Hydrological Modeling
description
Transcript of Hydrological Modeling
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Hydrological Modeling
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Overview
•Introduction•Watershed delineation•Automatic delineation•Flow length
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Introduction
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Watershed management
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Definition of watershed
• “The region draining into a river, river system, or body of water”
-American Heritage Dictionary
• The upstream area of any given point on the landscape
• Physically defined by drainage point and upstream area
• Also known as basin, sub-basin, catchment, and contributing area
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Watershed delineation
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How it works
•Water always flows downhill•For any point on a grid representing a landscape,a drop of water can be traced downhill direction of flow is known for every DEM cell
•For any point on a grid representing a landscape,a flow pathway can be traced back uphill
flow accumulation is known for every DEM cell
•Uphill back-tracing proceeds to a ridgeline or to the edge of the grid
•Termination of uphill back-tracing defines watershed boundary
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Watershed delineation
Steps (with ArcToolbox):1. Create a depressionless DEM
2. Calculate flow direction
3. Calculate flow accumulation
4. Create watershed Pour points
5. Delineate watersheds
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1. Creating a depressionless DEM
• DEM must eventually drain off edge of grid• Areas of internal drainage will result in unprocessed areas
• FILL routine fills in sinks or cuts off peaks creating a new grid with no drainage errors
elevation
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2. Flow direction
• Every cell flows into another cell or off the grid edge
• Flow direction is calculated as the direction of steepest downward descent
• Flow direction is calculated for each cell, resulting in a new grid theme
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2. Flow direction
flow moves out of a cell in one of 8 directions
direction of flow is saved as a code number
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2. Flow direction
north-flowing cells
coded as 64
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2. Flow direction
Flow direction grid
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3. Flow accumulation
• Each cell has been coded for direction of flow
• Cumulative flow is calculated from flow direction
• Output grid is created where values are the number of upstream cells
• Lower accumulation values are ridge tops
• Higher accumulation values are valleys & stream channels
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3. Flow accumulation
1
3
5
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3. Flow accumulation
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single class legend
shows high flow cells
3. Flow accumulation
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Fit depends on accuracy of the DEM and stream layers
3. Flow accumulation
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4. Watershed “Pour points”
• Watersheds are defined by outlets (pour points)
• Pour points should be placed in high-flow pathways
• Basins will be generated from pour point to ridgeline or to upstream sub-basin
• Pour points should be numerically coded per sub-basin
• Pour points must be converted to a grid layer
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4. Watershed Pour points
•Zoom in to place pour point in center of high-flow cell
•Create as many pour points as necessary
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5. Delineating watersheds
•Preliminary steps are completed–Filled DEM–Flow direction–Flow accumulation–Pour points created & converted to grid
•Run tool to create watersheds
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5. Delineating watersheds
Watersheds represent area upstream from Pour points and terminate at ridgelines, uphill sub-basin boundary, or edge of the grid
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5. Delineating watersheds
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Automatic delineation
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Automatic delineation
Pour points automatically selected by “intersection” of highest-flow pathways and grid edge
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Automatic delineation
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Flow length
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Flow lengthFlow distance for every cell to outlet
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Flow lengthFlow distance for every cell to closest stream
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Flow length
Euclidean distance vs. flow distance
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Homework
Read: Hydrological Modeling & Watershed Delineation, Map Layouts
Study for the exam
Presentations:
3/5 Luke, Jennifer, Josh
3/7 Chris, Prati, Carolyn