HYDRUS 2D

HYDRUS is a popular package for simulating water and solutes movement in variably-saturated porous media. It is widely used by designers of drip irrigation systems to simulate the active wetting pattern. However, there are many design parameters of the domain and for the flux calculation; other than the acceptable simulations, some configurations might cause underestimation or over-estimation of the wetting pattern. Comparative assessments of the surface drip simulation parameters were performed. We evaluated the effect of different domain geo-metries, element shapes, coordinate systems, emitter discharges, and soil textures on the accuracy and stability of the simulation. The results showed that the 3D simulations are more successful and reliable than the 2D simulations in terms of mass balance error. For the 2D domains, the error increases as the texture goes finer, and for axisymmetric domains than the Cartesian ones. We found a relationship between the flux and the soil hydraulic conductivity that their quotient should not exceed 1.8 and 3.2 for axisymmetric and cartesian coordinates. Our outcomes show that the infiltration stage took more time than the redistribution stage because of the variation in water content that delays the convergence at infiltration. Additionally, we found that the range between the saturated and the residual water content of the soil causes instability of the simulation as it gets higher. The differences between 2D simulations and the equivalent 3D slices were highly dependent on the soil texture, the heavier the larger; and dependent on the simulation phase, where the redistribution phase has fewer differences than the infiltration phase. Finally, we have determined the best matching 2D flux profile to each 3D profile and revealed the source of variation between 2D and 3D profiles where it was mostly-affected by the emitter's location, the soil texture, and simulation phase. In part 2 of this paper, we expanded the study for subsurface drip simulations and showed the difference between the two systems' designs.

Historically in Argentina, cattle grazing occurred in extensive lands, in natural pastures. However, in the last few decades the advancement of soy-cultivated land reduced land surfaces devoted to cattle grazing. From the 90´s, cattle production increased but to the expense of a new production type, feedlots-FL or animal feeding operation (AFO), which is used in intensive animal farming for finishing livestock, notably beef cattle, prior to slaughter
The steady and widespread growth of FLs has generated environmental concerns not only in Argentina.
As a consequence of cattle manure accumulation on FLs pens and uncontrolled effluents flows, FLs constitute potential point and diffuse contamination sites for soils and waters. Manure produces contamination within the pens as well as in nearby land as manure is flushed by precipitation
water and runoff. The effluent so generated is composed of particulate and dissolved substances that can infiltrate into the soil profile and move downward towards the saturated zone, jeopardizing its preservation as a potable water resource. The effluent is a hiper-concentrated source of organic carbon, nitrogen, phosphorus, sulfur, magnesium, potassium, among other constituents.
High NO3--N concentrations found in soils and waters around FLs , its evaluation and the analysis of processes associated to its transformations constitute an issue of great interest for groundwater resources protection. Consequently, the objective of this thesis was to identify how the interaction between physical processes (dilution from infiltrating water, salts concentration from evapotranspiration) and biological processes (nitrification, desnitrification) interact and affect the dynamics of NO3--N and other solutes in fined-textured soils, under different hydrologic conditions at the surface and soil moisture conditions within the soil profile. Specific objectives were aimed at designing appropriate spate/time monitoring strategies in order to capture the fundamental processes that affect NO3--N concentrations within soils.
A FL in Santa Fe State, Argentina, was selected for the investigation. In order to fulfill the objectives, three point sites located adjacent to a feedlot pen were instrumented with suction lysimeters, soil matric potential sensors and temperature sensors up to 1 m depth.
First, physical variables for each soil horizon were determined. Afterwards, numerous field campaigns
were undertaken. The monitoring period spanned for about 28 months, during which soil solution samples were extracted in coincidence with precipitation events. Hydrodynamic and meteorological variables were continuously recorded along the study period.
Water samples collected at the suction lysimeters were analyzed in-situ for certain physicochemical parameters and later transported to the lab for analytical determination of nitrates NO3--N, chloride Cl- and hardness. Finally, numerical simulations for flow and Cl- , NO3--N transport in the unsaturated zone in a 1D/ 2D configuration were performed with HYDRUS.
For flow a single porosity approach was used, while for solute transport, and due to the very fined structure soil prone to cracks/macropores formation, a mobile/immobile zone approach was implemented.
Field observations/interpretation and numerical simulations allowed identifying the relative role of specific processes affecting water movement through the soil profile, the dynamics of nitrate due to mixing processes, dilution, physical dispersion, vegetation consumption and biological processes, as well as the difficulties derived from structural changes associated to fine-textured soils in relation
to the aforementioned processes.

This is the second part of the companion papers assessing HYDRUS simulations of drip irrigation. While the first part focused on surface drip simulations, this part focused on the subsurface simulations and how they differ from the surface simulations. We evaluated the effect of different domain geometries, emission element shapes, coordinate systems, emitter discharges, and soil textures on the accuracy and stability of the simulation. The results showed that all the 2D and 3D simulations were done with a mass balance error of less than 0.9%, in contrast to the surface 2D cases that have large error values. The flux-conductivity ratio for subsurface simulations does not have the same influence on the mass balance error as in the surface simulations. We found extreme differences in the simulation speeds that were attributed to the emitter's location (the subsurface is faster), the domain axes (axisymmetric is faster than Cartesian), soil texture (the lighter is faster), the elapsed time or stage (the redistribution is hugely faster than redistribution). For the 2D-3D comparisons, we found that the 2D domains mostly underestimate the 3D slices. Finally, we found that the flux profiles of the 2D domains perfectly match the flux profiles of the 3D domains, in contrast to the surface comparisons. Hence, we can use 2D simulations reliably for subsurface drip, but not for the surface drip.