Comparison of different soil water extraction systems for the prognoses of solute transport at the field scale using numerical simulations, field and lysimeter experiments

Weihermüller, Lutz

Dissertation / PhD Thesis/Book

Comparison of different soil water extraction systems for the prognoses of solute transport at the field scale using numerical simulations, field and lysimeter experiments

Weihermüller, L. (Corresponding author)FZJ*

2005
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 3-89336-402-1

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Umwelt / Environment 55, XXII, 145 S. (2005) = Universität Bonn, Diss., 2005

Please use a persistent id in citations: http://hdl.handle.net/2128/490

Abstract: Sampling of water and solutes in soils is of major importance to understand water movement and solute transport in soils and to validate mathematical models. Porous cups are widely used to extract soil water for monitoring solute transport. However, it is not yet clear how the suction cup influences the matrix potential in the surrounding soil and which part of the soil is sampled. This research was designed to numerically evaluate the activity domain, the extraction domain and sampling area of a suction cup under constant infiltration. A finite element model (HYDRUS-2D) was used to simulate the effect of various applied suctions at two infiltration rates on the water status in three soils (clay loam, sandy clay and sandy soil). Particle tracking was used to track the streamlines which define the sampling area and extraction domain of the suction cup. In general, the activity domain, the extraction domain and sampling area of the suction cup depend primarily on the soil hydraulic parameters and the upper boundary, and secondarily on the applied suction. The results showed that the activity domain, the extraction domain and the sampling area are largest for highest ambient hydraulic conductivities. The activity domain and the sampling area also decrease with increasing infiltration rates. Further, the extraction domain of the suction cup depends strongly on the duration of water extraction. Soil heterogeneity seems to play a minor role with respect to the activity domain and sampling area of the cup, but a major role in the amount of extracted water and solute breakthrough. In a second step, a lysimeter and field experiment equipped with porous ceramic plates, tensiometers, TDR-probes and suction cups was conducted over a 427 day period to evaluate differences in soil water sampling and solute transport for a conservative tracer and the test compounds Methabenzthiazuron (MBT) and Ethidimuron (ETD). In general, the two lysimeters at the lysimeter station at the $\textit{Forschungszentrum Jülich}$ showed comparable leaching behaviour over time, reflected by the tensiometer, TDR and drainage measurements. In comparison to the lysimeters the sampling pits at the test site Merzenhausen indicate lower water contents and amounts of extracted water over the whole sampling period. These differences can be traced back to variability in the climatic data (e.g. precipitation and evaporation) caused by microclimatic distinctions at the two locations. The results of the field and lysimeter experiments show that no complete tracer breakthrough could be determined at a depth of 120 cm for the sampling pits and the lysimeters as a consequence of the short sampling time and the dry summer in 2003. As a result of retardation the breakthrough of the test substances MBT and ETD is not complete even for the samplers at 40 cm depth. Therefore, bromide breakthrough at 120 cm depth and MBT/ETD breakthrough for 40 and 120 cm depth were just described in qualitative terms. The variability in bromide tracer breakthrough at 40 cm depth for the different samplers at the two locations is reflected in the variability of the mean pore water velocity, $\nu$, and dispersivity, $\lambda$. On the other hand, no transport parameters could be determined for the pesticide breakthrough at all locations. In general, larger ETD recoveries were measured compared to the MBT for all samplers, whereby high peak concentrations in the samplers can be traced back to single events. The differences in the amount of extracted water, bromide masses, and as a result variability in transport [...]

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