By Joan Klipsch, HEC

Scenario

You have a ResSim Watershed (model) that represents a large river system with multiple tributaries holding several reservoirs. Operationally, the reservoirs in this watershed are rarely, if ever, operated together as a single large system. Instead, the large watershed actually holds two or more smaller watersheds, within which the reservoirs are operated together to meet system objectives. So, to improve usability, you have decided to break up the large watershed (model) into two or more smaller tributary watersheds (models).

A Recommended Procedure

After having performed this task several times with different approaches, I believe that the following steps are the simplest and the least error prone. Here's the overall plan:
You'll make a copy of the original watershed and clean it up to use as the template for each of the tributary watersheds. In the template watershed, you will identify the network and one or more associated alternatives that you will use as the basis for your tributary models and their alternatives. Then you'll identify or create a simulation that you will later use to verify each new model and its alternative(s).

In each tributary watershed, you will rename the network you selected and then delete all the elements from it that are not part of the tributary model you are creating. Once you have the network cleaned up, you will verify that the alternative you selected has correctly updated itself to the new state of the network. You will make a copy of that alternative, giving it an appropriate name for the new model and add that alternative to the simulation you identified earlier. In the simulation, you will compute the new alternative and compare its results to the results generated by the original alternative.

You could quit at this point and your new model would be in a good state to move forward. However, there's one last detail you should take care of – the watershed configuration(s). You see, the watershed configuration(s) still reflect the large watershed. For completeness, you should go to the Watershed Setup module and edit the configuration that your network was based on. The objective is to remove all the reservoirs and diversions that are not used in your network. This can be done by deleting them completely from the study, or by simply removing them from the configuration's Project List in the Configuration Editor. In addition, your watershed still has a lot of common computation points that are not needed by the smaller tributary model; consider deleting these extraneous points from the watershed schematic using the Computation Point tool.

Prepare the Template Watershed

Make a copy of the original watershed

• Make a copy of the watershed in a new location on your computer.
• Launch ResSim and Open the new copy of the watershed
• Go to the Reservoir Network module and review the list of networks. Identify the network you will use as the basis for your new tributary model. If you have more than one network to choose from and the choice is not clear, Open the Alternative Editor to identify the alternative you want to use as the basis for your tributary model's alternatives. That alternative's network is the one you need.
• Go to the Simulation module and Open an existing simulation in which the alternative you have chosen as the basis for your tributary models' alternatives computes successfully and produces validated results. If you don't have such a simulation, create one and make sure your selected alternative computes and produces valid results.
• Now, delete any other simulations that exist in the watershed. You don't want to carry all those simulations with you into the smaller sub-watersheds, right? For details, see "Cleaning up an HEC-ResSim Watershed" at https://www.hec.usace.army.mil/software/hec-ressim/documentation.aspx

• Exit ResSim and zip up the template watershed so that you have a backup.

Create the Tributary Watershed(s)

Make a copy of the template watershed for each tributary watershed to want to create.

• If your ResSim watershed is part of a CWMS watershed, use the File, SaveAs option available in the CWMS 3.2.1 (or later) CAVI to make each copy.
• Otherwise, you can make a copy of the ResSim watershed and rename it by following these four steps:

1. Exit ResSim (or the CWMS CAVI) - if you currently have it open. Open a Windows File Explorer and make a copy of the full watershed folder.
2. Rename the folder you just created to reflect the name of one of the sub-watersheds.
3. Rename the watershedName.wksp file in the root folder of the watershed to match the new name of the watershed folder.
4. Using a text editor (such as Notepad), edit the newWatershedName.wksp file and change the name of the watershed on the first line of the file to match the new name of the watershed folder.

Edit the Network and the Alternative(s)

• Launch ResSim and open one of the tributary watersheds you created.
• Go to the Reservoir Network module and Open the network you identified as the basis for your new tributary model.
• Delete all model elements from the network's map schematic that are NOT part of the tributary basin you are creating. Be sure to delete elements in the following order:
  1. Reaches
  2. Diversions
  3. Reservoirs
  4. Junctions
• Save the network.
• Open the Reservoir Editor. Edit each reservoir to clean up any possible breaks in the operations. Remember, you just got rid of a whole host of elements whose model variables may have been referenced in the definition of one or more rules, IF-Blocks, or zones in the remaining reservoirs. Start at the upstream-most reservoirs and work your way down.
  • Review each zone. If the zone is a function of a model variable for an element that is no longer in the model, determine if a modified form of the zone is still needed
  • Review each rule. If the rule is a downstream control rule OR is a function of a model variable for an element that is no longer in the model – determine if some modified form of that rule is still needed. If so, make the necessary changes; if not, Delete it. Note that the "necessary changes" may require a new rule of a different type to approximate the intent of the operation.
  • If you have any scripted rules, review the text of each rule carefully to determine if the script accesses any model variables for elements that are no longer in the model. If so, determine if the state variable is still required (with modifications). If so, make the necessary changes; if not, Delete it.
  • Review the conditional expressions in each IF-Block. If they use a model variable for an element that is no longer in the model, determine if a modified form of the conditional expression or the whole IF-Block is still needed. If so, make the necessary changes; if not, Delete it.
  • Be sure to work your way through each operation set of the reservoir before moving on to the next reservoir. You'll find there are fewer and fewer rules you will need to spend time on since, if you modified the rule in another operation set, it is already modified for the current one. And, if you deleted (rather than removed) a rule while working with another operation set, it will be deleted from all operation sets it had been used in. And, in ResSim 3.3 and later, the same is true with IF-Blocks.
• When you are done cleaning up the reservoirs, close the Reservoir Editor and Save the network.
• As needed, clean-up the diversions in a similar manner; review each diversion's demand definition and revise as needed to remove/replace any references to model variables for elements that are no longer in the watershed.
• Open the State Variable Editor and review the text of each state variable's scripts carefully to determine if the script accesses any model variables for elements that are no longer in the model. If so, determine if the state variable is still required (with modifications). If so, make the necessary changes; if not, delete it. Be careful – there may be rules, IF-Blocks, zones, or diversions that may be a function of or may reference this state variable so keep track of the state variables you delete so that you can correct or delete the references in your reservoirs and diversions as needed. When done, close the State Variable Editor and Save the network.
• Open the Alternative Editor and select the alternative you identified earlier. Click through each relevant tab for that alternative.
  • You should notice on the Operations tab that there are entries ONLY for the remaining reservoirs in your network; all entries for reservoirs that have been deleted are gone.
  • Similarly, on the Lookback tab, only those reservoirs, diversions, and state variables that are still in the model will appear.
  • And, on the Time Series and Observed tabs, only input for the existing reservoirs and junctions will be listed.
  • Thus, you won't need to make any changes to the alternative to deal with the deleted elements.
  • Now, make a copy of the alternative using SaveAs. You will use this copy to test the revised network. Then close the Alternative Editor.

Verify your New Alternative(s)

• Change to the Simulation module and Open the existing simulation you worked with in the first step of these instructions. Edit the simulation and add your new alternative. Compute the alternative. If the compute is successful, compare selected results from the current alternative with the previous results from the original alternative. If you made any operational changes, such as deleting or modifying a rule, zone, IF-Block, diversion, or state variable, the results may differ; it will be up to you to validate that the differences are due to your operational or physical changes. If, however, you model does not computer successfully, then the two most likely causes are a network connectivity problem or a model or state variable link that has become broken.
• If your watershed has multiple networks, each of which is used by one or more alternatives, determine if these additional alternatives and their networks are still needed in your new, smaller watershed. If so, repeat these instructions for each alternative-network combination. If not, delete the extraneous alternatives and networks, in that order.
• When done, Save the watershed, Exit ResSim and zip up the watershed you just completed to provide yourself a "known good state" backup of the watershed.
• Repeat these instructions for each sub-watershed you wanted to create from the original large watershed.

Good Luck and Happy Modeling!