Irreversible soil degradation due to soil salinity and sodicity, and the role of reduced hydraulic conductivity feedback

Abstract:

Soil sodicity is characterized by a high relative amount of sodium cations in the soil, usually measured by the sodium adsorption ratio (SAR) or by the exchangeable sodium percentage (ESP). It negatively affects the soil’s physical properties, causing slaking, swelling, and dispersion of clay, which lead to decreased hydraulic conductivity and infiltration rate. Soil sodicity is especially problematic in arid and semi-arid regions, where low-quality waters, such as treated wastewater or sodic/saline groundwater, are often used to meet agricultural demands.

The dynamics of sodicity in the soil is intertwined with that of salinity, i.e., the total concentration of salt in the soil, and the soil water balance. We present a model for the coupled dynamics of relative soil water content s, salinity C, and sodicity ESP and investigate the effects of irrigation with water of good and bad quality on the soil. We explicitly account for the major feedback on the soil moisture dynamics due to decreased saturated hydraulic conductivity at low values of C and for high values of ESP, and show that it leads to a bifurcation of steady-state solutions along a control parameter axis (irrigation rate, SAR of irrigation water, etc).

We study theoretically such bifurcations, which are related to a runaway sodification process, and determine the conditions in which there is an irreversible soil degradation and the time scales associated with it. Finally, we discuss different soil remediation strategies based on the optimal control theory applied to the proposed system for the coupled water, salinity and sodicy dynamics.

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