IIRC, the materials cost for building long haul fiber is a couple dollar per meter or less, especially in China.
If you're already building a massive structure like a canal, embedding conduits into the infrastructure is a tiny marginal cost. And Fiber maintenance is very, very low in protected environments (such as the electric-fence protected canal).
Not only that, you could easily build two conduits, one on each side of the canal, creating route diversity.
So for laying hundreds of miles (let's say 1000km) of fiber where construction is already happening, the total cost would be a few million USD.
They list the maximum range as 100 Km, with accuracy of +/- (0.8 m + 0.001% x Distance). So measuring to a break at maximum range, you'll be within 1.8 meters.
- Maintenance: It could be very difficult to find and fix a fiber break. AFAIK, there's no easy way to determine where a break in the fiber is, you generally end up replacing the entire broken segment, which in this case could be miles.
- Reliability: If the fiber breaks, all the upstream sensors go down. Especially since a fiber break could also mean a pipeline break, you just handicapped yourself at the most critical time.
- Cost: Hundreds of miles of fiber, properly shielded from the elements, must cost a lot. (Of course, getting wireless signal to thousands of sensor stations would also be pricey, and I can't say how they'd compare.)