Nicole Meyer
Exercise 3: Flow Path Mapping with ArcHydro
Q1: When opening the attribute table for the vector catchment polygons, nh_Catchment, there are 305 subcatchments formed in a 3500 drainage area.
2
2
3150000𝑓𝑡
2
𝑥
9.29 𝑥 10 −6 ℎ𝑒𝑐𝑡𝑎𝑟𝑒𝑠
1𝑓𝑡 2
= 29.3 ℎ𝑒𝑐𝑡𝑎𝑟𝑒𝑠

This is a 30m resolution DEM. So the drainage area is (30m * 30m) *(2000~3500 pixels) *0.000 1 m2/ha
= 180 ~ 315 ha (-1.0)
Q2: There are several assumptions mad when working with a model. For example, when using the flow direction tool you assuming that if two points have the same elevation value that they are connected.
Also, in NED the value of any elevation is an estimate and the value of streams are an estimate. Streams can shift direction and 1mm off in digitizing can correspond to 100m in error. Also, in these GIS models you are assuming that no pits are real. In reference to the stream delineation, the lower the value of the drainage area, the more detail (branching) you will see on the map (see below). Through trial and error you have to see which value best corresponds to the NHD stream network flow line. After comparing several values I chose 3500 as a reasonable drainage area.
The assumptions are
1) We based flow direction on the D8 system. So we are assuming that water does not flow in split paths but just flows in one D8 direction from a higher elevation in the DEM.
2) It does not account for water infrastructure that re-directs flows.
Ways to improve are
1) D-inf flow direction
2) burning process by vector stream data
3) considering urban water drainage infrastructure (-3.0).

Q3.) Make impervious cover zonal statistics map with catchment boundary shape file

Make land cover zonal statics map with catchment boundary shape file
What is the range of % impervious area for the catchments:
The average for the zonal mean for impervious cover over all catchments was 6.49. (Every time I tried to create a zonal histogram, ArcMap continued to crash)
The range of mean % impervious cover is 0 – 60.7 % (-2.0)
(19/25)