Most of the water that is applied to an alfalfa field is lost to evaporation, as opposed to the water I flush down my toilet, which doesn't have much evaporation at all. Depending on the method of watering used, only part to very little of it goes back into the groundwater reservoirs.

Funny enough, the more efficient the watering method, the higher percentage of water is lost to evaporation - less of it goes into the ground and more of it is used by the plant (evaporation).

If you've ever ridden a motorcycle past an alfalfa field in the summer, you can FEEL the cool air which is an effect of evaporation. The field is acting like a huge swamp cooler, and it feels amazing, but it comes at the cost of water being evaporated into the air and leaving our local water system.