[GRASS-user] r.sim.water - interpretation of output

[Emilie Botten]

Hi everyone,

This is my first question to this list. I hope this is the right place to ask. I have been testing r.sim.water on a small area but I struggle to interpret the output, especially the depth. The results show water depths that are significantly deeper than what the surrounding terrain would suggest is possible. It seems that the water does not spread out as expected. I have played around with the
 inputs to try to understand how the various inputs affect the simulation, but I don't feel secure in my understanding of the tool inputs.

I have the following questions:
1. When the water depth is unrealistically high relative to the surrounding terrain, which input parameters to adjust?

2. How to know if the tool has reached a steady state?

3. I never get a log file from the observation points. What should the input for the observation points be?

** Here are some details about the parameters I used: **
- DTM resolution: 0.5 m x 0.5 m (X: 1700, Y: 1416) 
- dx, dy: created with r.slope.aspect
- rain_value: 50 mm/hr
- man or man_value: tried both a raster (ranging from 0.025-0.5) and the default value 0.1
- observation: vector file with points. Tried both 2D and 3D
- nwalkers: 1000, 10000, 100000, 1000000, 3000000, 6000000
- niterations: 30 min, 60 min
- output_step: 2 min, 5 min
- diffusion_coeff, hmax, halpha, hbeta: all default values

** From running the model with my own classification of Manning's number: **
- min elevation: 0 m
- max elevation: 43.22 m
- mean source rate (rainfall excess or sediment): 0.000014 m/s or kg/m²s
- mean flow velocity: 13.631142 m/s
- mean Manning's: 0.030061
- Number of iterations: 49063
- Time step: 0.01

The error output is 0 when I increase nwalkers. I see that the default Manning value 0.1 creates a more realistic water depth output, but still the depth at the deepest of a water accumulation can be 2x what is possible from the surrounding terrain heights. The same issue occurs when testing with terrain 1 m x 1 m.

I have read the help page for r.sim.water and all other pages and PDFs I could find on the topic. Some say to increase niterations, and that will help the simulation reach a steady state, but from my understanding that would simulate a longer rainfall event not iteration towards steady state. Is that correct? 

Is there anyone with insights who can help me better understand this tool?

Thank you in advance for your help!

Best regards,
Emilie