Soil water tension
The soil water tension or suction tension describes the energy conditions in the pore water . There is a functional relationship between the soil water tension (pressure in the pore water) and the degree of filling of the pore space with water (degree of saturation ), which is why this is used as a state variable to characterize the hydraulic availability of the soil water. The relationship between soil water tension and soil moisture is characteristic for the pore size distribution and ultimately for the water storage capacity of different soils.
Suction tension
The suction tension , physically a mechanical tension in the pore water, results from the capillarity of the grain structure in the soil and the surface tension of the wetting fluid . The soil water tension can be derived microscopically as a discontinuity of the phase pressure at the phase boundary between the wetting and the non-wetting phase. Correspondingly, in a soil column that stands in pressure-free (ground) water and is not irrigated, the higher the water, the more fine, coherent pores there are in the grain structure, which allow the water to rise against gravity through capillarity. (Rain) water, on the other hand, from coarser pores will flow into the lower free (ground) water. The smaller the pore radii still wetted in the grain structure during drainage, the higher the suction stresses in the soil.
According to Hagen-Poiseuille's law , a laminar flow through a pipe decreases with the fourth power of the radius of the pipe; therefore, with increasing suction tension in the soil, not only does the water content decrease, but also the water conductivity (even drastically).
The hydraulic potential is used as a measure of the energy ratios of the pore water to describe the movement of water in the soil and groundwater . The groundwater surface often serves as a reference point in soil physics. By convention, positive pore water pressures prevail below the groundwater surface. Pore water pressures that are measured above the groundwater level are represented as suction stresses ("suction") with the opposite sign .
Potential concept
Potentials are expressed in the dimension of an energy. To characterize the energy state, potentials are often related to a specific volume, mass or weight. In groundwater hydraulics and soil physics , they are almost always standardized to weight, which formally results in the dimension of a length for potentials ("meter of water column"). The suction tension of one meter clearly corresponds to the load that a 1 m high column of water would exert on a membrane. The inherent energy (e.g. lifting work) that holds the water against gravity in the soil matrix is expressed with the matrix potential. In soil science, the matrix potential is given as the decadic logarithm of the soil water tension expressed in cm water column, analogous to the pH value, as the pF value . A soil water tension of −100 hPa corresponds to the amount according to the pressure of a 100 cm high water column, i.e. the pF value 2.0.
Water tension curve
There is a characteristic relationship between the water tension and the amount of water (expressed as volumetric or gravimetric water content) in the soil, which describes the water storage properties of the soil as a water tension curve. The figure shows the functional curves of the matrix potentials for sand (Ss), silt (Uu), silty loam (Lu) and clay (Tt). Based on the course of the curve, the pore size distribution of a soil and the amount of water available to plants can be determined depending on the water content. The water tension curve, which characterizes the distribution of the pore sizes, is also decisive for the hydraulic permeability . Often attempts are made to determine the decrease in hydraulic permeability as the degree of saturation decreases from the relationship between soil water tension and soil moisture .
Hysteresis
Between the drainage ( desorption ) of a saturated soil and the irrigation of a dry soil, there are differences in the course of the water tension curve of a soil, which represent a typical hysteresis . This results in different energetic levels (matrix potential) for the same degree of filling (water content), depending on the previous history of irrigation or drainage. The reasons for this are complex and not fully understood. One reason, however, is that during desorption the coarse pores are drained first, then the fine pores, while with slow irrigation the fine pores begin to fill first due to capillary effects. The trapped air also behaves differently in both cases.
Measurements and measurement
The soil water tension is read off in mm water column or mm mercury column or given as negative pressure in hPa . Pressure measuring devices consisting of water-filled permeable ceramics, which are in hydraulic contact with the soil water, tensiometers serve as measuring devices . Pressure cells measure the water pressure in the tensiometer, whereby it is assumed that this is in equilibrium with the pore water.
Conversion examples
In the hydrostatic state , a groundwater level of 60 cm below the soil surface corresponds, for example, to a soil water tension of −60 hPa or a pF value of 1.8. The pF value 4.2 of the permanent wilting point PWP corresponds to a soil water tension of −1.5 MPa
See also
swell
- F. Scheffer, P. Schachtschabel: Textbook of soil science. 15th edition, Spektrum Akademischer Verlag. 2002, ISBN 3-8274-1324-9 , pp. 151-185.
- Karl Heinrich Hartge: Introduction to Soil Physics. F. Enke, Stuttgart 1978, ISBN 3-432-89681-6 , pp. 132-140.
Web links
- UNSODA Model - Database of Unsaturated Hydraulic Properties
- SWRC Fit adapt soil hydraulic models to measured soil water tension curves