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.
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?
305 subcatchments were formed. The drainage area was defined as 3500 cells, each cell is 20 ft
by 20 ft, so 3500 cells * 400 ft2/cell * (9.290304 × 10-6) hectares/ft2 = 13 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?
Stream network made from DEM, which is just elevation. An important assumption is that
elevation can be used to map the stream network. The network is made purely from a DEM,
which uses the elevation values to map the stream. This is where problems like pits and sinks
occur, so they must be filled. The problem is that by filling these pits, we are really simplifying
the terrain. There could be storm water holding areas, which could account for the pits, or
various other things in the topology of the area that causes the streams to not have as much
connectivity as this model might suggest. We are also use D-8 flow direction mapping, which is
a problem because it only sees streams as lines instead of something that has width. It also only
allows one direction of flow, whereas a stream might have multiple flow directions. Another
issue is that it is very difficult to make the stream network derived from this data match the
actual flow lines from vector mapping. The data will always be a little off, but that is to be
expected with models.
To improve the model to better match field conditions, one could find out specifically which
sinks not to fill, and find a way to allow the streams to flow in more than one direction, such as
using D-infinity as opposed to D-8 flow direction. This would allow a more natural flow of the
stream to find more natural catchment boundaries.
Task 2: Derive catchment level statistics of land cover
1. Zonal statistics (mean) for impervious cover for all catchments – map and table using
hydroid
2. Zonal statistics (majority) for land cover for all catchments – map and table using hydroid
3. Zonal statistics histogram for land cover for all catchments – table using hydroid
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?
According to the tables and the mean percentage map, the highest percentage of impervious land
cover is 60.7195% and the lowest is 0%.