Water retention curve

The soil water tension or suction pressure describes the energy of the interstitial water. There is a functional relationship between the base voltage of water ( water pressure in the pores ) and the degree of filling of the pore space with water ( saturation ), so this is used as a state variable, in order to characterize the hydraulic availability of soil water. The relationship between soil water tension and soil moisture is characteristic of the pore size distribution and, ultimately, for the water storage capacity of different soils.

Soil water potential

The suction pressure, a physical stress in the pore water, the result of the capillarity of the grain skeleton in the ground, and the surface tension of the wetting fluid. Microscopically, leaves the soil water tension derived as a discontinuity of the phase pressure at the phase boundary between the wetting and non-wetting phase. Accordingly, the water rises in a soil column, which is in pressure-free ( ground) water and is not irrigated, the higher are present, the more fine, coherent pores in the soil skeleton, which can ascend the water against gravity by capillarity. ( Rain) water from coarser pores, however, drain into the deep free standing ( ground) water. The smaller the drainage at a still wetted pore radius in the grain structure, the higher suctions are in the ground.

According to the law of Hagen- Poiseuille takes a laminar flow through a pipe with the fourth power of the radius of the tube from; therefore increases with increasing soil moisture tension in the soil not only the water content, but also the water conductivity ( this even drastically ).

To describe the movement of water in soil and groundwater is used as a measure of the energy ratios of the pore water, the hydraulic potential. The reference point is used in soil physics often the groundwater surface. By convention prevail below the groundwater surface positive pore water pressures. Pore ​​water pressures measured above the water table, are represented with the opposite sign, as the suction pressure ( "suction" ).

Potential concept

Potential can be expressed in the dimension of energy. For the characterization of the energy state potentials are often related to a specific volume, a mass or a weight. In the groundwater hydraulics and soil physics, they are almost always normalized to the weight, resulting in potential for the formal dimension of a length contract (' meters of water "). Clearly corresponds to the suction pressure of one meter of stress that would put a 1 m column of water on a membrane. With the matrix so the inherent potential energy (e.g., lifting work ) put, which keeps the water against the gravity into the soil matrix. The matric potential is indicated in soil science as decimal logarithm of the amounts expressed in inches of water soil water tension, similar to the pH when pF value. A soil water tension of -100 hPa, corresponding in amount to the pressure of a 100 cm high water column, so the pF value 2.0.

Water tension curve

Between the water tension and the amount of water (expressed as a volumetric or gravimetric water content ) in the bottom, there is a characteristic relationship that describes the water tension curve, the water storage properties of the soil. The figure shows the functional profiles of the matrix potential for sand ( Ss), silt (Uu ), silty loam (Lu) and tone (Tt ) are plotted. Based on the progression 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 pore sizes, which also applies to the hydraulic permeability. Attempts are often made to determine from the relationship between soil water tension and soil moisture the decrease in hydraulic conductivity with decrease in the degree of saturation.

Hysteresis

Between the drainage ( desorption) of a saturated soil, and irrigation of a dry base, there are differences in the course of the voltage curve of a water base, which represent a typical hysteresis. Thus (water content ) are obtained for the same degree of filling different energy levels (matrix potential ), depending on the history of irrigation or drainage. The reasons for this are complex and not completely understood. One reason is that in the desorption first the coarse pores will be dewatered, the fine pores, while starting from the slow first irrigation the fine pores to be filled by capillary action. Also, the trapped air acts differently in both cases.

Metrics and measurement

The soil water tension is read in mm water column or inches of mercury or specified as a negative pressure in hPa. When instruments are used pressure measuring devices consisting of water-filled permeable ceramics which are in hydraulic contact with the soil water tensiometer. Load cells measure the water pressure in the tensiometer, assuming that it is in equilibrium with the pore waters.

Conversion Examples

In the hydrostatic state, a groundwater level of 60 cm below the soil surface corresponds, for example, a soil water tension of -60 hPa or a pF value of 1.8. The 4.2 pF value of the permanent wilting point PWP corresponds to a soil water tension of -1.5 MPa