Most people refer to water droplets on grass as 'dew.' Dew is condensed water vapor that forms at a certain temperature (dew point). Grass itself, when healthy, also secretes some water at night known as guttation. Grass that is dry, does not do this. The end result is a weak dew pattern. You can see above in the picture, a fairway that has a weak dew pattern. As golf course managers, this is clue number one that a given area is in need of irrigation. Therefore, it is important for me and my assistants to scout the fairways prior to mowers removing the dew. This gives us a pretty good indication of what is going to happen that day.
All plants and grass work very simply. The roots of the plant take up water (just like we drink it) and then the plants release water through stomates to cool themselves off (like we sweat). When there is no more water for the roots (dry soil) the plant wilts. This triggers dormancy in some plants, death in others. The release of water is known as evapotranspiration. Our goal as turf managers is to simply replace what is lost on a given day. There are different models to estimate this. If a playing surface loses the equivalent of .10" water/day, it is my goal to add .10" of irrigation water. Of course, this does over simplify it by it gives you the basic overall philosophy. There are a number of other challenges that prohibit the movement of water into the soil that we also deal with.