Christina Hurley
Exercise 3
Map 1: New Hope Catchment polygons with Vector Drainage lines and NHD Flowlines
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 344 subcatchments formed in my watershed partitioning. I used a 3100 pixel drainage area to define the beginning of each channel. 3100 pixels x 900m^2 (30m x 30m cells) = 2,790,000 m^2 = 279 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?
The Arc Hydro Model only accounts for natural water systems, based on the digital elevation model (Terrain Processing). It does not account for water infrastructure that re-directs flows, nor does it account for geological features, such as groundwater systems. Furthermore, we based flow direction on the D8 system, which allows a water on one cell to flow into only one of eight directions (E,SE,S,SW,W,NW,N,NE). This creates a one-pixel wide, unique path for streams. In addition, we had the ability to define a stream based on a drainage area we felt was appropriate (trying to match as closely as possible to the NHD Flowline). This process is somewhat subjective to the individual’s preferences, and assumes that the NHD Flowline is accurate.
To improve watershed delineation, we would need to use a model that takes urban water drainage infrastructure into account. In addition, a model that considered factors such as level of imperviousness (curve number), soil saturation, and groundwater storage should be considered. Also, a more accurate representation of the field could be achieved by using a D-infinity drainage direction system, so that a single, unique path is not assumed. We saw an example of this with the TAS GIS Wetness Index in class.
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?