The total amount of water in the earth system and its partition and movement among major earth realms have been topics of speculation and investigation during more than a century. Engineers regularly calculate channel storage and storage changes for many river segments as an aid to water management. Although these calculations represent only a small sample of the millions of miles of river channels around the world, they suffice to show that total channel storage, important though it may be locally, is insignificant on the world scale. Animal bodies are largely water, and plant tissue also contains much water. Plants have an important role in the water cycle through the process of transpiration. The pores in granular geological formations, fissures and joints in hard rocks, and solution cavities and channels in limestone are examples of voids within masses of rock and sediment that can store and transmit water.