Last Gridcell/Sub routing question

[mee067]

This is just a question. MESH used not to run the LSS for the very last gridcell that has NEXT=0. Newer code, not sure since which release, started to run the LSS for that last gridcell. To make things equivalent to old code, one would set all GRU fractions for that cell to zero for the LSS not to run. Maybe this is related to vector setups as the workflow does not add that additional sub-basin which will have a Rank with NEXT=0 as GK used to do.

I know that last gridcell was added to enable adding its runoff and routing it to the outlet with all the incoming flow.

Now the LSS will generate runoff for that last grid or sub which is not an extra. Does the routing still work for it while its NEXT = 0?

[dprincz]

The routing will only run over 1:NAA, but it will put the outflow from NAA into NA. I don't believe runoff over NA is routed (meaning qo only includes qi from upstream cells). The way to do that is to set NAA=NA. It'll print a warning if that's the case, but I don't believe it will halt the run (I think setups ported from GEM-Hydro usually have the condition NAA=NA).

This is a good consideration, as runoff over the downstream-most subbasin in vector setups might not see full routing (e.g., incorporating runoff over that subbasin itself). It could be something for someone to investigate.

[mee067]

From what I gather, this setting of NAA=NA would be inside the code, right?

For large basins that have one outlet at the end, runoff from the last grid/sub may not add much but for smaller basins, and for a setup with multiple outlets, excluding runoff of the last grid/sub or not routing it may be significant.

[dprincz]

I'm not sure NAA=NA should be an assumption set by the code. It could be set by the fields in the drainage database (e.g., by having no cell with NEXT==0, which could be handled by the workflow.