In soil, water that is readily available to plant roots is capillary water.Capillary water is held in the capillary pores (micro pores). It is retained on the soil particles by surface forces. It is held so strongly that gravity cannot remove it from the soil particles. Gravitational water refers to free water that moves through the soil due to the force of gravity. It moves rapidly out of well drained soil and is not considered to be available to plants. In hydrology, bound water is an extremely thin layer of water surrounding mineral surfaces .Hygroscopic water refers to water absorbed from the atmosphere and held very tightly by the soil particles, so that it is unavailable to plants amounts sufficient for them to survive.