The USGS Water Science School
If you live in an area where ample rain falls all year, you won't see many aqueducts like the ones pictured here. But there are many areas of the world, such as the western United States, where much less rainfall occurs and it may only occur during certain times of the year. Large cities and communities in the dry areas need lots of water, and nature doesn't always supply it to them.
Some parts of the western U.S. do have ample water supplies, though. So, some states have developed ways of moving water from the place of ample supply to the thirsty areas. Engineers have built aqueducts, or canals, to move water, sometimes many hundreds of miles. Actually, aqueducts aren't a high-tech modern invention—the ancient Romans had aqueducts to bring water from the mountains above Rome, Italy to the city.
Can you see something about the aqueduct in these pictures that causes some water to be lost in transit? In places where the climate is hot and dry, a certain portion of the water flowing in the aqueduct is bound to evaporate. It would be more efficient to cover the aqueduct to stop loss by evaporation, but the cost of covering it must be weighed against the value of the evaporated water.