The concept of field capacity has not been established enough. In the present paper, it is intended to make a clear definition of field capacity based upon the kinetic point of view.The concept proposed by the author is : Field capacity must be defined as “the water regime, of the soil layer in which plant root could distribute, at the point where the amount of water to be drained off from the layer, naked and without evaporation, decreases to the level of the amount of water lost from the layer with plant covering by evapotranspiration.”
To clarify the characteristics of drainage process, the relationships between layered conditions of soils and the hydraulic gradient changes with time during drainage were investigated.
Under well-drained condition, the soil of uniform structure and the soil underlaid by a layer of smaller unsaturated permeability show the follwing tendencies.
1. At the beginning of drainage process, the hydraulic gradient in the drying zone is constant and about 1.0 regardless of depth, and subsequently it decreases with time.
2. The decreasing velocity of hydraulic gradient is larger for soils with larger percentage of pore space corresponding to the water suction from 30 to 100cm.
3. The hydraulic gradients at field capacity ranges from 0.1 to 1.0.
By using the values of unsaturated permeability of each soil layer, a method of determining the distribution of water content and water suction at field capacity is proposed on the basis of the author's definition and his experimental results.
Good results were obtained when the proposed method was applied to some field soils.
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