Evaluating Surface Losses in GSSHA

CEEn-531 Dr. Nelson

By this time, you should have your basic GSSHA model working. I know you have put a lot of efforts to have your first GSSHA model and hopefully you feel confident in the results you are receiving when running it. However, if you would like you may download my model which I know is working reasonably well. Get the project file Here and do not forget to edit the *.prj and *.cmt files for the path after extracting and before running the model.

In this assignment, we will be start from this basic model and experiment with different infiltration methods as we did in HMS.

NOTE:  You will make some more index maps in this assignment.  Be sure that when you save your GSSHA projects that an index maps is not selected or WMS will crash.

Parameters

You will analyze various infiltration parameters in this exercise and it is important for you to understand how each of those parameters is determined and what watershed properties can be used to "index" these parameters.

1. No infiltration: This is our control case from which we will be able to compare other models.  In this analysis, you will not consider any infiltration parameters. The overland flow surface roughness (necessary for runoff calculation) is characterized by Manning's coefficient. You can get the information about Manning's n in the Assignments links below.

2. Infiltration considered: Here you will incorporate both infiltration as well as roughness characteristics. GSSHA provides to variations of a physics based infiltration model; Green and Ampt and Green and Ampt Redistributed. The parameters for Green and Ampt are: Hydraulic conductivity (Ks), Capillary Head (ψf) Porosity (θs) and Field Capacity.  Two more parameters are added when you use Green and Ampt Redistributed method which are Pore Distribution Index (λ) and  Residual Saturation (θr). The GSSHA reference manual discusses hot to estimate these parameters (Page 92, Parameter Estimates, GSSHA Reference Manual).

Aaron also recently sent a few tables that may be helpful in determining parameters. Link 1 Link 2.

Model Setup

Be sure you have a basic model of GSSHA that has surface runoff working with streams defined.  You can use yours or download mine, but if you download mine be sure to change the directory paths in the .prj and .cmt files.  Your GSSHA model should:

Assignment

  1. Uniform Index map and uniform roughness

    1. No infiltration

      The index map being uniform, you will have only one land use defined in your watershed and thus you will have single roughness value. This is basically just running your base GSSHA model. We are performing this analysis so as to match the case with HMS analysis.

      • Precipitation:                  2.5 inches, Type I temporal distribution

      • Manning's roughness:     Determine an "average" value based on the land use downloaded and used for the previous HMS models. These two links you may find helpful  ( Link1 Link 2), as well as page 67 of the GSSHA reference, or other standard hydraulics text for Manning's n values.

      • Run GSSHA to determine the runoff when no infiltration is considered (except for the variable surface roughness this model will be identical to the one you ran in the precipitation assignment for the Type I storm).

       

    2. Green and Ampt Method

       In GSSHA, there are two forms of Green and Ampt (GA) equations. GA method is updated to Green and Ampt with soil moisture redistribution (GAR). The input parameters are basically the same but the algorithm in GAR redistributes the initial soil moisture until there is no rainfall and this maintains the continuity in the soil moisture when the next rainfall event occurs. (page 104, GSSHA reference manual). You have already experienced the methods of determining Green and Ampt parameters from the land use and soil information for HMS. We will use similar parameters here, but you can see, the index map is uniform representing same land use and soil type all over the watershed, so you will have single roughness parameter and a single set of infiltration parameters, making this model similar to what was run in HMS.

      We will use the GAR method for this case and perform some sensitivity analysis with the GAR parameters. In GSSHA | Job control, you can choose the infiltration method to be Green and Ampt with soil moisture redistribution. In the figures to your right (seen from the Mapping Tables dialog), you can see the parameters for GAR method.  Besides these parameters you will also need to define the initial moisture, done separately from the GAR parameters in the mapping table setup.

      • Precipitation:                  2.5 inches, Type I temporal distribution

      • Manning's roughness:     Determine based on the same roughness index used in the no infiltration option.

      • Infiltration parameters:    Determine based on the uniform index map (select GSSHA | Mapping table, and switch to Infiltration tab, Generate ID's).  We want to use a uniform set of values so we can compare more closely with the results of HMS.  See page 92 of the GSSHA reference, with the symbol definitions at the top of this page to help you.  Also, the table below compare the different parameters used for HMS and GSSHA.  Enter .1 for all soils for field capacity for this assignment.

      • In the GSSHA Job Control under Output Control turn on the output of the Cumulative Infiltration Depth, Infiltration Rate, and Soil Moisture.  These results can help you get an idea of what is happening to rainfall lost to infiltration.

      • Initial Moisture: In the mapping table input you define initial soil moisture separately.  If the soil is saturated this will equal the porosity.  Use a value that approximates what you used for HMS.  Remember that for HMS you entered a soil moisture deficit (porosity minus initial soil moisture), but for GSSHA you will enter porosity and soil moisture separately.

      • Run GSSHA so that you capture the hydrograph properly.

      • Rerun GSSHA by increasing and decreasing the values of K by 25%

      • Rerun GSSHA by increasing and decreasing the values of Porosity by 25%

      • Rerun GSSHA by increasing and decreasing the values of Capillary head by 25%

      • Rerun GSSHA with initial moisture = porosity, and initial moisture = 0.

    Note: The following table shows different parameters for GA method as used in HMS and GSSHA

    Parameter Named in HMS as Named in GSSHA as
    1 Initial Loss Interception, Initial depth, Retention depth (These values are used collectively with the Overland flow and infiltration models) and we won't use them for our GSSHA model yet.
    2 Moisture Deficit = Porosity - Initial moisture Porosity
    3 Initial Soil Moisture
    4 Suction Capillary Head
    5 Conductivity Hydraulic Conductivity

    Other parameters like pore distribution index and field capacity are also needed in GSSHA. These parameters are the functions of soil properties and initial soil moisture content. Table 10 on Page 94 of GSSHA reference manual will be of much help in determining these parameters.

2.   Spatially defined soils using a non-uniform Index map

For this case, you will create a new index map from the SSURGO soils data (converted to a WMS Coverage).  In the previous case (case 1), you had simplified the watershed properties by using uniform index map (which matched the HMS model better). Here you will have multiple soils to define the GAR parameters in the mapping tables.

Under GSSHA | mapping table, select the Infiltration tab, Generate ID's and enter suitable GAR parameters for the different soil properties. Generally, the roughness parameters are derived from land use data and the infiltration parameters from soil data, but it is possible that some parameters are a function of both even though we are simplifying this to just use the soils (Table 10, Page 94 of GSSHA reference manual).

  1. Green and Ampt with soil moisture redistribution

3.   Spatial variations in land use

Here, we will see how the land use pattern affects the infiltration through the watershed. Using the coverage overlay calculator in WMS, you can determine the percentage of watershed area covered by different land use and soil types (In the figure to the right, you can see how WMS calculates percentage area for different land uses)

According to the USGS land use classifications LUCODE attributes with values between 10-19 are urban areas.  For this part of assignment, calculate coverage overlay for your watershed and see which portion of land (and what percentage) are urban areas.  If an urban area is "directly connected" there is no infiltration (this is the inherent assumption in models like HMS), but if it is not then it runs off onto land uses that are pervious (something GSSHA can handle). Now, we will see how this consideration affects the hydrograph. 

a. Comparison of land use changes in HMS and GSSHA

i) Go back to your Green and Ampt HMS project and in percent impervious area field, enter the percentage of urban land use as impervious . Adjust your Green and Ampt parameters if you feel it is necessary. Then run HMS, we will use the results to compare with GSSHA.

ii) For GSSHA we want the soil parameters to be the same as HMS everywhere (one index value) except the urban areas.  The easiest way to do this is to use the Land Use map you downloaded to define the infiltration parameters.  For ID's that are not urban use the values you determined for part 1 that most closely match HMS, then for the IDS that are urban make the hydraulic conductivity = 0 and the capillary head = 0 (impervious). Save your project and run GSSHA with the all other parameters same.

Compare how HMS and GSSHA treat imperviousness .

To turn in:

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