The soil moisture status can be expressed in terms of soil moisture content and water potential. What is the relationship between them? The relationship between the two can be represented by the curve formed by the soil moisture content and the corresponding free energy logarithm PF, that is, the soil moisture characteristic curve. Different texture soils have similar characteristic curves. Under the same suction, the finer the texture, the greater the water content; at the same moisture content, the finer the texture, the greater the suction. For plants, the effectiveness of sand is higher under the same moisture content, but sand absorbs less moisture and the maximum moisture content is lower. The soil water potential can be measured using a soil water potential meter so that it can reflect its data very well.
Water potential and soil moisture content are not always single-value relationships, and their characteristic curves do not coincide during the dehydration process. The same soil moisture content under suction is higher in the dehydration process than in the water absorption process and is called hysteresis. The two complete characteristic curves are called the main lines of hysteresis. When the soil starts to drain from semi-humid, or when partially dehydrated soil is re-wetted, the suction and soil moisture advance along the middle curve between the two main lines. The hysteresis is related to the texture, the sand is obvious, and the clay is not obvious.
Through the analysis of data measured by the soil water potential meter, the reason for this can be explained as:
(1) The "ink bottle effect" caused by the geometric inhomogeneity of individual pores. That is, water must pass through the radius during the drainage process.
(2) In the wet process, the curved surface along which the moisture advances along the capillary tube is larger than the contact angle when the dewatering process retreats, that is, the curvature radius is larger and the suction force is smaller.
(3) The air enclosed in the soil mass during wetting reduces the water content of the soil.
(4) Soil swelling and shrinkage and aging during the process of getting wet and drying out, and the gradual dissolution of air in soil water, or the release of dissolved gases all have an impact on the formation of hysteresis.
Water potential and soil moisture content are not always single-value relationships, and their characteristic curves do not coincide during the dehydration process. The same soil moisture content under suction is higher in the dehydration process than in the water absorption process and is called hysteresis. The two complete characteristic curves are called the main lines of hysteresis. When the soil starts to drain from semi-humid, or when partially dehydrated soil is re-wetted, the suction and soil moisture advance along the middle curve between the two main lines. The hysteresis is related to the texture, the sand is obvious, and the clay is not obvious.
Through the analysis of data measured by the soil water potential meter, the reason for this can be explained as:
(1) The "ink bottle effect" caused by the geometric inhomogeneity of individual pores. That is, water must pass through the radius during the drainage process.
(2) In the wet process, the curved surface along which the moisture advances along the capillary tube is larger than the contact angle when the dewatering process retreats, that is, the curvature radius is larger and the suction force is smaller.
(3) The air enclosed in the soil mass during wetting reduces the water content of the soil.
(4) Soil swelling and shrinkage and aging during the process of getting wet and drying out, and the gradual dissolution of air in soil water, or the release of dissolved gases all have an impact on the formation of hysteresis.
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