Soil salinity and sodicity are serious environmental hazards, with the potential to limit agricultural production and cause destructive soil degradation. These concerns are especially high in dry areas, which often rely on saline and sodic irrigation water to support agriculture. To assess long-term soil degradation risk, we introduce the Salt of the Earth (SOTE) model, which describes the dynamics of soil water content, salinity, and sodicity, as driven by irrigation and rainfall. The SOTE model incorporates how changes in salinity and sodicity affect saturated soil hydraulic conductivity, K s , on a soil-specific basis. The model was successfully validated against results from a multiyear lysimeter experiment involving different irrigation water qualities and precipitation. We evaluated the impact of shorter rainy seasons on the dynamics of soil degradation in a Mediterranean climate. Critical degradation risk, indicated by reductions in K s greater than 20%, increased from 0% to 3% when the rainy season was shortened from 130 to 80 days. Alarmingly, when irreversible degradation is allowed for, overall risk increases to 68%. Assessing the effect of irrigation water on different soils textures, we found that while greater clay fractions are usually more susceptible to dispersion, accurate risk assessment hinges on soil water dynamics. SOTE is amenable to large-ensemble simulations of stochastic climatic conditions, for which trends in the statistics of salinization and soil degradation can be identified. As such, SOTE can be a useful land management tool, allowing planners to understand the risk of long-term soil degradation given irrigation practices, soil qualities, and climate conditions.
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.
Models for the effect of salinity and sodicity on saturated soil hydraulic conductivity, Ks, have yet to consider hysteresis. Ignoring hysteresis limits our ability to assess the risk posed by irrigation with saline and sodic water, such as treated wastewater (TWW). We introduce SOTE 2.0, the first model to consider hysteresis in Ks, as driven by different climate and irrigation regimes. The new model integrates the SOTE 1.0 model for salinity and sodicity dynamics with a model for the effect of saline and sodic water on Ks that explicitly includes hysteresis. SOTE 2.0 is used to demonstrate how hysteresis significantly alters our understanding of degradation and rehabilitation. SOTE 2.0 relies on weight functions to highlight soil‐specific differences in degradation and rehabilitation patterns. While TWW irrigation can be crucial to mitigating water scarcity, simulations show that salinity and sodicity have the potential to irreversibly damage soil structure, as measured by declines in Ks. Compared to the McNeal model used by Hydrus and others, SOTE predicts up to 50% degradation risk in settings where the McNeal model predicts none. The SOTE model also predicts slower rehabilitation: up to 100 days, compared to 0 days when using the McNeal model. Results highlight the difference between susceptibility and risk, showing that the probability of degradation is not solely dependent on initial susceptibility to degradation. To fully characterize a soil, we must also know its propensity to rehabilitation.
Abstract. Declines in soil saturated hydraulic conductivity (Ks) as a result of saline and sodic irrigation water are 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 our 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 novel 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, 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.
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