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Zelnik, Y. R. ; Mau, Y. ; Shachak, M. ; Meron, E. High-integrity human intervention in ecosystems: Tracking self-organization modes. PLOS COMPUTATIONAL BIOLOGY 2021, 17.Abstract
Author summary Human intervention in ecosystems is motivated by various functional needs, such as provisioning ecosystem services, but often has unexpected detrimental outcomes. A major question in ecology is how to manage human intervention so as to achieve its goal without impairing ecosystem function. The main idea pursued here is the need to identify the inherent response ways of ecosystems to disturbances, and use them as road maps for conducting interventions. This approach is demonstrated mathematically using two contexts, grazing management and vegetation restoration, and compared to remote sensing data for the latter. Among the surprising insights obtained is the beneficial effect of grazing, in terms of resilience to droughts, that can be achieved by managing it non-uniformly in space. Humans play major roles in shaping and transforming the ecology of Earth. Unlike natural drivers of ecosystem change, which are erratic and unpredictable, human intervention in ecosystems generally involves planning and management, but often results in detrimental outcomes. Using model studies and aerial-image analysis, we argue that the design of a successful human intervention form calls for the identification of the self-organization modes that drive ecosystem change, and for studying their dynamics. We demonstrate this approach with two examples: grazing management in drought-prone ecosystems, and rehabilitation of degraded vegetation by water harvesting. We show that grazing can increase the resilience to droughts, rather than imposing an additional stress, if managed in a spatially non-uniform manner, and that fragmental restoration along contour bunds is more resilient than the common practice of continuous restoration in vegetation stripes. We conclude by discussing the need for additional studies of self-organization modes and their dynamics.
Kramer, I. ; Bayer, Y. ; Adeyemo, T. ; Mau, Y. Hysteresis in soil hydraulic conductivity as driven by salinity and sodicity - a modeling framework. HYDROLOGY AND EARTH SYSTEM SCIENCES 2021, 25, 1993-2008.Abstract
Declining soil-saturated hydraulic conductivity (K-s) 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 K-s, 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 K-s that explicitly includes hysteresis. As such, the idea that a soil's history of degradation and rehabilitation determines its future K-s 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 K-s is reversible for a given soil. We demonstrate the feasibility of this procedure by presenting experimental results show-casing the presence of hysteresis in soil K-s and using these results to calculate a weight function. Past experiments and models on the decline of K-s due to salinity and sodicity focus on degradation alone, ignoring any characterization of the degree to which declines in K-s are reversible. Our model and experimental results emphasize the need to measure ``reversal curves'', which are obtained from rehabilitation measurements following mild declines in K-s. 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.
Kramer, I. ; Mau, Y. Soil Degradation Risks Assessed by the SOTE Model for Salinity and Sodicity. WATER RESOURCES RESEARCH 2020, 56.Abstract
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.
Mau, Y. ; Zelnik, Y. ; Meron, E. Drought resilience of dryland ecosystems under spatial periodic forcing. In EGU General Assembly Conference Abstracts; EGU General Assembly Conference Abstracts; 2018; Vol. 20, pp. 9249.
Kramer, I. ; Mau, Y. Salinity and Sodicity Dynamics in Soils: A Simplified Model. AGU Fall Meeting Abstracts 2018.
Mau, Y. ; Porporato, A. Optimal control solutions to sodic soil reclamation. Advances in Water Resources 2016, 91, 37 - 45. Publisher's VersionAbstract
We study the reclamation process of a sodic soil by irrigation with water amended with calcium cations. In order to explore the entire range of time-dependent strategies, this task is framed as an optimal control problem, where the amendment rate is the control and the total rehabilitation time is the quantity to be minimized. We use a minimalist model of vertically averaged soil salinity and sodicity, in which the main feedback controlling the dynamics is the nonlinear coupling of soil water and exchange complex, given by the Gapon equation. We show that the optimal solution is a bang–bang control strategy, where the amendment rate is discontinuously switched along the process from a maximum value to zero. The solution enables a reduction in remediation time of about 50%, compared with the continuous use of good-quality irrigation water. Because of its general structure, the bang–bang solution is also shown to work for the reclamation of other soil conditions, such as saline–sodic soils. The novelty in our modeling approach is the capability of searching the entire “strategy space” for optimal time-dependent protocols. The optimal solutions found for the minimalist model can be then fine-tuned by experiments and numerical simulations, applicable to realistic conditions that include spatial variability and heterogeneities.