Removing water means that the remaining water is more tightly bound than before and therefore the water activity is lower. Lowering the temperature results in reducing the rate of movement of the water and this acts to lower the water activity. It must be kept in mind that when products warm up, there water activity will rise too and then leave that product prone to whatever negative reactions occur within the new water activity range. Adding ingredients can result in lowering water activity by lowering the energy state of absorbed water. The energy that would be needed to remove the bound water form the food is called water potential.
Water potential is an equilibrium measure.
This often is not the case with foods. Sometimes however, the water is moving through the system so slowly that equilibrium cannot be achieved in the foods normal lifttime. In this case it can for practical purposes be said that the water is bound. For example, it is hypothesised that when foods enter the glassy state the movement of water is so slow that it is effectively bound. Water potential is a quantitative measure of the binding energy of water in food.