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To improve the quality of parameter optimization, estimability analysis has been proposed as the first step before inverse modeling. When using field data of irrigation experiments for the determination of soil hydraulic parameters, wetting and drying processes may complicate optimization. The objectives of this study were to compare estimability analysis and inverse optimization of the soil hydraulic parameters in the models with and without considering hysteresis of the soil water retention function. Soil water pressure head data of a field irrigation experiment were used. The one-dimensional vertical water movement in variably saturated soil was described with the Richards equation using the HYDRUS-1D code. Estimability of the unimodal van Genuchten–Mualem hydraulic model parameters as well as of the hysteretic parameter model of Parker and Lenhard was classified according to a sensitivity coefficient matrix. The matrix was obtained by sequentially calculating effects of initial parameter variations on changes in the simulated pressure head values.Optimization was carried out by means of the Levenberg-Marquardt method implemented in the HYDRUS-1D code. The parameters α, Ks, θs, and n in the nonhysteretic model were found sensitive and parameter θs strongly correlated with parameter n. When assuming hysteresis, the estimability was decreased with soil depth for Ks and αd , and increased for θs and n. Among the shape parameters, αw was the most estimable. The hysteretic model could approximate the pressure heads in the soil by considering parameters from wetting and drying periods separately as initial estimates. The inverse optimization could be carried out more efficiently with most estimable parameters. Despite the remaining weaknesses of the local optimization algorithm and the inflexibility of the unimodal van Genuchten model, the results suggested that estimability analysis could be considered as a guidance to better define the optimization scenarios and then improved the determination of soil hydraulic parameters.
An artificial catchment was constructed to study initial soil and ecosystem development. Our research site is the hydrologic catchment area “Chicken Creek”, 20 km south of the city Cottbus (Germany), which is left to undirected succession. The approximately 6-ha catchment area was built in 2005 of coarse-textured quaternary sediments and forms a back- and foot-slope that flattens out to a pond. A clay liner, 2-3 m in thickness, seals the catchment at the bottom. The initial “soil” layer consists of sandy quaternary sediments, approximately 2–3 m in thickness, that overlay the clay liner. The sediments of the artificial system are in the very initial stage of soil formation. As a key process, the pore structure dynamics in the soil at the surface strongly influences erosion, infiltration, matter dynamics, and vegetation establishment. Little is known, however, about early macropore formation. This presentation focuses on observations of the general development of the catchment, with emphasis on soil structure modifications, such as soil surface crusts, structure formation from ground beetles, moss vegetation and its effect on water flow.
We visualized the soil structure and flow patterns using a combination of neutron radiography, neutron tomography (NT) and X-ray computed tomography (CT) experiments. All techniques were used to describe preferential pathways as a soil
structure with CT and the process of preferential flow of water with NT. For the first time, neutron radiation was applied on undisturbed soil cores to visualize water distributions in natural soils on three examples containing heterogeneities of different origin. Our observations demonstrate relatively high abiotic and biotic dynamics of soil pore structure in the soil surface even during the very early development stages of the catchment. The structure formation influences runoff and infiltration by forming sealing layers or preferential flow paths, due to biotic activity.