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)
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.
The graph above is at a 2300 threshold stream definition.
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?
There were 451 subcatchments formed in my watershed partitioning. I used 2300 to define the
beginning of the stream.
(400 ft^2)(0.000009 hectares/ft^2)(2300 pixels) = 8.28 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?
Key assumptions= The drainage area number that we choose was completely accurate. We
couldn’t match up the streams exactly with the nd_flowlines and since our streams weren’t
completely accurate when we performed the stream delineation and segmentation, then our
subcatchment polygons aren’t completely accurate either.
Improvements= Better define the drainage network. Instead of just choosing a minimum area,
we could also choose a minimum pixel length.
Task 2: Derive catchment level statistics of land cover
1. Zonal statistics (mean) for impervious cover for all catchments – map and table using
hydroid
From the table, the mean overall mean is 6.5.
2. Zonal statistics (majority) for land cover for all catchments – map and table using hydroid
From the table, the majority is land cover type is 41, which is deciduous forest. There is also a lot
of “developed, open space” (or 21 in the eastern region) .
3. Zonal statistics histogram for land cover for all catchments – table using hydroid
I wasn’t able to generate a zonal histogram in ArcMap, I tried numerous different ways and
restarted the whole lab, but I couldn’t get it to function properly. I kept getting an error that said,
“ERROR 010240: Could not save raster dataset with output format GRID.” So instead, I opened
the table for the majority landcover pixels and exported the data to an Excel file. I created this
graph in Excel that shows the landcover and the number of pixels each type has in the catchment
basins.
Pixels per Land Type
250000
200000
150000
100000
50000
0
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?
The range is anywhere from 0 to 58.7% impervious surface.