Hydrological Modeling
description
Transcript of Hydrological Modeling
Hydrological Modeling
Overview
•Introduction•Watershed delineation•Automatic delineation•Flow length
Introduction
Watershed management
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
Watershed delineation
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
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
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
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
2. Flow direction
flow moves out of a cell in one of 8 directions
direction of flow is saved as a code number
2. Flow direction
north-flowing cells
coded as 64
2. Flow direction
Flow direction grid
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
3. Flow accumulation
1
3
5
3. Flow accumulation
single class legend
shows high flow cells
3. Flow accumulation
Fit depends on accuracy of the DEM and stream layers
3. Flow accumulation
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
4. Watershed Pour points
•Zoom in to place pour point in center of high-flow cell
•Create as many pour points as necessary
5. Delineating watersheds
•Preliminary steps are completed–Filled DEM–Flow direction–Flow accumulation–Pour points created & converted to grid
•Run tool to create watersheds
5. Delineating watersheds
Watersheds represent area upstream from Pour points and terminate at ridgelines, uphill sub-basin boundary, or edge of the grid
5. Delineating watersheds
Automatic delineation
Automatic delineation
Pour points automatically selected by “intersection” of highest-flow pathways and grid edge
Automatic delineation
Flow length
Flow lengthFlow distance for every cell to outlet
Flow lengthFlow distance for every cell to closest stream
Flow length
Euclidean distance vs. flow distance
Homework
Read: Hydrological Modeling & Watershed Delineation, Map Layouts
Study for the exam
Presentations:
3/5 Luke, Jennifer, Josh
3/7 Chris, Prati, Carolyn