rainfall-Runoff) attempt to model the Hydrologic Cycle at the watershed scale.Watershed? Natural System with all major components of cycleHydrologic Cycle? (Water Cycle) Water DistributionModel? Abstraction of reality using Mathematics

The Major Processes of a Model

InfiltrationUnsaturated Zone flowGroundwater (saturated zone flow)Overland FlowRiver FlowEvapotranspirationRainfall is usually the major input in Hydrologic Models it is not modeled!

Classification of ModelsBased on mathematical representation of processes Conceptual ModelsPhysically-based Models

Based spatial nature of input/output of modelsLumped Rainfall, Runoff, ET, GW flowDistributed Rainfall, Runoff, ET, GW flow

Conceptual Vs Physically BasedConceptual Models Idealization of processes as stores, buckets, parameterizations simplified equations representing mass, momentum, energy.

Physically-based Models rigorous numerical solution of partial differential equations governing flow through porous media, overland and channel flows.

Lumped Vs Distributed Models

Distributed - The spatial and temporal variation of meteorological, hydrological, geological and hydrogeological data across the model area is described in gridded form for the input as well as the output from the model Lumped - The spatial and temporal variation of meteorological, hydrological, geological and hydrogeological data across the model area is described as one aggregated value for input and output!All models are lumped at the finest scale!

Data Requirements for ModelsRainfall Data (Major Input)Soils Data (Infiltration, Runoff)DEM channel network (River routing)Vegetation Data (For ET)GWT Data (Saturated zone flow)Historical Rainfall-Streamflow Data (Calibration)Evaporation Data (ET)

More Physically-based means more data requirementMore Conceptual requires less data!Data acquisition is an investment that needs to be commensurate with the model

Sources of DataRainfall Gage, Radar (NWS), SatelliteSoils STATSGO, USDADEM USGS, Topo maps, SatellitesVegetation Satellites, USDAStreamflow USGS ReliabilityReady availabilityCost $$$Resolution (space, time)

Is all data needed? Physically-based Insatiable needs but potential high returnsConceptual modest needs with modest returnsCalibration needs historical data. What is Calibration?What to do in ungaged regions where there is No Data? Transfer experience. PUB

What affects $$ operational costs of a Model? The multiple needs of the TVA river reservoir systemLevel of data needs Data is expensiveLevel of physical complexity (Physically-based means qualified personnel; Rational Method no-brainerScale of the problem being addressed larger/heterogenous more $$$

Some Models Out thereHEC-1PRMSHSPF (Fortran)MIKE 11Make your own survey if you wish (Consider, DSS Riverware, BASINS)For a comprehensive guide refer to: VP Singh Computer Models for Watersheds

HEC-1Developed by Hydrologic Engineering Center (HEC) of US Army Corps of Engineers

Flood Hydrograph Package single storm even simulation. Lumped model

Loss Function approach SCS, Green Ampt, HoltanData Needs sub-basin delineation, rainfall, runoff, routing parametersBased on Unit Hydrograph or kinematic for runoff routingOther capabilities Snowfall/melt, dam safety, pumping, diversionsCustomer Support? Try HEC, San Diego, CA.

PRMS Precipitation-Runoff Modeling SystemDeveloped by USGS more of an educational tool to build your own modelsLumped/sub-basin continuous simulation see HandoutData needs (depends) precip, streamflow, DEM, minmax air temp, radiation, vegetation..(see handout) Code in F77Capabilities - manyCustomer Support? Try USGS

HSPF Hydrological Simulation Program - FortranUSGS multi-use modelCan do water quality.Mainly for land-use change, reservoir operations, flow diversions etc.Data needs A lot (see Hand-out)Software In Fortran 77Capabilities A lot (see hand-out)Customer Support? Try USGSReal Example 62,000 sq mi tributary area in Chesapeake Bay.

MIKE 11Danish Hydraulic Institute DHIVery sophisticated physically-based, high-end modelData needs a lot (dependent of in-situ measurementsSoftwares commercial (and expensive)Capabilities Many (see hand-out)Customer Support? Try DHI or VendorsReal Example River Jamuna, Bangladesh

MIKE SHEUSGS educational, conceptual/physicalData needs?SoftwaresCapabilitiesMerits/Demerits (can/can not do)Customer Support?Real Example -

What is the best Model?This is situational It All Depends!Primary objectives$$ budgetLevel of Tolerable Accuracy/precisionComplexity and uniqueness of the problemTime frame for deliveryData availabilityIts a highly complex question whose answer is sensitive to a multiplicity of factors

Homework Problem Given watershed,Given dataGiven $$, deadlineChoose your model (make strong arguments in favor of your choice over others)