How do cows find water, assuming that there is no previous knowledge on the cow's part?
There is no substantive evidence that cows can smell water (or riparia for that matter). It seems to be generally agreed upon that cows either stumble upon water or follow trails to water; however, some believe that cows sense water through smell.
To my knowledge, there is no hard evidence that cattle have the ability to smell/sense water.
Cattle that are inexperienced in a particular pasture usually spend a good amount of time exploring their new surroundings often using existing livestock/wildlife trails. Logically, these trails usually lead to water. In addition, cattle, much like many other animals, do not exert more energy than needed, so they prefer gentler terrain. This terrain often contains the water sources in a pasture.
With that said, it is a good practice to show naive cattle the reliable water source(s) in a pasture at turnout, particularly in extensive rangeland pastures where the distance between water sources may be great and/or located in the uplands.