CEEn 531 - Water Resources Engineering

Dr. Nelson

CN Computation Assignment

Complete the WMS tutorial on Time of Concentration Calculations and Computing a Composite CN then read the following set of questions and write half a page paragraph about what you have learned.

Using the data for your watershed delineate the boundaries and compute a composite CN with the best the avialable land use and STATSGO or SSURGO soils data.  Compute the percent overlay of land use and soils and using the results be sure that you have defined in your landuse.tbl file the necessary information to accurately compute CN.

Vector Data Mapping Table with all USGS LUCODE's.

Grid Data Mapping Table with all USGS LUCODE's.

Hydrological Parameters from Digital Data 

  1. What are the two main classes of input parameters most hydrologic models require?

  2. How does the application of the GIS concept of “map overlay” help in computing composite hydrologic parameters?

  3. What are the required GIS data for the computation of each of the following: composite curve numbers, runoff coefficients, and Rainfall?

  4. Which watershed produces more runoff, watershed A with CN 94, watershed B with CN 63? Why?

Extra Questions:

Here are some extra questions for you to think about.

  1. To be able to overlay a soil map to compute hydrological parameters, what is the key attribute needed to be in the attribute table of the map? What is it for the land use map?

  2. In WMS, in order to compute composite curve numbers, we will need to read in a table relating land use and soil type to curve numbers. Why do we need to do that? 

  3. What are the two methods that WMS offers to compute time of travel?

  4. What is the relationship that WMS uses between lag time and time of concentration?