In an unlined irrigation canal—especially in areas like California where water shortages are already a concern—water velocity can cause issues with the irrigation system as well as wastage. Water velocity is affected by things like capillary pressure, rough surfaces and heavy weed growth. Weed growth in and of itself can lead to water loss as the weeds soak up the water from the irrigation canal. Low velocity caused by rough surfaces and weed blockages can cause undue amounts of seepage as water soaks into the earth surrounding the canal, and it can also lead to excess evaporation from pooled water. Additionally, low flow leads to water not reaching the crops that it should—and that will cause problems with growth and harvesting.
Velocity that is too high in an unlined canal—speeds above 0.7 meters per second—will lead to erosion that can alter the shape of the canal. You may find that canal walls collapse, or that the canal widens while becoming shallower. This can lead to too much water flowing through the canal, wasting any reserves you have stockpiled in retention ponds. There are ways to optimize the velocity of irrigation canals. Liners are the most common. Even gravel on canal sides will increase velocity while reducing erosion, and concrete makes for a common liner that gets around some of these issues. Your best bet, however, will be a flexible liner that supports high velocity flows without worrying about erosion or other types of damage.
