Modelling the change in conductivity of soil associated with the application of saline-sodic water

Modelling the change in conductivity of soil associated with the application of saline-sodic water

[Abstract]: Scarcity of fresh water has led to use of low quality waters (high sodicity and salinity) that were considered unsuitable for irrigation in the past. Mismanagement of irrigation using this water can increase the potential for soil degradation and limit crop production in the long term. Irrigation using highly saline-sodic water requires appropriate management to avoid long term development of sodicity and salinity problems. The main factors that control the sodicity and salinity problems are maintenance of sufficient
leaching and avoidance of soil structure degradation due to sodicity. The management options are determined by complex factors such as soil type and condition, water quality, irrigation practice and crop type.

Investigating the management options for using highly saline-sodic water in irrigation experimentally is costly and time consuming. However, it could be done using an
appropriate modelling tool that can handle the degradation of soil structure due to sodicity along with the chemical reaction system within the soil profile. UNSATCHEM has been widely used to model sodicity and salinity effects under irrigation. It has a feature to deal with soil structure degradation along with water and solute movement, major ion chemistry, CO2 production and movement and heat transfer under sodic conditions. It uses a hydraulic conductivity reduction function to
relate the change of chemical properties to the change in hydraulic properties of the soil. However, the evaluation of the hydraulic conductivity reduction function under
high sodicity during simulation has not been done. Hence, the core of this research project has been to improve quantification of soil structure degradation under sodic
conditions and enhance the modelling of water and solute movement under sodic conditions. The hydraulic conductivity reduction function incorporated in
UNSATCHEM was evaluated using data obtained from soil column experiments.

Columns of two local soils were used in an experiment to investigate the effect on soil structural stability of different amendments to highly saline-sodic water rich with
bicarbonates (EC = 4.6 dS/m and SAR = 117). The column experiments were used to examine the effect of reducing water pH to different levels using sulphuric acid and
combined gypsum and dilution treatments. It was found that reducing the pH of highly saline-sodic water did not enhance soil structural stability as the water applied has
naturally high relative sodium concentrations. However, the application of diluted highly saline-sodic water amended with gypsum showed no significant effect on soil structure and permeability. It is concluded that different amendments associated with appropriate irrigation management can be applied to sustain irrigation and prevent long term salinity and sodicity problems.

The data from the column experiments was used to evaluate the quantification of the soil structure degradation in UNSATCHEM. The resultant simulations for the soil
columns showed that the estimated outflow and hydraulic conductivity were less than the experimental measurements, which suggested that the soil structure degradation
was not accounted for properly. The sodicity effect was accounted for in UNSATCHEM by a reduction function, which is a combined function of the McNeal (1968) clay swelling model and the Simunek et al. (1996) pH effect equations. The empirical pH effect equation accounts for the reduction of the conductivity due to increasing pH and clay swelling. The evaluation of UNSATCHEM under highly sodic conditions suggests that the hydraulic conductivity reduction function is limiting the UNSATCHEM performance.

Consideration of the first term of the hydraulic conductivity reduction function (i.e. the McNeal (1968) clay swelling model) has highlighted the weaknesses of the McNeal model and led to develop a generic clay swelling model (GCSM). Calibration of the GCSM using the data of McNeal showed good agreement between the estimated and measured relative conductivity data. Further calibration of the GCSM using relative conductivity data obtained for five local soils also showed good agreement between the model estimation and the measured data.

Coding of the generic clay swelling model into UNSATCHEM and re-simulating the column experiments showed that the modelling process is improved compared with the
UNSATCHEM version containing the McNeal (1968) clay swelling model. However, the outflow and conductivity values produced were still less than measured values. This result suggested that further investigations are required to identify the effect of pH on the change of hydraulic conductivity, cation exchange capacity, and the exchangeable sodium percentage. Further research is also required regarding bicarbonate chemistry during
application of highly saline-sodic water.