Modeling risks of salt-induced irreversible soil degradation

Abstract:

Degradation of soil hydraulic conductivity resulting from the use of saline and sodic irrigation waters is a major environmental danger, especially in dry areas. Limited water resources in these regions encourages the use of low-quality irrigation water -- often high in salt content -- for agriculture. While the mechanisms of how salinity and sodicity lead to reductions in hydraulic conductivity (e.g., slaking, swelling, clay dispersion) are the focus of a vast body of scientific literature, the rehabilitation process is far less understood. Existing models treat degradation and rehabilitation in the hydraulic conductivity as reversible. The scant experimental evidence that exists, however, suggests that these processes feature hysteresis, i.e., the system follows different paths for degradation and rehabilitation. We introduce the SOTE model, a minimalistic model designed to study the long-term dynamics of soil water content, salinity, and sodicity, as driven by irrigation practices and climatic conditions. We integrate the SOTE model with a novel framework in which a soil’s history of degradation and rehabilitation are used to assess future response. This framework is based on the Preisach model of hysteresis and soil column experiments in which we measured changes in saturated hydraulic conductivity in soils of different compositions under varying levels of salinity and sodicity. In these experiments, both the EC and SAR of the input water were ramped up and down in a controlled manner, in order to reveal the hysteretic response of Ks. We use the integrated version of the SOTE model to explore the effect of irreversibility on the risk of soil degradation. When the potential for hysteresis is taken into account, risk of long-term degradation from a typical irrigation regime and climate conditions in Israel increases from 20% to 35% in a span of 10 years. When comparing current climate conditions to expected shorter rainfall seasons, degradation risk increases by 25% over 10 years. Our results emphasize the importance of considering irreversibility when assessing salinity and sodicity induced degradation risks, which no existing models do.