Routing of waterbodies in SWAT+

[Swetasree Nag]

Dear SWAT+ User Group,

I am working on a SWAT+ model where my study area includes over 900 waterbodies (reservoirs, lakes, ponds, etc.). I seek your guidance on the following points to ensure proper representation and routing of these waterbodies:

1. The SWAT+ manual mentions that reservoirs should be located on streams. However, many of my waterbodies are not directly on streams. How can I incorporate these into the model? If I initially define them as wetlands, and later change their hydrology type to reservoir using the SWAT+ Editor (while manually assigning an outlet), will this approach work effectively? If not, could you kindly suggest an alternative?

   To address this, I have generated streams at a finer resolution to connect as many waterbodies as possible. However, some waterbodies still remain unconnected. At what lowest threshold can streams be generated in SWAT+ to reasonably represent flow paths? I understand that using a finer threshold may produce streams that do not exactly match real-world conditions but could still serve as flow paths. Please correct me if I am wrong or if there is a better approach.

2. For accurate routing and interaction between waterbodies, would a grid version of SWAT+ be more suitable than a non-grid setup? I understand that in the grid model, routing occurs between neighboring cells, allowing upslope and downslope interactions. I would appreciate your suggestion on which approach would be better in this scenario.

3. For waterbodies not located on streams, if I manually define outflow, can these contribute to the aquifer or corresponding HRU through overland flow when conditions permit? How should I determine and specify such outflow for these cases?

4. Lastly, I would like to understand how SWAT+ calculates the volume of lakes and how it links with overland flow generation.

I look forward to your insights. Thank you for your time and support.

Best regards,
Swetasree Nag

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[Natalja C.]

Hello,

1. Regarding lakes: SWAT+ does not require lakes to be located on streams. The current documentation is a legacy from SWAT2012 and will be updated soon. While I'm not sure how to incorporate these off-stream lakes directly within the QSWAT+ interface off the top of my head, the QSWAT+ manual provides guidance on this. In any case, after completing the GIS setup, you can always manually adjust the feature connectivity in the Editor by, for example, removing unwanted connections to streams. Since I'm not familiar with the area or its hydrology, I cannot recommend a specific solution for minimal thresholds. Your current approach might be viable but may require the post-processing step I mentioned.

2. Regarding gridded models: Gridded models have limited usability for this purpose. For accurate interaction between land features, SWAT+ allows the routing of HRUs to other HRUs. As far as I know, you cannot create this setup with the standard interfaces; you will need to adjust the routing manually or by using post-processing tools. We have developed an R-based tool for such applications called SWATBuildR (see associated publications from the OPTAIN project for more details). But be prepared for quire some time to generate the required inputs in vector files. 

3. Regarding water routing: I want to stress that you can only route from land features to other land features, or from land to water features, not the other way around. This means HRUs can contribute flow to other HRUs (like I mention in item 2), or HRUs can "drain" into reaches or lakes. However, water from ponds, lakes, or reservoirs (if you model your wetlands this way instead of as wetland HRUs) cannot be routed to HRUs. They (water features) can only be routed to streams (or other water features) or left unconnected. Seepage to aquifers is possible.

4. Regarding the code: I suggest going through the source code, the most recent version of which is always available on GitHub. Look for subroutines that contain "res" for reservoirs, such as res_control.

Best,

Natalja