Surface water hydrology and groundwater hydrology are coupled together in natural ecosystem. While it is easy to say so, it is not easy to model both of them simultaneously.
In my earlier posts I have covered a lot on related topics including MODFLOW, PRMS and GSFLOW.
For example: https://www.changliao.us/2018/03/paper-discussion-002.html https://www.changliao.us/2018/03/paper-discussion-001.html https://www.changliao.us/2015/09/integrated-groundwater-and-surface-water-01.html https://www.changliao.us/2016/04/surface-water-hydrology-modeling-002.html
Today I will discuss some other issues related to SWAT-MODFLOW. SWAT is another widely used surface hydrology model and there are ongoing efforts trying the couple MODFLOW with SWAT. https://swat.tamu.edu/software/swat-modflow/
Unlike PRMS, SWAT in general does NOT use grid based approach to run simulation. Instead, SWAT uses subbasin and HRU to represent the watershed. And this difference may cause a list of challenge for us. I will discuss a few of them in details below.
Because SWAT and MODFLOW are not fully coupled from the mathematic perspective, some balance may not be achieved. Let’s temporally ignore the issue brought out by HRU and MODFLOW grid overlap. Both model simulate the river routing, but only the SWAT routing result is used as “true” output.
Conceptually, these two model behave different as well. For SWAT, the routing is defined by the fig file and the stream segment numbering is not important. However, in MODFLOW, the order matters, even if you are using RIV package.
“Note 5: Reach information is read in sequential order from upstream to downstream, first by segments, and then sequentially by reaches. If segments are not numbered sequentially in downstream order, then the inflow to a segment during the current MODFLOW iteration will be the outflow from an upstream segment calculated during the previous iteration. Lagging the inflow by one iteration does not change the solution for flows into and out of a segment after MODFLOW converges; however, this approach may require an additional iteration for the model to converge. Reaches must be listed and read sequentially because the order determines the connections of inflows and outflows within a stream segment.” (SFR2 user manual)
Even though after the model converges, the difference will be not significant, it will still bring uncertainty in the following simulation.
Therefore, it is obvious that these two model use different index system to identify stream networks. It is inevitably we can not use SWAT numbering as the input for MODFLOW and addition efforts will be required to link them together.
However, there are some solutions to this issue: We can just go ahead and prepare SWAT and MODFLOW model as usual. After that, we can manually link them using the indices. This method requires less modifications on the original model but we need to generate new stream ID for MODFLOW. We can change the ArcSWAT results so that the SWAT stream ID are sequentially numbered. The SWAT stream ID can then be used directly by MODFLOW. This method requires good understanding of the ArcSWAT mechanism. Given that ArcSWAT is not open source, we need to check SWAT input/output carefully to see whether the modification is successful or not. We can also change the SWAT fig and related files and then generate new Stream ID for MODFLOW. In this case, we do not have to modify ArcSWAT. But we have to check through SWAT input to make sure all related files are updated. Because SWAT is open source, this is straightforward. Method 2 and 3 require more efforts. In short term, method 1 should be used.
Updates: in the RIV package, the order does not matter because the MODFLOW does not calculate the river routing at all!