Simulating preferential soil water flow and reactive solute transport using the Lagrangian Soil Water and Solute Transport Model (LAST)
Authors: Sternagel Alexander , Loritz Ralf , Wilcke Wolfgang , Zehe Erwin 
Published: March 2020

&lt;p&gt;Recently, we proposed 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 is based on a Lagrangian perspective on the movement of water particles (Zehe and Jackisch, 2016) carrying solute masses through the subsurface which is separated into a soil matrix domain and a preferential flow domain (Sternagel et al., 2019). 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 either be derived 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 pores 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.&lt;/p&gt;&lt;p&gt;The LAST-Model was evaluated using tracer profiles and macropore data obtained at four different study sites in the Weiherbach catchment in south Germany and additionally compared against simulations using HYDRUS 1-D as benchmark model. The results generally corroborated the feasibility of the model concept and particularly the implemented representation of macropore flow and macropore-matrix exchange. We thus concluded that the LAST-Model approach provides a useful and alternative framework for simulating rainfall-driven soil water and solute dynamics and fingerprints of preferential flow.&lt;/p&gt;&lt;p&gt;This study presents an extension of the model allowing for the simulation of reactive solute transport. Transformation kinetics are considered by transferring mass from the parent to the child components in each water particle according to the corresponding reaction rates, which is limited by the compound solubility. A retardation coefficient is not helpful in the particle-based framework, as the solute mass is carried by the water particles and travels thus by default at the same velocity. Ad- and desorption are explicit represented through transfer of dissolved mass from the water particles at a given depth to surrounding adsorption sites of the soil solid phase and vice versa. This may either operate under rate-limited or non-limited conditions. Adsorbed solute masses will be considered to be degraded following first-order reaction kinetics. The retardation process delays the solute displacement and enables a suitable time scale for the degradation process, which must be smaller than the time scale for the re-mobilization of the solutes. The proposed extension will be benchmarked against observations of pesticide transport in soil profiles and at tile-drained field sites.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Zehe, E., Jackisch, C.: A Lagrangian model for soil water dynamics during rainfall-driven conditions, Hydrol. Earth Syst. Sci., 20, 3511&amp;#8211;3526, https://doi.org/10.5194/hess-20-3511-2016, 2016.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Sternagel, A., Loritz, R., Wilcke, W., and Zehe, E.: Simulating preferential soil water flow and tracer transport using the Lagrangian Soil Water and Solute Transport Model, Hydrol. Earth Syst. Sci., 23, 4249&amp;#8211;4267, https://doi.org/10.5194/hess-23-4249-2019, 2019.&lt;/p&gt;