Laboratory equipment
With laboratory measurements, you determine various parameters under defined boundary conditions, which gives you a high repetition rate of the tests and thus a well-founded statistical evaluation.
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Things to know about laboratory equipment
Some important parameters for the hydraulic characterization of a site / soil material can only be derived from laboratory experiments. Laboratory measurements allow the determination of various parameters under defined boundary conditions, which allows a high repetition rate of the experiments and thus also a well-founded statistical evaluation of the determined parameters. One of the most important material-describing parameters are the grain size distribution and the porosity, which in addition to the determination of the soil type, also provide information about the quality of the soil, its water balance characteristic, as well as its suitability as arable land or soil.
Further essential soil-hydraulic properties are the retention characteristics and the description of the conductivity of the soil as a function of the degree of saturation. Both characteristics can be determined by means of laboratory experiments.
Grain size distribution
The grain size distribution indicates the proportions of the grain size classes of a soil. On the basis of this distribution of the proportions, it is not only possible to derive the soil type but also to make assumptions about the hydraulic behavior and the transport of the material (e.g. increased sorption behavior in the case of highly clayous soils). The distribution of the grain sizes is determined for particles with a diameter greater than 0.063 mm by means of a dry sieve analysis. After pre-wet separation of the fine particles (wet sieving), the distribution of fine fractions is determined by a sedimentation analysis.
Porosity
The porosity of a soil is the ratio of the pore volume to the total volume of a soil sample. It plays an important role in the water and material transport behavior of a soil. The porosity can be derived from the particle density and the bulk density of a soil. The particle density can be determined by means of a pycnometer.
Retention curve
The retention curve of a soil describes the non-linear relationship between the water content and the pressure head in the soil. For this reason, important characteristic values can be derived, such as the water content at the permanent wilting point PWP of the plants (conventionally at pF = 4.2, where pF = lg (pressure head [cm]), the water content at field capacity FK (conventional at pF = 1.8) or the amaount of plant available water (FK-PWP).
The course of the retention curve up to pF = 3 can be determined by means of the evaporation method (ku-pF-apparatus) or via multistep-outflow experiments ( e.g. with a hanging water column). Pressure plate experiments are usually used for the range of the retention curve with pF> 4.
Conductivity characteristic
The conductivity characteristic of a soil describes the non-linear relationship between the water content (or spressure head) of a soil and the hydraulic conductivity. The retention characteristic is used together with the conductivity characteristic for the description of the hydraulic behavior of a soil and provides a basis for the parameterization of physically based water transport models. The hydraulic conductivity of a soil is greatest under saturated conditions.
As the water content decreases, the hydraulic conductivity of the soil decreases as the effective cross-section area for the flow decreases, while the friction and the tortuosity increase at the same time. The determination of the conductivity function can be determined simultaneously with the retention curve by using the ku-pF apparatus.
FAQ: Hydraulic Conductivity
Saturated hydraulic conductivity (Ksat) describes the ability of a fully saturated soil to transmit water. It is one of the most important hydraulic soil properties and strongly influences infiltration, seepage, groundwater recharge, and surface runoff.
Hydraulic conductivity is determined in the laboratory on undisturbed or prepared soil samples under defined hydraulic conditions. Permeameters are often used for this purpose; they measure the water flow through a soil sample at a controlled hydraulic gradient. In the constant-head method, the hydraulic gradient is kept constant during the measurement, resulting in stable and reproducible measurement conditions. In the falling-head method, the gradient changes during the measurement due to the decreasing water column.
The UGT permeameter based on the Ketelsen method operates according to the constant-head principle and enables automated measurement of hydraulic conductivity over a wide range of values. The system detects the hydraulic properties of the sample and automatically adapts the measurement to both well- and poorly permeable soils. The lower measurement limit is a kf value of 0.15 cm/d.
The UGT Hood-Permeameter by Hartge also operates on the constant-head principle and enables the determination of saturated hydraulic conductivity in undisturbed soil samples under very small hydraulic gradients. By using gradients that closely resemble natural conditions, the conditions of water movement in the soil are better represented than in measurements with high pressure gradients.
Hydraulic conductivity influences nearly all water movements in the soil and is therefore a key input parameter for hydrological models. Precise measurements significantly improve the simulation of infiltration, evaporation, groundwater recharge, and substance transport. A realistic determination of hydraulic conductivity is therefore crucial for reliably modeling water flows in the soil and improving the accuracy of hydrological models.
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