Simulating preferential soil water flow and tracer transport using the Lagrangian Soil Water and Solute Transport Model

Abstract

Abstract. We propose an alternative model concept to represent rainfall-driven soil water dynamics and especially preferential water flow and solute transport in the vadose zone. Our LAST-Model (Lagrangian Soil Water and Solute Transport) is based on a Lagrangian perspective of the movement of water particles (Zehe and Jackisch, 2016) carrying a solute mass through the subsurface which is separated into a soil matrix domain and a preferential flow domain. The preferential flow domain relies on observable field data like the average number of macropores of a given diameter, their hydraulic properties and their vertical length distribution. These data may be derived either from field observations or by inverse modelling using tracer data. Parameterization of the soil matrix domain requires soil hydraulic functions which determine the parameters of the water particle movement and particularly the distribution of flow velocities in different pore sizes. Infiltration into the matrix and the macropores depends on their respective moisture state, and subsequently macropores are gradually filled. Macropores and matrix interact through diffusive mixing of water and solutes between the two flow domains, which again depends on their water content and matric potential at the considered depths. The LAST-Model is evaluated using tracer profiles and macropore data obtained at four different study sites in the Weiherbach catchment in southern Germany and additionally compared against simulations using HYDRUS 1-D as a benchmark model. While both models show qual performance at two matrix-flow-dominated sites, simulations with LAST are in better accordance with the fingerprints of preferential flow at the two other sites compared to HYDRUS 1-D. These findings generally corroborate the feasibility of the model concept and particularly the implemented representation of macropore flow and macropore–matrix exchange. We thus conclude that the LAST-Model approach provides a useful and alternative framework for (a) simulating rainfall-driven soil water and solute dynamics and fingerprints of preferential flow as well as (b) linking model approaches and field experiments. We also suggest that the Lagrangian perspective offers promising opportunities to quantify water ages and to evaluate travel and residence times of water and solutes by a simple age tagging of particles entering and leaving the model domain.

Simulation of reactive solute transport in the critical zone: A Lagrangian model for transient flow and preferential transport

“…The LAST-Model (Sternagel et al, 2019) is based on the Lagrangian approach of Zehe and Jackisch (2016), which was introduced to simulate infiltration and soil water dynamics in the partially saturated zone using a non-linear random walk in the space domain. The results of test simulations (cf.…”

“…2). A comparison of HYDRUS 1-D and the LAST-Model based on plot scale tracer experiments showed that both models perform similarly in case of matrix flow dominated tracer transport; however, under preferential flow conditions, LAST better matched observed tracer profiles indicating preferential flow (Sternagel et al, 2019). 40 https://doi.org /10.5194/hess-2020-527 Preprint.…”

“…Ewen, 1996a, b;Bücker-Gittel et al, 2003;Davies and Beven, 2012;Zehe and Jackisch, 2016;Jackisch and Zehe, 2018). Sternagel et al (2019) proposed that these water particles may optionally carry solute masses to simulate non-reactive transport. Their Lagrangian Soil Water and Solute Transport Model (LAST) combines the assets of the Lagrangian approach with an Euler-Grid to simulate fluid motion and solute transport in heterogeneous, partially saturated 1-D soil domains.…”

“…While the results of Sternagel et al (2019) demonstrate the feasibility of the Lagrangian approach to simulate conservative tracer transport even under preferential flow conditions during one-day simulations, a generalization of the Lagrangian approach to reactive solute transport and larger time scales is still missing. In this context it is important to recall that the use of spatially discrete control volumes (Euler-Grids) to calculate concentrations of 5 water and solute particles in Lagrangian models can suffer from the problem of artificial over-mixing (e.g.…”

“…Appendix). We refer to Sternagel et al (2019) for a more detailed description of the model and results.…”