> It seems like it would be more effective to simply put all the solar panels in a field, and construct a cheap shade over the entire canal no?

No because now you're using land that could be used for other things, which is exactly what this avoids. This takes existing land that loses water and costs money, and makes it lose less water and make money.

That’s true of everything you build. The cost of land has to be taken into account, but land is much cheaper in some areas than others.

You are assuming that land is expensive and forgetting that building and maintaining panels over long a canal is harder than building over a large plot.