Having to rent a cherry picker and being able to do maintenance only during a limited time per year significantly increases the cost of maintenance compared to an installation you can trivially access from ground level year-round. Canals being long and narrow don't exactly help either.
Sure, it can be done, but strictly from the PV perspective it doesn't make any economic sense compared to a regular solar farm when there's still plenty of land available. Similarly, from a "preventing evaporation" perspective going for a plain corrugated metal roof might not be much cheaper in installation, but it will be significantly easier to maintain.
Just because two thing can technically be combined, doesn't mean it makes sense to do so.
Sure, it can be done, but strictly from the PV perspective it doesn't make any economic sense compared to a regular solar farm when there's still plenty of land available. Similarly, from a "preventing evaporation" perspective going for a plain corrugated metal roof might not be much cheaper in installation, but it will be significantly easier to maintain.
Just because two thing can technically be combined, doesn't mean it makes sense to do so.