How to fill missing input data

Hello everyone,
I am trying to calculate stream metabolism for a reach that does not have depth or solar radiation data.
I would like to hear your thoughts on my approach as I know that metabolism is very sensitive to depth values.

for depth, this is what I did
- Found a USGS gage 6 miles upstream (the river is relatively flat and there is no big difference in cross-sections as it is a manmade channel)
- Used the field data from the USGS and related Gage Height to Water depth
- confirmed that the relationship is statistically significant (p < 0.01)
- from the time-series gage height data, I calculated water depth and assumed that the water depth at the gage is similar to the water depth at the DO station location.
- water depth data has several gaps and I don't know what is the best approach to fill the gaps (interpolate? average? average monthly?)

for solar radiation,
- I downloaded solar radiation data from the NREL website
- the data has a lot of gaps and it does not make sense as the model takes the light data to determine when the day starts and when it ends.
- heard from some friends that you can come up with light data based on sunrise sunset info?

Thanks in advance!!!
Karoline