Exercise 3
: Flow path mapping with Arc Hydro
Due Date: February 24, 2012
Report: ex3_youronyen_report.doc in your personal working directory
Dataset:
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Ned_nh: A DEM (elevation) raster covering the New Hope Creek area.
nhdflowline_nh: NHD stream network line data of New Hope Creek area
Road_nh: road network vector data
Nlcd2006: 2006 National Land Cover Data of New Hope Creek area
Nlcd2006_imp: 2006 Impervious proportion (0-100) for each grid cell
All datasets of this exercise are in
\\isis.unc.edu\html\courses\2010spring\geog\591\001\exercises2012\ex3.
Open arc hydro
: Customize  Toolbars  Check “Arc Hydro Tools”
Task 1: Derive stream network and associated catchments
1. Select Base DEM Grid: ned_nh
2. Fill DEM
- In Basic Grid Analysis in sequence from top to bottom, click "Fill Pits."
- Name output filled grid “ned_nhfil.”
- Check “Add layer(s) upon completion.”
3. D8 flow direction
- Input: filled DEM
- Output D8 flow direction grid: ned_nhfdr
4. D8 contributing area
- Input: D8 flow direction grid, ned_nhfdr
- Output D8 contributing area: ned_nhfac
5. Stream delineation 1
- Click “Stream Definition” and choose a “reasonable” drainage area. Look at the
extent of the “nhdflowline” stream network to help with this choice.
- Output defined stream network: nhStr_(drainage_area) (fill in your threshold drainage
area)
- Experiment with input numbers for “number of cells” field. Will probably be 2000 or
3000. You want it to reasonably match the “nhdflowline.”
Here is a small portion of my map with drainage area of 2500.
6. Stream segmentation: nh_Strlnk_(drainage_area)
7. Catchment Grid Delineation: nh_Str(drainage_area)_cat
8. Catchment polygon processing to form vector catchment polygons: nh_Catchment
9. Drainage Line Processing to form vector stream network: nh_DrainageLine
10. Make map with 1) vector stream (nh_DrainageLine) and 2) catchment boundary
(nh_Catchment) that you made in Task1, and 3) nhdflowline.
Q1. How many subcatchments were formed in your watershed partitioning? What was
the drainage area in hectares that you used to define the beginning of each channel?
422 subcatchments were formed in my watershed partitioning, according to the number of entries
in the Attribute Table of nhCatchment. I used a threshold drainage area of 2500 pixels. Each
pixel has area 20’ x 20’ because the resolution of the DEM is 20.’ This means that each pixel has
area 400 square feet. There are 9.29 x 10-6 hectares per square foot. Multiplying 2500 pixels by
400 square feet per pixel and then converting to hectares results in a threshold drainage area of
9.29 hectares.
Q2. Remember this is all a model. What key assumptions were made, and how would you
want to improve the watershed delineation to better match field conditions?
A key assumption we made was that it was alright to construct the stream network based on the
DEM, which is an elevation model. This didn’t take into account any actually-observed stream
paths. In order to better match field conditions, we could burn known streams into the DEM first.
Task 2: Derive catchment level statistics of land cover
1. Zonal statistics (mean) for impervious cover for all catchments – map and table using
hydroid
Mean impervious cover table for first 22 catchment entries
2. Zonal statistics (majority) for land cover for all catchments – map and table using hydroid
Majority land cover table for first 22 catchment entries
3. Zonal statistics histogram for land cover for all catchments – table using hydroid
Part of histogram.
Q3. Make impervious cover and land cover zonal statistics maps with your catchment
boundary shape file. According to the zonal maps and tables, what is the range of %
impervious area for the catchments?
Impervious cover zonal statistics map:
Land cover zonal statistics map:
The range of % impervious area for the catchments is 0% - 60.7%. I determined this from the
mean % impervious by zone table and by looking at the range of values displayed on the mean %
impervious by zone map.