Estimating pollutant loadings – SIMPLE



WinTR-55 Small Watershed Hydrology Model

Hydrologic Modeling Workshop

General background

WinTR-55 is a single-event rainfall-runoff hydrologic model for small watersheds. The model generates hydrographs for both urban and agricultural areas and at selected points along the stream system. Hydrographs are routed downstream through channels and/or reservoirs. Multiple sub-areas can be modeled within the watershed. A watershed is composed of sub-areas (land areas) and reaches (major flow paths in the watershed). Each sub-area has a hydrograph generated from the land area based on the land and climate characteristics provided. Hydrographs from sub-areas and reaches are combined as needed to accumulate flow as water moves from the upland areas down through the watershed reach network. The accumulation of all runoff from the watershed is represented at the watershed Outlet. The model uses the SCS Curve Number method to calculate runoff. A weighted curve number is calculated based on the proportion of landuse types in each sub-watersheds as well as the dominant soil hydro group.

Step 1. Getting Started.

1.1 Open TR-55 by clicking on the Start menu. Go to All Programs ( Engineering Applications ( WinTR-55 ( WinTR-55. Click on start when the program opens.

1.2. Fill out the top of the front page of the project with the following information:

User: Your name

State: Vermont

Project: Base Potash

County: Chittenden

Sub-areas expressed in: Hectares

Dimensionless Unit Hydrograph:

1.3. Download and open the file: TR55_Potash.xls. This file contains all of the summarized data for each of the subwatersheds that you will need to use in the TR55 model. Although, the GIS work has been done for you in this example, in the next exercise, you may update this input data with your GIS-based watershed characterization.

1.4. When you first open TR55_Potash.xls, you will see a number of subwatersheds with stream reaches numbered from 1-7 (click on the tab ‘Map’). Look at the subwatersheds and note which ones flow to a stream reach and which one flows to the watershed outlet. Enter the names of the Sub-watersheds in the Sub-area Entry and Summary section in WinTR-55 the list at the bottom of the TR55 front page. Also enter the reach to which the area drains (rather than the reach within the watershed itself). For example, Subarea 4 drains to Reach 6. Don’t enter anything in the area, CN, or Tc columns.

Step 2. Enter landuse data

In TR55, go to Project Data ( Landuse Details. Click on each of the subareas (in the dropdown menu at the top left) that you have already defined on the front page and fill in the landuse data according to the Landuse tab on the excel spreadsheet. You will need to put your areas under the correct Soil Hydro Group. In the Landuse&Soil Tab you will see that we’ve classified each subwatershed to be dominated by one of the 4 Hydro Groups. In a real project, you would want to divide the landuse types by each of these soil hydro groups but we will make it easy here by assigning them all to one group.

To make things simpler, please follow the following landuse classifications to convert the landuse types defined in GIS to the more detailed types in TR55.

|GIS Lanudse |TR55 Landuse |

|Barren |Open Space – poor |

|Water |None |

|Residential |Residential Districts ( 1 acre |

|Commercial |Urban districts ( Commercial and business |

|Industrial |Urban districts ( Industrial |

|Transportation |Impervious Area ( Streets and roads ( Paved; open ditches |

|Airport |In subarea 6, assume that 100 ha of the transportation layer is the airport (can you see it on the |

| |map?). Enter this as Paved parking lots, roofs, and driveways. |

|Other Urban |Residential districts ( 1/8 acre (this is to get the right IC at 65%) |

|Misc Ag |Other Ag Lands ( Brush ( Good |

|Forest |Other Ag Lands ( Woods ( Good |

|Wetland |None. |

|Row Crop |Cultivated Ag Lands ( Row crop ( Straight row (SR) ( good |

|Pasture |Other Ag lands ( Pasture, grassland or range ( Good |

Note that the data should be entered in hectares. When you are finished, click ‘Accept’. Notice that the model has now filled in the weighted CN for each Subarea as well as the total area on the front window.

Step 3. Time of Concentration

There are several ways to calculate Time of Concentration, which is the time it takes for water to get from the furthest hydrologic point in the subwatershed to the outlet. If you know a lot about the drainage of the area, including which areas are drained naturally, which have storm drains and which produce sheet flow then you can use the Time of Concentration calculator in the WinTR-55 model. Since we don’t have the required level of detail to use this function, we will compute Time of Concentration using a Curve Number method described by the Natural Resources Conservation Service. The equation is:

[pic]

Where:

Tc = Time of Concentration

L = hydraulic length of watershed in feet

CN = curve number

S = watershed land slope in percent

This equation has been entered into the Excel file under the tab, Time of Concentration. You will also find the length (longest flow path) and slope for each watershed area. Enter the Curve Number (CN) and the Time of Concentration will be calculated. Copy these values into the first page on the WinTR-55 model.

Step 4. Reach data

Enter data about the stream reaches that receive water in the model (Reach 3,6, and 7). Go to Project Data ( Enter the data for length, slope, and Manning’s N according to the data provided in the ‘Reach’ tab of the excel spreadsheet. Save the entire project as Potash_Current.

Step 5. Run

You are now ready to run your model. Go to the Run menu. Click to run a 2-Yr, 5-Yr, and 50-Yr storm. When you first run the model you automatically get a Hydrograph Peak/Peak Time Table. Click on the [pic] to view a hydrograph of all of the subareas (click each sub-area in the dialog box). You can make a hydrograph for each different storm type.

Step 6. Create current scenario dataset

We now want to create a more detailed data set to save and compare to another scenario in the future. You could do this for all the storm types and subareas, but for now just re-run one storm type: an actual storm on June 26th 2006. Open the rainfall data for June 26, 2006 that has already been summarized for you (Rainfall_6_26_06.xls). Notice that the storm duration is approximately 24 hours and that the English units (HI-hundredths of inches) have been provided in SI units. Open the Storm Data window in TR-55 and replace the 1-yr rainfall return precipitation amount to match the actual rainfall on 6/26/06. Find the output table containing time and flow data (in TR20 reports). This text table gives you detailed runoff data, over time for each subarea and outlet. Copy the data for Outlet.

Paste the data into a new excel worksheet and rename it Current_scenario. You can delete all of the columns except the first (Time in hours) and the second (Flow in cfs). The other cfs flow readings are spread between each time series value and we can fill these in ourselves using excel.

Graph, using a Scatter Plot, the hydrograph (Time v. Flow).

Step 7. Pre-Development Scenario

Now we will re-run the model for a less-developed, slightly forested scenario.

Save a new copy of the project and call it Potash_Forest. Open the Landuse Details menu again. For each sub-area, click ‘Clear’ then enter the entire area (not including water or wetlands) of the watershed into the Other Agricultural Land ( Woods category. Make sure to input the area under the correct Soil Hydro Group.

Recalculate the Time of concentration for the new weighted curve numbers then rerun the model, again using the actual storm data from 6/26/06.

Copy the output just like you did in the Current_scenario. Plot the hydrographs for the current and forested scenarios together in excel. What do you notice?

Step 8. Assess Model Performance

Now we will test the current scenario model with real precipitation data for the storm event in June 2006. Real-time gage data has been available for Potash Brook since 2004 (USGS). We will then compare the modeled discharge to the measured discharge near the outlet of Potash Brook.

Open the gage data for Potash Brook (Discharge_6_06.xls). This is the raw data file provided by the USGS.

Now graph both the modeled and actual discharge for Potash Brook for the June 26th storm event. (Watch units! Watch time synchronization!)

How does the model perform? What are the differences between the model output and actual discharge in terms of water quantity and timing of the peak flow? What would be your next step in assessing the model’s performance? What could we change in the model to improve its performance?

Recommended Readings:

Part 630 Hydrology, National Engineering Handbook. 1997. USDA, NRCS. Chapters 12 and 15.



WinTR-55 User’s Guide

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