Gravitational water refers to the amount of water held by the soil between saturation and field capacity. Several terms are used to describe the water held between these different water contents. These terms illustrate soil from its wettest condition to its driest condition. This water is held as films coating soil particles. Unavailable water is the soil water content that is strongly attached to soil particles and aggregates, and cannot be extracted by plants. At the permanent wilting point, a plant will wilt and not recover. Permanent wilting point is the soil water content when plants have extracted all the water they can. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity.
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Free drainage occurs because of the force of gravity pulling on the water. Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours. The water content in the soil at saturation is equal to the percent porosity. Saturation is the soil water content when all pores are filled with water. The following discussion will consider water content on a volumetric basis. Since 1 gram of water is equal to 1 milliliter of water, we can easily determine the weight of water and immediately know its volume. The volumetric expression of water content is used most often. Water content can be quantified on both a gravimetric (g water/g soil) and volumetric (ml water/ml soil) basis. Water held in a soil is described by the term water content. Infiltration and permeability describe the manner by which water moves into and through soil. If the percolating water carries chemicals such as nitrates or pesticides, these water reservoirs may become contaminated. Water that percolates deep in the soil may reach a perched water table or groundwater aquifer. It also refers to the movement of water below the root zone. This is the mechanism by which water reaches the subsoil and rooting zone of plants. Permeability is generally rated from very rapid to very slow ( Table 2.4). Permeability also varies with soil texture and structure. Organic matter, especially crop residue and decaying roots, promotes aggregation so that larger soil pores develop, allowing water to infiltrate more readily. Low infiltration rates lead to ponding on nearly level ground and runoff on sloping ground. Infiltration rates can be near zero for very clayey and compacted soils, or more than 10 inches per hour for sandy and well aggregated soils.
It also serves as the storage compartment for water. Pore space in soil is the conduit that allows water to infiltrate and percolate. Movement of water into soil is called infiltration, and the downward movement of water within the soil is called percolation, permeability or hydraulic conductivity. Soil acts as a sponge to take up and retain water.
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