Error in exchange waterbalance coupler Ribasim-MF-MS #1863
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SnippenE
added
coupling
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@rleander made a reproducer in Python that I translated into Julia so I can directly test changes in the core. Here it is: https://gist.github.com/visr/78a864974619f9d445a3a0e1226310b0. It outputs figures like the following:
These lines should both stay at 0 and 25 where they start. The error gets worse over time. Resetting the cumulatives like in #1864 seems to help. But then how often should we do this and who should do this in BMI context? It is probably not a real solution, just like how forcing smaller timesteps or lower tolerances also helps reduce the problem. Changing the solver algorithm to Rodas5P does seem to help, showing no divergence even if I run the bucket model for 30.000 days, hence #1869. It seems like the cumulative flow states are difficult for some algorithms. Though I also wouldn't rule out that we can do more in the core to alleviate this. |
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This is with QNDF, only changing the reltol from 1e-5 to 1e-7. PR to make that the default incoming. |
With #1819 the state goes further away from 0 with time. That meanse we rely more on the relative tolerance than the aboslute tolerance. Reducing the default `reltol` by two orders of magnitude is enough to get only negligible leaky Terminals with occasional very brief leaks of 1e-8 m3/s. It is also enough to minimize the infiltration error to acceptable levels as can be seen in #1863 (comment). For HWS this slows down simulation from 9.1s to 12.3s, which isn't too bad for a two orders of magnitude reduction in relative tolerance. https://docs.sciml.ai/DiffEqDocs/stable/basics/faq/#What-does-tolerance-mean-and-how-much-error-should-I-expect
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Is this without or without resetting to 0? It kinda seems like it's showing growing oscillations:
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Indeed it's still growing over time, just slower so it's not so much a problem in practice. That is also consistent with your comment in #1874 (comment). I wrote a new issue for follow-up work: #1897. This is running the gist 30.000 days rather than 300:
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With #1912 divergence is very slow (I subtracted 25 from the bottom plot):
I wouldn't be surprised if this is now dominated by floating point truncation errors. |
After the refactor in waterbalance error #1808 test in coupler are giving unexpected results. Could be related to the changes in how fluxes are integrated on Ribasim side.
Possibly the same as #1851.
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