Hysteresis in soil hydraulic conductivity as driven by salinity and sodicity – a modeling framework

Abstract

Abstract. Declining soil-saturated hydraulic conductivity (Ks) as a result of saline and sodic irrigation water is a major cause of soil degradation. While it is understood that the mechanisms that lead to degradation can cause irreversible changes in Ks, existing models do not account for hysteresis between the degradation and rehabilitation processes. We develop the first model for the effect of saline and sodic water on Ks that explicitly includes hysteresis. As such, the idea that a soil's history of degradation and rehabilitation determines its future Ks lies at the center of this model. By means of a “weight” function, the model accounts for soil-specific differences, such as clay content. The weight function also determines the form of the hysteresis curves, which are not restricted to a single shape, as in some existing models for irreversible soil processes. The concept of the weight function is used to develop a reversibility index, which allows for the quantitative comparison of different soils and their susceptibility to irreversible degradation. We discuss the experimental setup required to find a soil's weight function and show how the weight function determines the degree to which Ks is reversible for a given soil. We demonstrate the feasibility of this procedure by presenting experimental results showcasing the presence of hysteresis in soil Ks and using these results to calculate a weight function. Past experiments and models on the decline of Ks due to salinity and sodicity focus on degradation alone, ignoring any characterization of the degree to which declines in Ks are reversible. Our model and experimental results emphasize the need to measure “reversal curves”, which are obtained from rehabilitation measurements following mild declines in Ks. The developed model has the potential to significantly improve our ability to assess the risk of soil degradation by allowing for the consideration of how the accumulation of small degradation events can cause significant land degradation.

The Sustainability of Treated Wastewater Irrigation: The Impact of Hysteresis on Saturated Soil Hydraulic Conductivity

“…Specifically, SOTE 2.0 integrates the Preisach framework for changes in K s , as presented in Kramer et al. (2021), which is unique in its ability to consider how a soil's history of degradation and rehabilitation will affect its future state. The Preisach framework is founded on the idea of hysterons, elementary switches that can be turned “on” and “off” according to predefined thresholds.…”

“…Kramer et al. (2021) adapted the original framework so that its inputs are salinity concentration ( C s ) and sodium adsorption ratio (SAR), while K s is the output. While SOTE 1.0, like all other existing models, is “memoryless,” that is, future states of the system depend only the present state, the dynamics prescribed by SOTE 2.0 depend on the whole of salinization and sodification.…”

“…In contrast to the models noted earlier, the framework developed by Kramer et al. (2021) is the only one that includes hysteresis in K s under changing water chemistry. We then use the integrated model, called SOTE 2.0, to highlight the clear effects that hysteresis has on K s dynamics, when modeling both degradation and rehabilitation scenarios (Section 3).…”

“…Our objective in this paper is to address this gap, by developing a model for the dynamics of soil salinity and sodicity, which is explicitly capable of considering hysteresis in K s . We begin in Section 2, by integrating SOTE 1.0 (Kramer & Mau, 2020)—a model for the dynamics of soil water content, salinity concentration, and relative sodicity—with a recently introduced module for irreversible changes in K s (Kramer et al., 2021). In contrast to the models noted earlier, the framework developed by Kramer et al.…”

“…This degradation is not always completely reversible, with experimental evidence indicating that changes in K s as a result of salinity are characterized by hysteresis (Adeyemo et al., 2022; Dane & Klute, 1977; Kramer et al., 2021). That is to say, while K s can be expected to decline under saline and sodic conditions, results do not suggest that an improvement in chemical conditions will cause K s to rise at the same rate that it declined.…”