Strategic use of alkaline irrigation water: soil structural constraints and predictive amendment management

Strategic use of alkaline irrigation water: soil structural constraints and predictive amendment management

Land application of marginal quality irrigation water is a progressively increasing practice in many agronomic regions throughout the world, due to freshwater limitation and increasing demand for food and fibre. The use of marginal quality water can increase the potential for soil degradation and limit crop production in the long-term. Irrigation with such water that contains an excess amount of alkaline ions requires a strategic and appropriate management to avoid the potential detrimental impacts on the soil resource through considering the quality of water, soil type and site condition, and irrigation practice. This research project mainly focuses on enhancing the current understanding of practical strategies for utilising alkaline water as an irrigation water resource. This is achieved via assessment of the deleterious impact of alkaline irrigation water on soil physicochemical properties, investigation of the ability of models to simulate alkalinity condition, and land amendments with acidifying materials.

The effects of pH on the soil structural degradation were investigated in the laboratory using nine Australian soils with contrasting properties. The soils were leached with irrigation water of varying EC, SAR and pH (6, 7, 8 and 9). The outcomes of this study indicated that the increase of pH results in the increase of net negative charge on clay particles, consequentially causing greater: exchangeable cation at the clay surface, negative zeta potential, clay dispersion and movement of dislodged particles into pore spaces. Subsequently, saturated hydraulic conductivity (Ks) reduction occurred at a greater magnitude. Results reinforced that the effect of pH on the Ks is soil-specific depending on the original pH, clay content and clay mineralogy of soils.

The Ks reduction data were used to develop a generalised linear function, similar to the current function presented in HYDRUS model — developed from three American soils. A nonlinear (pedotransfer) function was also produced based on these nine local soils using the Levenberg–Marquardt optimisation algorithm, considering pH and electrical conductivity (EC) of the applied irrigation water, as well as the soil clay content. Comparison of the observed Ks reduction with the predicted outputs of these functions indicated that the models performed objectively well, successfully describing Ks reduction due to the pH. The nonlinear function improved the estimation of the pH scaling factor for Ks reduction to operate as a function of soil specificity in the HYDRUS model, and needs to be considered in future HYDRUS model developments and use.

The functions developed for HYDRUS were developed under saturated hydraulic conditions in short columns, meaning that it was prudent to validate their performance for both unsaturated and longer soil columns using acidic, neutral and alkaline soils. The hydraulic conductivity data from the column experiments were used to validate the developed generalised and nonlinear functions as suitable for variably saturated conditions and superior to the existing HYDRUS function. Hydraulic conductivity prediction was greatest for the nonlinear (pedotransfer). The HYDRUS model was reasonably able to predict the change in EC and SAR, but unable to simulate pH and alkalinity appropriately. This suggests that the soil hydraulic conductivity reduction scaling factor function requires updating in HYDRUS to serve the majority of soils, preferably such that soil-specific nuances can be captured.

To strategically utilise alkaline water as an irrigation source, an investigation to improve current threshold electrolyte concentration (CTH) semi-empirical equations via incorporation the alkaline anion (HCO3) was undertaken. The current semi-empirical disaggregation model approach of Ezlit et al. (2013) is only based on the sodium and calcium system, without considering the adverse effects of alkalinity (i.e. HCO3) to reduce Ks. The results indicated that an increase in HCO3 results in Ks reduction, even at low concentration (as low as 100 mg L-1), and it is dependent on soil type. The results also demonstrated that there was a great association between Ks reduction for non-alkaline and alkaline irrigation water solution SAR and adjusted SAR (SARadj) for up to 30% Ks reduction. This suggested that HCO3 can be successfully incorporated into the current disaggregation model to determine CTH (≤20% Ks reduction) to provide an indication of alkalinity effects on soils without having to conduct experimentation for CTH beyond the current methodology of Ezlit et al. (2013).

Finally, the usefulness of marginal quality alkaline water was investigated within the field in order to demonstrate that there are suitable treatment options that can be land applied, and to investigate the capability of HYDRUS to model the expected outcomes. This study addressed the efficacy of gypsum and sulphur under irrigation with alkaline groundwater, on two Dermosol soils in New South Wales, Australia. The results indicated that the addition of sulphur source is an efficient strategy to address the alkalinity of irrigation water. The application of gypsum and sulphur performed well to reduce pH, alkalinity and SAR, and improved soil solution electrolyte concentration as well as increasing cotton yield. This supported the improvement in soil structure and permeability based on the predicted CTH analysis for EC and SARadj. The predicted results demonstrated that the HYDRUS model suffers from some issues in predicting pH, HCO3, EC and SAR within the soil profile overtime, but remains useful in helping design land amendment strategies provided model limitations are reported with presented strategies by practitioners.

This thesis research clearly highlighted the potential for strategic use of alkaline irrigation water, resulting in useful discussion identifying the limitation of guidelines and regulations. The work has highlighted the debility of the current CTH determination and the HYDRUS model to predict soil structural degradation under alkalinity conditions, providing outcomes and pathways to improve both. This study culminates by providing a framework for the strategic use of alkaline water for irrigation, with suggestions for Australian guidelines to consider alkalinity under a general use approval (GUA) and beneficial use approval (BUA) system. The GUA and BUA system simultaneously provides irrigators freedom to operate and protects the environment as a general approach, as well as providing an application based system to irrigate on a soil specific basis where receiving environment and land amendment strategies can be demonstrated as suitable.