Contents

Calibrate iTree Hydro. 1

Initial Parameters. 2

DEM Data. 2

Land Cover Data. 3

Delineating Impervious and Tree Cover. 3

Specify File Paths. 7

Soil Parameters. 7

Running The Parameter Conversion Macro. 8

Modeling in iTree Hydro. 10

Comparing Data. 10

 

 

First, use iTree Hydro (you will need to create an account to download the application) and the instructions given in the iTree Hydro Users Manual to calibrate the model for the watershed that most closely matches the smaller subwatershed you will to working with.� This may be much larger than the subwatershed you intend to look at (see graphic to right), since there are not always stream gauges available for the smaller subwatersheds, but the closer it is in size and location, the more accurate it will likely be.� Save the project and keep it open.�

Click here to open the Parameter Conversion Excel file (you may need to enable macros to use it).� Select cells A2 to A34 and copy and paste to a new .txt file in notepad (see graphic to right).� Copy the calibrated values from the Configuration window in iTree Hydro to the third section in the text file (�Initial Hydrological Parameters�).� You should do this by replacing the �#� before the label with the calibrated number�make sure there is still a space between the number and the label.� Save this .txt file and note the full file path (e.g.: �C:\Documents\filepar.txt�).� You will add the other information later.�

Next, obtain a high-resolution DEM (such as LiDAR) for your area of interest.� The Parameter Conversion macro is meant to work with 1-m spatial resolution data.� If it is a different spatial resolution, resample the raster image using ESRI ArcGIS or similar GIS software to a 1-meter cell size.� Next, delineate the subwatershed using the DEM and similar methods as described in the iTree Hydro User�s Manual.� Extract by mask for the area of your watershed.� Do not worry about finding the area of the subwatershed, since it will be automatically generated when the grid is run through the macro.� Convert the layer to an ASCII file and rename to .dat file format for input into iTree.

 

Land cover data should be auto-delineated or obtained from an online source.� The macro currently uses input in the format of the NASA-Arc land cover data made at SUNY ESF in 2002.� This lists trees as 1, short vegetation as 2, bare soil as 3, water as 4 and impervious as 5.� This data layer will also need to be resampled to the correct cell size and extracted for the subwatershed�s area.� Be sure the projected coordinate system matches the one used with the DEM.� Once projected, resampled, and extracted, convert to an ASCII file.

 

Impervious and tree cover should be manually delineated for the subwatershed.� You may do this by ground surveys or by �tracing� the areas in ArcGIS.� To �trace� it in GIS, you will need two aerial images�a leaf-on and a leaf-off.� Leaf-on images can be obtained from USDA-NRCS's Geospatial Data Gateway.� Chose the Interactive map link on the right hand side of the page.� Select the area you want, giving yourself enough room so you won�t cut any part of your subwatershed off.� Accept the area and select the most recent �National Ag. Imagery Program Mosaic� under the Ortho Imagery heading.� Aerial images are usually taken during leaf-off times, and so can be obtained from most online aerial image sources, such as the New York State GIS Clearinghouse.� Add these data layers to ArcGIS and project to the proper coordinate system (same used in DEM and the rest of the layers).� Cell size should not be an issue, although make sure you can make out distinct shapes and tell what what�s impervious and what�s a tree.� Follow these instructions to make the layers:

1.      Create a feature class (Data Management Tools�Feature Class�Create Feature Class) with the same coordinate system as the DEM (See right).� You can select the �Import�� option when you chose your coordinate system at the bottom of the window when you�re making the feature class.� This ensures it is the same.�� Open the feature class for editing using the editing toolbar.�

 

2.      With the leaf-off image showing as the top-most layer, trace any areas that are impervious with polygons (using the �create new feature� option).� The main ones to be looking for are houses, sidewalks, roads, and parking lots.� It�s a good idea to go slightly beyond the bounds of your subwatershed so you don�t have to go back and edit again later.

 

3.      Go into the attribute table and make sure all the �Id� numbers are �1� (see below).� After you have finished adding all the impervious areas and changed the Id numbers to 1, save your edits and stop editing.�

 

4.      Convert the feature layer to a raster(Conversion Tools� To Raster� Feature to Raster, see below), making sure the cell size is 1 meter (it will automatically chose some fraction of the extent by default to be the cell size).� Make sure it says �Id� under the Field option.

 

5.      You will want a �background� value of zero for areas that are in the watershed but not impervious.� Multiply the DEM raster layer by zero (Spatial Analyst Tools�Math�Times) then create a mosaic with the �zero� layer and the impervious layer (Data Management Tools�Raster�Raster Dataset�Mosaic to New Raster).� Make sure you select the method that will ensure the impervious layer values are chosen over the zero values (either FIRST option or LAST option, depending on the order you put the layer in), as well as the proper cell size and coordinate system (see below).

 

6.      Extract the impervious layer with the zeros background so that it is the same extent as the DEM and land cover layers you extracted.� Convert to an ASCII file.�

7.      Repeat steps 1 through 6 to create the tree cover layer, only use the leaf-on image as background and trace trees instead of impervious surfaces.�

These two layers are meant to be more reliable and accurate delineations than the land cover layer.� In the macro, they are assumed to be true where values equal 1 for at least one of them and all other cells� values are obtained from the land cover data.� So, if you miss some trees or some impervious areas they are likely to be accounted for in the land cover data.� However, the macro will also use the impervious layer to fill in your watershed with hypothetical rain gardens, edging the impervious surfaces first.� A sub-par delineation may show the addition of rain gardens to have less effect on the subwatershed, due to rain gardens not being placed near the roads and sidewalks where they should be placed.

 

Now that you have all the input files you need, enter their file paths into the �filepar.txt� file from Initial Parameters.� You should only replace the sample file paths between the brackets and change nothing else.� Enter the percent of the watershed you wish to be converted to rain gardens by replacing the �#� between brackets in the first section with the number (e.g.: to simulate 10% of the watershed as rain gardens, it should read ��watershed: <10>�).� This should be less than the percent of the watershed that is pervious (usually no more than 50%) and something that is reasonably attainable but significant (somewhere between 5% and 25% is suggested).� Also enter valid file paths for the 4 output files.� Only the parameter files are ones you will be using in your comparison, but the tree and rain garden files may be interesting to look at is you want to see how the watershed was modified for the hypothetical addition of rain gardens.� Regardless, they have to be 4 distinct file names in existing folders.�

 

Before running the model, look at the soil parameters (saturated soil transmissivity, wetting front suction, surface hydraulic conductivity, and wetted moisture content)� and make sure they make sense in your subwatershed, since these particularly can have a profound effect on the results of the simulation and are given as a spatial average for the whole watershed through the calibrated values.� See table below for values typically used for different soil textures.� It is suggested that clay loam or a texture more towards clay is used for urban areas.� The values for a �sand� soil texture are factored in when simulating the addition of rain gardens.� Also check the relation of soil transmissivity vs. surface hydraulic conductivity.� These two values are related by soil thickness (, where T0 is soil transmissivity, Di is soil depth or thickness, and Ks is hydraulic conductivity).�� If the soil depth seems too deep or too shallow for your area, you may need to look around for the soil depth or give your best estimate.� If it seems about right, then multiply by the hydraulic conductivity to get the transmissivity value you�ll use.� Change these four values in the filepar.txt file, save all changes, and you�re ready to run the Parameter Conversion macro.�

 

Typical Values for the Green and Ampt Model; (Adapted from Wurbs and James's Water Resources Engineering, pg 484.)

 

Open �Parameter Conversion.xlsm� and make sure macros are enabled (click �Options�� button if you see the Security Warning above the formula bar and click �Enable this content�).� If everything in the filepar.txt file is ready to go, click the red START arrow (see top screenshot, below). You will be asked to enter the location of the filepar.txt file.� The macro may take several minutes to run, depending on how large your subwatershed is.� If it�s having trouble fitting the requested percentage of rain gardens, a notification will come up asking permission to continue filling the pervious areas non-discriminately, so keep an eye on it if you think you�re close to the max.� You will see numbers changing on the screen (see bottom screenshot, below), but when it�s done it will return to the same as when you opened it.� It�s going through a lot of numbers so try to be patient, even if it occasionally says it�s �Not Responding� because it will work through it eventually.

 

 

 

Open iTree and start a new project.� Also open the base case parameter file in the location that you specified in the filpar.txt file.� Enter all information in the parameter file when necessary and the location of the subwatershed�s DEM file, starting with the Input�Raw Data Input window and moving to the Input�Configuration.�

NOTE: You won�t be calibrating this time, so stream gauge data doesn�t matter�you just need to enter a file name in that slot to continue.� For weather data, you can use whatever best suits your needs, but you may want to consider looking at a design storm based on Intensity-Duration-Frequency curves available from the National Weather Service you just need to use the same weather data for the base case and the rain garden case.�

After you have entered all Raw Data and Calibration inputs, run the model by clicking Reports�Water Flow Reports�Current Scenario.� Export the chart output to an Excel file then repeat the process with the parameters in the rain garden parameter file.�

 

Perhaps the easiest way to compare the data is by exporting the overall totals only and putting the output for the two scenarios in one Excel sheet.� Then create a bar graph to visually compare the runoff totals (the last 4 columns) or compare the values themselves.� Try repeating the process for different percentages of the watershed as bioretention basins (just change the percentage in filepar.txt and run the macro again, then iTree with the new parameter file).� And see how the results change as you increase or decrease it. The base case parameter output file from the Parameter Conversion macro will always be the same as long as the other inputs are the same, so you don�t need to change that file path in filepar.txt, but it is suggested you change the others to reflect the percentage you are using.�