Percolation
PERCOLATION - is the process by which water from the unsaturated storage gets into the saturated layer by means of gravitational and capillary forces.
Soil porosity is calculated as the fraction of pore space in a unit
volume of soil.
Some of the water gets attached to soil particles and stays in the soil. The proportion of water that cannot be removed by gravitation is called field capacity: In this case, as shown in the Figure, some water remains attached to the soil particles even after most of it has been drawn downwards by gravitation. Porosity and field capacity are important characteristics of soil. They define the maximum amount of water that soil can hold and the minimum amount that stays in the soil after a rainfall event. Additional amounts of water can be sucked out by the action of roots which is the transpiration process.
Similarly, the soil moisture content is defined as
Soil moisture content is the amount of moisture related to the total volume of the unsaturated layer. In reality the unsaturated layer is not homogeneous and therefore U varies with depth. Since in our model we assume the whole unsaturated zone to be uniform we will think that Vtotal is the total volume of the unsaturated layer and Vwater is the total amount of water in this layer. In saturated soils U = P, that is all the pores are filled with water Vwater = Vpores; and in completely dry soils U = 0. Note, however that due to percolation U can get no lower than F, the field capacity.
Water moves with different speed through different soil types. As in the
infiltration process the rate of percolation
(also called hydraulic conductivity hc) can vary from
1 m/day in sandy soils to
1 mm/day in clay. The percolation rate is also affected by the soil
moisture content:  Percolation rate factor as function of U. If U < F, the moisture is at field capacity and percolation p is 0. It tends to 1 when U approaches P. As the water percolates down it adds to the amount of water already present in the unsaturated storage. It takes only P-U water to fill in the unsaturated storage so that it becomes saturated. 
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The soil consists of material particles with air in between. These voids or
pores can be potentially taken by water. When all the pores are filled, the
soil is called saturated and vertical movement of water is very much slowed
down. While the pores are not filled the water is pulled down by the
force of gravitation and the capillary effects until it reaches the
saturated layer. In this case the vertical motion is dominant.