Effects of irrigation water salinity and sodicity on infiltration and lucerne growth over a shallow watertable

Abstract

Irrigation using saline sodic groundwater is a major strategy to manage salinisation from shallow watertables in the irrigation areas of south-east Australia. There is concern that this strategy will increase soil sodicity and induce a decline in soil physical properties that affect infiltration. Laboratory experiments have shown that the saturated hydraulic conductivity of soils may decrease when a saline–sodic soil is leached with low salinity water. This paper evaluates the field significance of these concerns to irrigation water management practices.

The effects of changing the irrigation water source from saline–sodic groundwater to low salinity channel water on the infiltration properties of a hardsetting red-brown earth and the yield of lucerne (Medicago sativa) were evaluated over a 3-year period. Four dilution strategies to use high-salinity (EC 6 dS/m) and high-sodicity [SAR 16 (mmol/L)^0.5] groundwater were compared. They were: (i) irrigation with groundwater in the spring then channel water for remainder of the summer irrigation season; (ii) irrigation with channel water in spring then groundwater for the rest of season; (iii) irrigation with diluted groundwater EC 3 dS/m for whole season; and (iv) alternative irrigations with groundwater EC 6 dS/m and channel water throughout the season. The control treatment was irrigated with low-salinity (EC 0.15 dS/m) channel water all season. The treatments were applied for 2 summer irrigation seasons then channel water was applied to all plots for another season. The site was underlain by a shallow watertable at 1.0 m. The final steady infiltration rate of each plot was measured each irrigation using capacitance water level loggers. This value was used as an index of soil structural stability to the water quality treatments.

The results show all groundwater treatments caused the soil to increase in salinity from EC_{e(0–0.15 m)} 0.6–0.9 dS/m to 3.8–7.3 dS/m and sodicity from SAR_{e(0–0.15 m)} 1.7–2.1 to 14.2–16.8 after 2 years of application.

The steady infiltration rate was not affected by treatment during this period. In the third year when all plots were irrigated with channel water there was a small decrease in the steady infiltration rate during irrigation in the alternating groundwater treatment.

The steady infiltration rates of the experimental soil were relatively low, varying from 4.9 to 7.0 mm/h for different water quality treatments. The most likely explanation of the small treatment effect is that infiltration in this soil is dominated by water entry via surface cracks. Soil analysis indicated that sufficient electrolyte was maintained in the matrix of the surface soil to prevent significant swelling and clay dispersion, even after many irrigations of channel water were applied.

Water balance estimates and changes in profile salinity indicated that the lucerne used significant quantities of water directly from the watertable, concentrating salt within the capillary fringe above the watertable to a maximum of 36 dS/m. A larger proportion of the water requirement appeared to be taken up directly from the watertable where saline irrigation water was also applied. This led to rapid profile salinisation and sodification from a combination of upward flux from the watertable and salt applied in the irrigation water.