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A Survey of Major Watershed (Rainfall-Runoff) Models
Knowing what is out there in the Workplace
Capabilities of each model Requirements of each model What is the best model? Examples
Ungaged 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
GOALS
Be aware of the Major Models out there and their basic workings
Understand 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 Cycle
All 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
Infiltration
Unsaturated Zone flow
Groundwater (saturated
zone flow)
Overland Flow
River Flow
Evapotranspiration
Rainfall is usually the major input in Hydrologic Models – it is not modeled!
Classification of Models
Based on mathematical representation of processes
Conceptual Models Physically-based Models
Based spatial nature of input/output of modelsLumped Rainfall, Runoff, ET, GW flowDistributed Rainfall, Runoff, ET, GW
flow
Conceptual Vs Physically Based
Conceptual 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 Models
Rainfall 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 requirement
More Conceptual requires less data!
Data acquisition is an investment that needs to be commensurate with the model
Sources of Data
Rainfall – Gage, Radar (NWS), Satellite
Soils – STATSGO, USDA
DEM – USGS, Topo maps, Satellites
Vegetation – Satellites, USDA
Streamflow – USGS
ReliabilityReady availabilityCost $$$Resolution (space,
time)
Is all data needed?
Physically-based – Insatiable needs – but potential high returns
Conceptual – modest needs with modest returns
Calibration –needs historical data. What is Calibration?
What to do in ungaged regions where there is ‘No Data’? Transfer experience. PUB
What affects $$ operational costs of a Model?
The multiple needs of the TVA river reservoir system
Level of data needs – Data is expensive Level of physical complexity (Physically-
based means qualified personnel; Rational Method – no-brainer
Scale of the problem being addressed – larger/heterogenous – more $$$
Some Models Out there
HEC-1 PRMS HSPF (Fortran) MIKE 11 Make your own survey if you wish
(Consider, DSS – Riverware, BASINS)
For a comprehensive guide refer to: VP Singh – Computer Models for Watersheds
HEC-1
Developed 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, Holtan Data Needs – sub-basin delineation, rainfall, runoff, routing
parameters Based on Unit Hydrograph or kinematic for runoff routing Other capabilities – Snowfall/melt, dam safety, pumping,
diversions Customer Support? – Try HEC, San Diego, CA.
PRMS – Precipitation-Runoff Modeling System
Developed by USGS – more of an educational tool to build your own models
Lumped/sub-basin –continuous simulation – see Handout
Data needs – (depends) precip, streamflow, DEM, minmax air temp, radiation, vegetation..(see handout)
Code in F77Capabilities - manyCustomer Support? – Try USGS
HSPF Hydrological Simulation Program - Fortran
USGS – 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 11
Danish 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 SHE
USGS – educational, conceptual/physical
Data needs?SoftwaresCapabilitiesMerits/Demerits (can/can not do)Customer Support?Real Example -
What is the best Model?
This is situational – It All Depends!
Primary objectives
$$ budget
Level of Tolerable Accuracy/precision
Complexity and uniqueness of the problem
Time frame for delivery
Data availability
It’s a highly complex question whose answer is sensitive to a multiplicity of factors
Homework Problem
Given watershed,Given dataGiven $$, deadlineChoose your model (make strong
arguments in favor of your choice over others)