Watershed Impact Assessment

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Watershed Impact Assessment

• An Introduction to Watershed Modeling using
• Nonpoint Source Pollution and
• Erosion Comparison Tool
• (N-SPECT)

Chris McColl Center for Spatial
Information RGIS USDA-CSREES
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Workshop Overview

• Introduction
• Tutorial 1 2
• Watershed Modeling Theory and Concepts
• Tutorial 3 4
• Watershed Delineation Background
• Watershed Delineation Practical
• Tutorial 5

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Workshop Objectives

• Become familiar with the N-SPECT
• Become comfortable with its application
• Develop an understanding regarding the
  theoretical framework that supports N-SPECT
• Understand how to compare and contrast watershed
  development scenarios using N-SPECT
• Learn how to delineate hydrologically correct
  watersheds how to check and fix them

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RGIS-Pacific Northwest

• Mission
• The mission of RGIS is to eliminate the digital
  divide facing rural America by promoting the
  transfer of geospatial technologies to
  under-served rural areas by
• providing geospatial tools, technologies, and
  training to empower local governments,
  organizations, and citizens to understand and
  participate in decisions that affect their
  environment, economy, and quality of life
• educating and training a cadre of people to apply
  geospatial technologies to rural issues

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RGIS-Pacific Northwest CSI
CSI Center for Spatial Information
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Linking Land Cover and Water Quality

• Land cover and water quality are directly linked
  through runoff generation.
• Runoff generation is controlled by
• Climate
• Vegetation
• Land cover and use
• Soil properties
• Topography
• Rainfall characteristics

Source Unknown
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Relationship of Urbanization and Runoff

• As urbanization increases, impervious areas
  increase
• Runoff volume and rate of conveyance to stream
  network increases
• Less groundwater recharge because of increases in
  imperviousness
• Impervious surfaces (e.g. transport roof tops)
  retain heat that is transferred to runoff,
  increasing stream temperatures.
• Transport-related imperviousness often exert a
  greater hydrological impact than rooftop-related
  imperviousness directly connected to storm
  drains!

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Runoff Pollutants

• During storms, accumulated pollutants are
  quickly washed off and rapidly delivered to
  aquatic systems
• Types of pollutants
• Sediment
• Cropland, construction sites, roadways, forest
  activities, and stream-bank erosion
• Nutrients
• Cropland, lawns and gardens, livestock
  operations, septic systems, an wildlife
• Pathogens
• Livestock, wildlife, septic systems, landfill
  sites, and urban runoff
• Toxic contaminants
• Roadways, croplands, lawns and gardens, and
  mining operations.

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Implications for management

• Land use planners, environmental planners,
  resource managers are faced with task of
  anticipating the impacts of their decisions.
• Want to make the most effective use of resources
  and mitigate for potential negative impacts
  alternative scenario analysis can support the
  process.
• Watershed modeling tools offer a powerful tool to
  comparatively assess the associated impacts of
  various alternatives upon regional resources.

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Watershed ModelsHow they can support management
decisions

• Evaluate resource use policies before
  implementation
• Alternative scenario comparison through
  quantitative assessment of impacts
• Directional change concerning environmental
  metrics
• Spatial analysis enabling managers to isolate
  problematic locations, pollution sources
• Development short-term and long-term impact
  assessments for specific pollutants
• Caution is required when applying mathematical
  models to describe complex hydrologic systems.

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Watershed Processes
Source USACE, 2003
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Types of Watershed Models

• Empirical versus physically based (eg. Curve
  Number vs Kineros)
• Deterministic versus stochastic (randomness)
• Lumped versus distributed
• Continuous versus event

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N-SPECT

• N-SPECT was developed as a screening level tool,
  not as an exact engineering tool.
• Provides spatial analytical support to resource
  managers seeking to understand the nonpoint
  source pollution and sedimentation associated
  with land use changes and management decisions.
• Analyses are approached from the watershed or
  subwatershed scale and estimate fluxes in
• surface water,
• pollutant, and
• sediment throughout the landscape.
• Estimates pollution and erosion within an area
  using land cover, soils, topography, and
  precipitation data.

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• Audience
• Coastal managers
• Land-use planners
• Scientists
• Teachers
• Development team
• Dr. David L. Eslinger, Jamie Carter, Margaret
  VanderWilt, Bev Wilson, Ed Dempsey, Andrew
  Meredith
• Major contributors
• Hawaii Coastal Zone Management Program
• NOAA Coastal Services Center (CSC)
• National Ocean Service Pacific Services Center
• Hawaiian management community

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Capabilities

• Estimating runoff depth
• Estimating pollutant loads and concentrations
• Identifying areas highly susceptible to erosion
  by water
• Estimating sediment loads and concentrations
• Assessing the relative impacts of land use
  changes with scenario analysis

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Tool Implementation

• N-SPECT requires
• ArcView 8.3 or ArcView 9.x
• ArcView Spatial Analyst extension
• Basic geographic information system (GIS) skills
• Land cover grid
• Digital elevation model
• Precipitation grid
• Set of land cover pollutant coefficients
• Water quality standards
• Soil type data
• Model Output
• Accumulated runoff, pollutant, and sediment load
  grids
• Pollutant and sediment concentration grids
• Pollutant assessment grid
• Ability to simulate land cover changes, such as
  developments

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Functions

• Rainfall-runoff model - USDA-SCS curve number
  technique
• Sediment yield model Universal Soil Loss
  Equation (USLE) uses both the MUSLE RUSLE
• Pollutant model Event mean concentration
  coefficients need Fortran program to develop
  custom coefficients.

Source NOAA-CSC, 2007.
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Limitations

• Omitted processes
• Atmospheric deposition
• Groundwater processes
• Stormwater drainage
• Stream diversions
• Snow melt
• Landslides
• No time dependency on
• Runoff dynamics
• Sediment redeposition
• Pollutant load

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Model Setup

• Data inputs
• Digital Elevation Model (DEM)
• probably most important!
• Land cover grid
• Rainfall grid
• Soils shapefile
• R-factor grid (annual erosion)
• Local pollutants coefficients
• Water quality standards

Source NOAA-CSC, 2006.
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Common Data Sources

• Land cover
• National Oceanic and Atmospheric Administration
  (NOAA) Coastal Services Center Land Cover
  Analysis
• Multi-Resolution Land Characteristics (MRLC)
  National Land Cover Database 2001
• U.S. Geological Survey (USGS) Land Cover
  Institute
• Topography
• NOAA Coastal Services Center Topographic Change
  Mapping
• USGS National Elevation Dataset
• Soil
• U.S. Department of Agriculture (USDA) Natural
  Resources Conservation Service (NRCS) Soil Survey
  Geographic Database
• R-Factor (rainfall-runoff erosivity factor)
• USDA NRCS Electronic Field Office Technical Guide
  (eFOTG)
• USDA NRCS National RUSLE2 Database
• Precipitation
• Oregon Climate Service PRISM Group
• NOAA National Climatic Data Center

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NOAA Coastal Services Center -Tutorial 1 2

• Tutorial 1 Baseline Analysis (Accumulated
  Effects) Basic walk through of the N-SPECT
  model, presents baseline approximations of
  various nonpoint pollution variables
• Tutorial 2 Baseline Analysis with Local Effects
  to produce outputs that represent the results
  of processes occurring in each individual pixel.