Rework horizontal component hydrology model

[vgro]

The current version of the hydrology model (horizontal flow) has several flaws and open questions, including:

• what happens to water at boundaries and in depressions (lowest point of grid)?
• how far and fast should water move across the grid, from grid cell to grid cell in one time step, or all the way down the hill to the lowest point in one time step?
• how do we account for time delay in water flows, especially below ground water entering the stream?
• what should the units/quantities of outputs be?
• does each grid cell have a fraction of 'stream' or do we implement a proper river channel approach? For example combining something like https://github.com/mdbartos/pysheds with a SWAT model
• if we implemented a river channel, how would this affect the other models, i.e. would we have to reduce the available grid area for plant growth?

NEXT Steps:

• @vgro and @lelavathy will read about options to discuss on field trip