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.
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.
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.
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.
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.