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A list of the soil properties that can be measured in the lab,as well as the equipment and methods used for that purpose, is provided below: :

•  Water retention curve
•  Hydraulic conductivity curve
•  Preferential flow measurements 

In the framework of the Quality Policy of the lab, all our devices concerning pressure, temperature and mass measurements are calibrated against national standards. Moreover, procedures, instructions and operating modes are written and updated for each type of measurement. In some cases, part of the samples are preserved and stored for additional characterizations (i.e. soil particles distribution).

Water retention curve

Water retention curve reflects the capacity of soil to store water against external forces. During the soil drying process, the remaining water in the soil is linked stronger and stronger to soil particles and retrieving this water requires more and more energy. This can lead to the occurrence of a water stress for plants cultivated on a dry soil. Roughly speaking, the matrix potential (noted) is the variable measuring the strength of the links between the soil water and the soil particles. It is an energy per unit volume and it can expressed in pressure units (pascal, Pa, or water hanging column, m). It is negative in the unsaturated zone, because extracting water from unsaturated soil requires to provide some energy to the soil-water system. The opposite of matric potential is called suction 

Water retention curve is the relationship between the matrix potential on one hand and the humidity of soil on the other hand. This soil moisture can be expressed on a volume basis (it is the ratio between the volume of water contained in the sample and the volume of the sample), referred to as density moisture (m³ m^-3), or on a mass basis (it is the ratio between the mass of water and dry soil sample), while talking about mass moisture or gravimetric moisture W (kg kg^-1). Retention curve is therefore the curve  ( ) ou  (W). 

To determine the retention curve, we use two types of devices. In the range of low suctions (typically between 0 and 1 m), we use suction tables or sand boxes (photo left): saturated samples are laid on a bed of pure sand (silica) connected to a water supply, the level of which can be adjusted. The distance between the sand layer surface and the water surface in the water supply bottle represents the suction. On the left photo, we see two types of samples: cylindrical cores sampled into the ground and "aggregates" obtained from the fragmentation of large and undisturbed soil clods. For high suctions (typically between 1 m and 150 m), we use Richards’ apparatus (see photo right): saturated samples are laid on porous plates with a high air entry value. These plates are put in enclosures in which air under pressure is injected: the applied pressure is equivalent to the suction.

                          ©EMMAH: Left: soil samples on sand boxes;  Right: view of the Richards’ devices used to high suction.

Hydraulic conductivity curve

Hydraulic conductivity curve reflects the capacity of soil to transmit water. It is sometimes called unfairly "permeability". By analogy we assimilate the pore network to a set of capillaries where water can flow. When the soil is completely saturated, all the pores are filled with water.

Hydraulic conductivity is maximum and is called hydraulic conductivity at saturation, usually noted K_sat or K_s. As the soil dries, larger diameter pores drain first: water will flow out of finer and finer pores during the drying process. Due to increasing friction forces and to a decrease of the flow section, water will drain with more and more difficulty. For a given pressure head gradient, the flow of water will be much lower for a dry soil than for the same wetted soil. Hydraulic conductivity is a highly decreasing function of the soil moisture content. It is a function noted K() or K().

Hydraulic conductivityis the ratio between the volumetric flow of water (expressed as m s^-1) across a given section and the driving force of this flow, which is the pressure head gradient (dimensionless). It is based upon the Darcy’s law. Initially established for saturated sands, this law is also used for unsaturated soil under the name “extended Darcy’s law”. In the International Unit System, the hydraulic conductivity is expressed in m s^-1.
We use two types of devices for measuring hydraulic conductivity. For a saturated sample, the hydraulic conductivity at saturation K_sat is obtained by using the constant head permeameter (photo left). In the unsaturated domain, we use the Wind’s method (photo right) to calculate the curve K() or K(). This method can be used under evaporation or infiltration conditions.

©EMMAH: Left: measurement of hydraulic conductivity with the constant head perméamètre, Right: measurement of hydraulic conductivity with the Wind's method (device and data processing software are marketed by SDEC France) (under license INRA).

 Preferential flow measurement

Retention and hydraulic conductivity curves are needed to model water flow in unsaturated soils by the Richards’ equation. However, in certain circumstances and for certain soils, the Darcy’s law does not apply. This is the case when high intensities rains arrive on a dry and cracked soil or on a soil containing a lot of macropores (e.g. earthworms’ burrows). This kind of flow is referred to as preferential flows or bypass flow. To describe the preferential flows, we use an equation of kinematic waves (see Di Pietro et al., 2003) as an alternative of the Darcy’s law.Parameters of the kinematic waves equation are called "cinematic" parameters and are obtained with other experimental devices. 

©EMMAH

Experimental setup for the determination of “kinematic” parameters to model preferential flow in structured soils

Contact

Scientific leader: Stéphane Ruy ,  +33 (0)4 32 72 22 37
Technical leader: Dominique Renard,  +33 (0)4 32 72 22 53