Q1. How Many Subcatchments Were Formed in Your Watershed Partitioning? What Was the Drainage

Jason McMahon

GEOG591

2/15/12

EX3

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 534 subcatchments formed in my watershed partitioning. I chose a minimum drainage area of 2000 pixels for my threshold of determining a stream channel. This is the equivalent of:

2000 pixels * (20ft*20ft per pixel) = 800,000 ft2

800,000 ft2 = 7.43 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?

One assumption made in this watershed delineation is that the specific drainage threshold is representative of actual real-world conditions. Because this number was derived based on judgment of similarity with the NHD dataset, its accuracy relies on that of the NHD dataset itself. Therefore, to improve the delineation, it would make more sense to observe actual behavior of the watershed to determine a threshold that is not reliant on and subject to the inaccuracies of an existing dataset. Perhaps field measurements could be taken to sample the average area upstream of flowing channels.

Another assumption made in this model is that water only flows in 1 of 8 directions, based on the D8 flow direction model. Perhaps a D-infinity model could be used in the future that would more accurately distribute the water over the DEM layer.

In addition, it was assumed that the DEM layer is a good representation of the actual topography in the New Hope Creek region. There may be inherent errors in the DEM layer that could have contributed to a decline in the quality of the watershed delineation. For instance, the removal of pits could have affected a major drainage contribution or a collection area that would have drastically changed the drainage and watershed pattern. In addition, the accuracy of traditional DEM layers can be questionable, considering that newer and more accurate technology such as LIDAR can now be utilized. Employing a LIDAR dataset would be a good way to enhance the quality of the watershed delineation.

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 of % impervious area for the catchments is 0% to 60.7195%.