The idea of covering irrigation canals has been around probably for thousands of years. Only recently I have seen this being done in Xinjiang in towns right next to the desert. The canals which have river water often flow under streets and through little caves and such, enabling them to grow grapes and such. The grapes are then also providing shade on paths and roads.
No, I am talking about both. If too much water evaporated, they wouldn't have enough to water all the plants. Both ideas, preventing evaporation, and guiding the water, are very old.
