Bug in accumulated runoff

[vgro]

Problem:

There is an assumption that all water from upstream runs through the grid cell in one time step, however, the way it is updated in the loop is adding too much water:

New_accumulated runoff(t1) = surface_runoff(t1, me) + previous_accumulated_runoff(t0, upstream cells)

then set
Previous_accumulated_runoff(t1) = new_accumulated_runoff(t1)

New_accumulated runoff(t2)
=  surface_runoff(t2, me) + previous_accumulated_runoff(t1, upstream cells) 
= surface_runoff(t2, me)  + surface_runoff(t1, me)  + previous_accumulated_runoff(t0, upstream cells) +  previous_accumulated_runoff(t1, upstream cells) 

So we keep adding the same water over and over again

Possible solutions:

1. move water only a shorter distance, like one grid cell per time step, make sure inflow and outflow is considered.

Flow_through_cell (t1)=next highest neighbour(t0)-me(t1)

2. Make water run through the whole grid, adjust how previous_accumulated is handled, I believe it should be the surface runoff from the previous timestep only. Maybe better to call it upstream_runoff, definitely explain variables better in documentation

New_accumulated runoff(t1) = surface_runoff(t1, me) + upstream_runoff(t0)

then set
net upstream_runoff(t1) = surface_runoff(t1, upstream cells)

New_accumulated runoff(t2) = surface_runoff(t2, me) + upstream_runoff(t1)

This means that the water will magically disappear at the lowest points with the new time step.

@lelavathy , @sallymatson and I started a discussion that we might want to move to a proper river routing implementation, which would make this problem go away (see #1034 ) but for now decided to fix the bug first.

[sallymatson]

Personally I'd lean toward solution 2 as the immediate bug fix. This keeps the functionality as is, and just makes it work as intended. Then, we can discuss if solution 1 would be a more appropriate system.. however I'm worried that given our scale, this would actually make water move way too slowly. In any chase, changing the implementation to something like that would be a bigger picture choice so in the meantime we can do the low hanging fruit option!