added 4-bucket model to documentation

[vgro]

This PR adds the schematic diagram of the 4-bucket model to the hydrology documentation see here .

@lelavathy I noticed that there is no inflow in the grid cell from the left side, I added a not but we might want to update the figure.

Fixes #937

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.22%. Comparing base (92145eb) to head (7034bce).
⚠️ Report is 1625 commits behind head on develop.

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@@           Coverage Diff            @@
##           develop     #938   +/-   ##
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  Coverage    94.22%   94.22%           
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  Files           79       79           
  Lines         6320     6320           
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  Hits          5955     5955           
  Misses         365      365           

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[lelavathy]

@vgro Just to confirm—how is lateral inflow from the left side of the grid cell handled in the current implementation? I thought all lateral/horizontal flow arrows (Q1–Q5) point only to the right, toward river discharge. If inflow from the left is part of the model logic, it might be worth reflecting that with incoming blue arrows on the left side of each relevant storage layer.
Let me know what you think, and I’m happy to help update the figure!

[vgro]

@vgro Just to confirm—how is lateral inflow from the left side of the grid cell handled in the current implementation? I thought all lateral/horizontal flow arrows (Q1–Q5) point only to the right, toward river discharge. If inflow from the left is part of the model logic, it might be worth reflecting that with incoming blue arrows on the left side of each relevant storage layer. Let me know what you think, and I’m happy to help update the figure!

The way it is implemented at the moment is that the total outflow from one grid cell is the outflow from the current time step(Q1-Q5) + the accumulated outflow from the previous timestep from all the grid cells that are upsteam. Thus means the is not exactly an inflow that changes the soil moisture but rather a throughflow that adds directly to the stream.

[lelavathy]

@vgro Just to confirm—how is lateral inflow from the left side of the grid cell handled in the current implementation? I thought all lateral/horizontal flow arrows (Q1–Q5) point only to the right, toward river discharge. If inflow from the left is part of the model logic, it might be worth reflecting that with incoming blue arrows on the left side of each relevant storage layer. Let me know what you think, and I’m happy to help update the figure!

The way it is implemented at the moment is that the total outflow from one grid cell is the outflow from the current time step(Q1-Q5) + the accumulated outflow from the previous timestep from all the grid cells that are upsteam. Thus means the is not exactly an inflow that changes the soil moisture but rather a throughflow that adds directly to the stream.

[lelavathy]

@vgro Just to confirm—how is lateral inflow from the left side of the grid cell handled in the current implementation? I thought all lateral/horizontal flow arrows (Q1–Q5) point only to the right, toward river discharge. If inflow from the left is part of the model logic, it might be worth reflecting that with incoming blue arrows on the left side of each relevant storage layer. Let me know what you think, and I’m happy to help update the figure!

The way it is implemented at the moment is that the total outflow from one grid cell is the outflow from the current time step(Q1-Q5) + the accumulated outflow from the previous timestep from all the grid cells that are upsteam. Thus means the is not exactly an inflow that changes the soil moisture but rather a throughflow that adds directly to the stream.

[lelavathy]

@vgro Just to confirm—how is lateral inflow from the left side of the grid cell handled in the current implementation? I thought all lateral/horizontal flow arrows (Q1–Q5) point only to the right, toward river discharge. If inflow from the left is part of the model logic, it might be worth reflecting that with incoming blue arrows on the left side of each relevant storage layer. Let me know what you think, and I’m happy to help update the figure!

The way it is implemented at the moment is that the total outflow from one grid cell is the outflow from the current time step(Q1-Q5) + the accumulated outflow from the previous timestep from all the grid cells that are upsteam. Thus means the is not exactly an inflow that changes the soil moisture but rather a throughflow that adds directly to the stream.

Does that mean the throughflow from upstream (i.e. accumulated outflow from the previous timestep) doesn’t enter any of the storage buckets (like surface storage or soil moisture), but instead bypasses them and contributes directly to streamflow?

If so, would it be helpful to explicitly show that in the diagram—maybe with a separate arrow labeled as “throughflow from upstream cells” going directly into the river discharge box? Also, is there any scenario where this throughflow would need to be added to the surface or subsurface storage (e.g. if there's a ponding or backflow condition)?

[vgro]

Does that mean the throughflow from upstream (i.e. accumulated outflow from the previous timestep) doesn’t enter any of the storage buckets (like surface storage or soil moisture), but instead bypasses them and contributes directly to streamflow?

If so, would it be helpful to explicitly show that in the diagram—maybe with a separate arrow labeled as “throughflow from upstream cells” going directly into the river discharge box? Also, is there any scenario where this throughflow would need to be added to the surface or subsurface storage (e.g. if there's a ponding or backflow condition)?

yes, such an arrow that indicates the through-flow would be great. Would you be able to update this and send it to me, or replace the file on GitHub directly?
The might be a scenario in the future where this changes, but I don't see this happening in the next 12 months; I can add some more explanation to the figure description.

[lelavathy]

@vgro it supposed to be red solid arrows indicate upward vertical flow, red dashed arrows show
downward vertical flow.

[vgro]

you're right, well spotted. I fixed it in #949