Dr. Nelson

OverviewYou have learned the basic modeling techniques and performed various sensitivity studies with the precipitation and infiltration methods in HMS and GSSHA. In this exercise we will examine how HMS develops the runoff hydrograph, which is often termed

Transformation.Our sensitivity studies will again center around the Judy's Branch Watershed and you can use the same data as you have previously. We will use different transformation methods and see how they affect output. You will perform similar analyses with GSSHA (in the next assignment) and at the end you will compare the response

Model Set upUse your basic Judy's branch WMS project with the SSURGO soil information and the same land use data you have been using.

Create land use and soil type coverages in WMS and determine the curve number for the watershed as usual when necessary.

Processes

1.Transformation method comparisonIn this part we will the see variation in output hydrograph as we change the transformation method. Create a HMS model with:

a. SCS Transformation Method

Watershed: Single Basin

Loss Rate Method: Green and Ampt ( Initial loss = 0.2 in, Saturated content = 0.45, Initial moisture = .25, suction = 12 in, Conductivity = 0.25 in/hr, Impervious = 0%)

Transform method: SCS

Lag time: Kerby Method for Overland Flow for Tc (WMS will convert this Tc to a Lag time as required by SCS)

Precipitation: 2.5 inches with a Type II temporal distribution.

Model Run: 36 hrs @ 6 mins interval

b. Clark Transformation Method

Watershed: Single Basin

Loss Rate Method: Green and Ampt ( Initial loss = 0.2 in, Saturated moisture = 0.45, Initial moisture = .25, suction = 12 in, Conductivity = 0.25 in/hr, Impervious = 0%)

Transform method: Clark

Time of concentration: Kerby Method for Overland Flow

Precipitation: 2.5 inches with a Type II temporal distribution.

Model Run: 36 hrs @ 6 mins interval

Compare the results for the two different methods.

2. Sensitivity Analysis of R in Clark MethodThe Parameters for the Clark Transformation method are Time of Concentration t

_{c }and Storage Coefficient R. InHMS, open the project file that you used for 1b where you used Clark Method (you might want to save it as with new name so that you will have previous project unchanged). Now underTransformtab, leave the time of concentration the same but change the value of storage coefficient:

By increasing by 100%

By decreasing by 50%

Compare the outflow hydrographs with normal value (Determined by WMS in part 1b) and these two cases.

3. Sensitivity of Lag TimeThe parameter for SCS Transformation method is lag time T

_{Lag. }InWMS, open project the file you used for creating HMS file for 1a where you used SCS method. In the Edit Parameter Window, when you compute basin data for SCS Transformation method, change the Lag time equation as:

Use any four different Lag time or Time of Concentration (remember WMS will convert Tc to Tlag if you pick a Tc method) equations available in WMS basin data time computation calculator. Select the equations suitably so as to match the watershed property/Location/Land use and or soil type, but don't be surprised if there is some variation.

Save the HMS Project file (or you can calculate the lag time in WMS, take that value and replace it in HMS) and run for each case.

Compare the outflow hydrographs from all the different cases.

4. MODClark MethodThe performance of MODClark method is affected by the parameters like initial abstraction, potential retention factor and grid size resolution. Along with these, the time of concentration and storage coefficient R also play a significant role. But as we already analyzed the sensitivity of time of concentration and R, we will not change these values. Here we will be analyzing the sensitivity of Initial abstraction, potential retention factor and grid size resolution.

From the WMS base project create a 100 by 100 MODClark grid cells. Enter all values to create a MODClark HMS project. Use following:

Watershed: MODClark Grids (100 by 100)

Loss Rate Method: Gridded SCS method

Initial abstraction ratio 0, 0.1 and 0.2

Potential retention factor: 1

Transform method: MODClark method

Time of concentration: Kerby Method for Overland Flow

Precipitation: 2.5 inches with a Type II temporal distribution.

NOTE: You must define this as a user hyetograph.Model Run: 36 hrs @ 6 mins interval

Run HMS for all three cases and compare the results

b. Now In WMS, delete the grid you had and create grids of 5 by 5, 25 by 25 and 200 by 200. Then, in each of those models use the following:

Watershed: MODClark Grids

Loss Rate Method: Gridded SCS method

Initial abstraction ratio 0.2

Potential retention factor: 1

Transform method: MODClark method

Time of concentration: Kerby Method for Overland Flow

Precipitation: 2.5 inches with a Type II temporal distribution.

Model Run: 36 hrs @ 6 mins interval

Run MODClark in HMS for all the different grid cells sizes. you already have results for 100 by 100. compare the results for four different grid resolutions.

Plotted hydrographs for each of the model runs above. Paste you outflow values into this template. Be sure to indicate which methods you chose for Lag time or Time of Concentration in part 3.To Turn in with Assignment 23 (think about these now):

Comparison between Clark and SCS method (1a and b)

Analyze the sensitivity of R in Clark Method (2)

Analyze the sensitivity of different Lag time equations for SCS method (3).

Analyze the sensitivity of initial abstraction ratio of MODClark Method (4a)

Analyze the grid resolution sensitivity in MODClark Method (4b)