An Introduction to

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An Introduction to Watershed Modeling Micha
el D. Smolen Daniel E. Storm Mike White Phil
Busteed Biosystems and Agricultural Engineering
Department Oklahoma State University
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What is a Watershed?

• All the land area that drains to a point on the
  landscape it can be any size.
• Delineated by topography.
• To determine the watershed boundaries, ask the
  question, "if a drop of rain lands at a
  particular location, which way will it go?"

Watershed Outlet
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What is a Watershed Model?
A watershed model is a mathematical
representation of the hydrologic cycle, applied
to the watershed.
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Watershed Model Processes

• Rainfall-runoff and pollutant transport
• Infiltration and ground water
• Evaporation and evapotranspiration
• Crop growth
• Soil and in-stream biological and chemical
  processes
• Topography, soils, land use, and land management

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Why Use Watershed Models?

• To evaluate the effects of changing land-use and
  management
• To target optimal locations for BMPs
• To Identify and quantify pollutant sources
• To determine the overall effect of watershed
  management on a receiving water body
• To extend the information derived from monitoring

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Why not just monitor the water?

• Advantages
• Direct measurement
• May identify the smoking gun if there is one.
• Disadvantages
• Expensive and logistically very difficult
• Requires long duration studies for statistical
  validity

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Why not monitor?

• Some things cannot be measured directly.
• In long term monitoring things change.
• A model allows you to isolate the part of the
  system that you are interested in.
• A model extends the information from monitoring.

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There are many models available - some are very
specialized.

• SWAT Soil and Water Assessment Tool, developed
  by USDA-ARS in Texas
• HSPF Hydrologic Simulation Package Fortran,
  developed by a Aqua Terra, consultant to EPA
• AGNPS Agricultural Nonpoint Source, developed
  by USDA-ARS in Minnesota
• Many others

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Soil and Water Assessment Tool (SWAT)
Modeling Objective To predict the effect of
management decisions on water, sediment, nutrient
and pesticide yields with reasonable accuracy on
large, ungaged river basins.
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Data Needed to Run SWAT

• GIS database - to describe the watershed
  (topography, soils, land use)
• Time series of weather rainfall, temperature,
  solar radiation
• Time series of management activities - grazing,
  tillage, planting and harvesting, applying
  fertilizer or pesticides

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SWAT Model Operation

• Watershed is divided into sub-basins based on
  topography
• Land use and soils information is used to define
  HRUs (Hydrologic Response Units)
• Runoff, sediment, and nutrients are routed from
  HRUs through the sub-basins and the stream
  network to the watershed outlet.

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Modeling theLake Eucha Basin using SWAT
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Lakes Eucha Spavinaw
Livestock Production

• Poultry Integrators Peterson, Simmons, Tyson
• 300 Farms
• 700 Poultry Houses
• 120 million Birds
• 92,000 tons Litter/Year
• Cattle inventory 39,000 hd

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Modeling Objectives

• Evaluate P loading from nonpoint sources
• Identify sources of P
• Determine relative contributions by source
• Determine point source loading
• Evaluate potential P loading reduction possible
  from BMP implementation and watershed management

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SWAT Input GIS Data
Topography USGS 124,000 DEM
Soils Oklahoma - MIADS Arkansas - County Soil
Survey
Land Use 1100,000 GAP Analysis Project
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Poultry House Locations and Soil Test Phosphorus
on Pastures in Lake Eucha/Spavinaw Basin
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Point Source just one

• The City of Decatur - the only point source
  considered.
• The point source contributes 24 of the total
  phosphorous entering Lake Eucha.

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Nutrient Loading Data from Monitoring
Data Used to Calibrate SWAT Model
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Three Gauges for Flow Calibration
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Modeling Results Phosphorus Load Lake
Eucha/Spavinaw Basin
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Total Phosphorus Load Allocation Lake
Eucha/Spavinaw Basin
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Best Management Practice Scenarios30-year
Simulation

• 3x current litter application rate increased STP
  from 250 to 1300 lb/ac.
• Continuing current litter application rate
  increased STP by 300 lb/ac.
• No application gave a decline of 35 lb/ac.

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Targeting CriticalSediment and Nutrient
Sourcesin Oklahoma Watersheds
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Targeting Critical Areas(Stillwater Creek
Watershed)
5 of area (in red) gives 75 of sediment load
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Modeling theFt Cobb Basin using SWAT
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Introduction

• Fort Cobb Basin 314 mi2
• Primary land use is row crop and small grain
• Ft Cobb Reservoir 4,100 acres
• Listed on Oklahomas 303(d) List

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Ft Cobb Watershed
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Modeling Objectives

• Evaluate sediment loading from peanuts and wheat
• Evaluate P loading from peanuts and wheat
• Evaluate potential P loading reduction from BMP
  implementation
• Peanuts with Winter Fallow
• Peanuts with Winter Cover
• Wheat with Conventional Tillage
• Wheat with Conservation tillage

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GIS Input DataTopography from 10 m DEM
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Sub-basins
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Methods/Input Land Use Data
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Flow Calibration and Validation

• Only one suitable flow gage at Cobb Creek near
  Eakley, OK (1995-2001)
• USGS gage below the reservoir dam was used to
  evaluate flow calibration (Cobb Creek at Fort
  Cobb)
• Flow validation, 1975-1989

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Flow Calibration Results
Validation Period, lt 6.5 relative error
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Modeling BMP Scenarios
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Uncertainty and Sources of Error

• Simplifying assumptions of the model
• Parameter error measurement error or
  calibration error
• Processes simplifications, empiricisms, or
  incorrect representations
• Model structure programming problems
• Input data errors
• Static characteristics e.g. soils, topography,
  land use
• Dynamic characteristics e.g. land use, climate,
  management
• Calibration data
• Knowledgeable and trained users essential!!!

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Modes of Use Absolute vs. Relative Prediction

• Absolute Prediction - What is the expected
  concentration or load?
• Permit writing
• Implementing water quality standards
• Setting TMDLs
• Relative Prediction
• Determining changes from a baseline condition
• Targeting critical source areas
• Evaluating What If scenarios
• Evaluating BMP implementation

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Modeling Employs our Best Science in a Rational
Approach