Water use and salinity dynamics in the soils are the crucial management factors influencing the productivity and long-term sustainability of almond and associated environment. In this study, HYDRUS-2D was calibrated and validated on measured spatial and temporal water contents and soil salinities (ECe) distributions under almond irrigated with different water qualities (ECiw) at different physiological stages. During two irrigation seasons (2014–15 and 2015–16), less saline irrigation water (average ECiw 0.78 dS/m) was substituted for recycled irrigation water (average ECiw 1.9 dS/m) in three phenologically different growth stages; pre-pit hardening, kernel growth, and post-harvest, along with no and full substitution during the entire season. Graphical and statistical comparisons (RMSE, MAE, ME, the Nash and Sutcliffe model efficiency, and the coefficient of determination) between measured and simulated values of water contents and ECe in the soil showed a close agreement in all treatments. The water balance data revealed that the seasonal crop evapotranspiration of almond (ETc) varied from 850 to 955 mm in different treatments over the two seasons which represented 68–79% of the water application. Trees irrigated with only less saline water through the two seasons (average ECiw 0.78 dS/m) showed 10% higher plant water uptake as compared to those irrigated with recycled water only (average ECiw 1.9 dS/m). Substituting less saline irrigation during the kernel growth phase, between pit-hardening and harvest, showed greater water uptake by almond and lower salinity buildup in the soil as compared to treatments that substituted less saline irrigation early or late in the season. For all treatments, the average daily root zone ECe (2.4–3.7 dS/m) remained above the level of the almond salinity tolerance threshold (ECe = 1.5 dS/m) throughout the period of investigation. Water use efficiency of almonds varied in a narrow range (0.21–0.25 kg m−3) for different treatments. Deep drainage below the root zone (2 m) varied from 22.4–31.1% of the total water application (Rainfall + Irrigation), which was episodic and insufficient to contain the salinity below the almond threshold. This study provided a greater understanding of soil water and salinity dynamics under almond irrigated with waters of varying qualities.

Riparian zones are essential to preserve water quality of rivers adjacent to large areas of irrigated agriculture. We used HYDRUS (2D/3D) to quantify the long-term (8 years) influence of crops (almonds, wine grapes and potato-carrot) irrigated with recycled water on water and solute exchange at the Gawler River interface in relation to vegetation buffer widths from 10 to 110 m. We found that under almond and annual horticulture the likely average annual water flow from the irrigated area to the river was nearly twice as much (2.1 and 1.8, respectively) that under wine grapes. The hydraulic exchange at river interface for different irrigated crops was found to be sensitive to the buffer widths. For wine grapes, almonds and annual horticulture, the average annual hydraulic balance reached an equilibrium at 20, 65 and 55 m buffer widths, respectively. Furthermore, for wine grapes, with a 20 m buffer width, the average annual load of salts became negligible. This study shows that buffer widths of 20, 60, and 40 m for irrigated wine grapes, almond, and annual horticulture, respectively, are needed to restrict the migration of salts to the river. Further refinements are possible by incorporating the influence of preferential flow paths, improved water stress response functions, and addressing the data limitations for calibration of the model for solute dynamics.