Applying HYDRUS to Flow in a Sodic Clay Soil with Solution Composition–Dependent Hydraulic Conductivity

Abstract

Management of sodic soils under irrigation often requires application of chemical ameliorants to improve permeability combined with leaching of excess salts. Modeling irrigation, soil treatments, and leaching in these sodic soils requires a model that can adequately represent the physical and chemical changes in the soil associated with the amelioration process. While there are a number of models that simulate reactive solute transport, UNSATCHEM and HYDRUS‐1D are currently the only models that also include an ability to simulate the impacts of soil chemistry on hydraulic conductivity. Previous researchers have successfully applied these models to simulate amelioration experiments on a sodic loam soil. To further gauge their applicability, we extended the previous work by comparing HYDRUS simulations of sodic soil amelioration with the results from recently published laboratory experiments on a more reactive, repacked sodic clay soil. The general trends observed in the laboratory experiments were able to be simulated using HYDRUS. Differences between measured and simulated results were attributed to the limited flexibility of the function that represents chemistry‐dependent hydraulic conductivity in HYDRUS. While improvements in the function could be made, the present work indicates that HYDRUS‐UNSATCHEM captures the key changes in soil hydraulic properties that occur during sodic clay soil amelioration and thus extends the findings of previous researchers studying sodic loams.

Using HYDRUS to simulate the dynamic changes of Ca²⁺ and Na⁺ in sodic soils reclaimed by gypsum

“…This may be because CaSO 4 application decreased soil solution pH, increased aggregation stability, and improved the soil hydraulic conductivity properties. The efficiency of Ca 2+ application to improve doi: 10.17221/14/2015-SWR permeability of sodic and saline-sodic soils was demonstrated previously (Suarez et al 1984;Reading et al 2012a). The change of soil K under different treatments with leaching time can be simulated by HYDRUS-1D, which reflects the measured result.…”

“…The UNSATCHEM model has recently been incorporated in the HYDRUS-1D software package (Simunek et al 2008. The HYDRUS-1D software may be used to analyze the movements of water and solute in unsaturated, partially saturated, or fully saturated homogeneous layered media (Simunek et al 2008;Reading et al 2012a).…”

Reactive Transport Modeling

“…Reading et al (2012a) used reactive transport modeling to address issues associated with irrigation and management of sodic soils, where chemical ameliorants are often used to improve infiltration and hydraulic conductivity and enhance the leaching of excess salts from the root zone. Describing such a system requires a model that accommodates solute transport and the requisite chemical reactions (i.e., cation exchange and mineral dissolution/precipitation).…”

“…Describing such a system requires a model that accommodates solute transport and the requisite chemical reactions (i.e., cation exchange and mineral dissolution/precipitation). As Reading et al (2012a) noted, however, the model must also describe physical changes in soil properties that accompany changes in soil chemistry; more specifically, changes in hydraulic conductivity that occur when sodium dominates the cation exchange complex. While several available models can be used to describe such basic chemical reactions, only UNSATCHEM and HYDRUS‐1D can address changes in hydraulic conductivity associated with shifts in soil chemistry (Šimůnek et al, 1996, 2008).…”

“…In their study, Reading et al (2012a) compared simulations using HYDRUS‐1D (ver. 4.14) to a series of recent laboratory experiments reported in Reading et al (2012b) that evaluated the use of gypsum to ameliorate a nonsaline sodic clay soil from North Queensland, Australia.…”