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.
<p>If not carefully managed, use of saline and sodic irrigation waters, a common practice in dryland regions, has the potential to cause significant harm to soils. Application of saline and sodic irrigation waters can lead to reductions in hydraulic conductivity, Ks, the mechanisms of which (e.g., slaking, swelling, clay dispersion) have been the focus of a vast body of scientific literature. The rehabilitation process, by contrast, is far less understood. Despite experimental evidence showing a significant degree of irreversibility, traditionally models have treated the degradation and rehabilitation processes as reversible. We demonstrate how this assumption obfuscates our ability to analyze the risk of long-term degradation and to estimate the resources and time required to rehabilitate. We achieve this by using the SOTE model &#8212; a minimalistic model that can be used to study dynamics of soil water content, salinity, and sodicity, as driven by irrigation practices and climatic conditions. Crucially, SOTE also accounts for the feedback between changing salinity and sodicity and soil Ks. This feedback includes irreversible changes in hydraulic conductivity, such that a soil&#8217;s history of degradation and rehabilitation informs its future state. We compare SOTE to existing models, which do not include hysteresis in Ks, and demonstrate that SOTE predicts more gradual rehabilitation of degraded soils, in line with the limited experimental evidence that has examined this question. For the test case of a degraded soil in a typical Mediterranean climate, SOTE forecasts that rehabilitation requires 50% more time and water resources, in comparison to models without hysteresis. This difference underscores the need to limit the risk of potential degradation, which SOTE also shows increases by 50% when hysteresis is accounted for. A sensitivity analysis indicates that SOTE is most sensitive to parameters connected to soil texture. The sensitivity analysis further indicates that our results are robust -- under all ranges of parameter values SOTE continues to forecast greater time requirements for rehabilitation and increased risk of soil degradation.</p>
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