Roughness length for rice fields - small drainage/irrigation channels

[Dhanraj]

Hi,

I am currently editing an SRTM map for an island, using the WAsP Map Editor.

On this island, rice cultivation is very dominating and there are several plots of rice fields that are separated by small drainage/irrigation channels. The width of these channels can vary between 3 m and 1 m. As such, I seek the following clarifications:

1. Is there a minimum width threshold of a water body for which roughness length becomes significant? If so, what is this minimum width threshold?

2. In my current map of roughness-change lines, is it required to indicate roughness changes for these small drainage/irrigation channels? If so, what would be a recommended value?

Thank you.

D.Bachai.

[Niels Gylling Mortensen]

Dear D. Bachai,
No, I have never heard of a minimum width threshold for water bodies, but one could make a small case study to explore this. However, I am quite sure that small meter-wide channels will not have any significant effect for a sizeable wind mast or wind turbine.
The recommended roughness for water bodies is always 0 m in WAsP. However, in your case I would suggest a low land roughness for your rice fields, see e.g. http://agrometeorologia.it/documenti/Rivista2015_1/estimation_of_zero_plane_displacement_height_and_aerodynamic_roughness_length_on_rice_fields.pdf
Best regards,
Niels

[Dhanraj]

Thank you, Niels