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

Models for the effect of salinity and sodicity on saturated soil hydraulic conductivity, K_s, 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 K_s, 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 K_s 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 K_s. 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.