Occurrence and measurement of salinity stratification in shallow groundwater in the Murrumbidgee Irrigation Area, south-eastern Australia

Abstract

Capillary upflow from shallow groundwater is a significant contributor of soil salinisation in irrigated areas and is highly dependent on water table position and salinity. Lysimeters have traditionally been used to quantify capillary upflow from shallow water tables and the potential salinisation risk. However, water table position and salinity are usually controlled to remain constant in lysimeter studies, while short-term variations in these parameters may exist under surface irrigated agriculture. Consequently, the extrapolation of calculated crop water uptake and potential soil salinisation processes from lysimeter studies to field conditions may be misleading in selected cases. A multilevel sampler was designed to collect water samples at 20 cm intervals to investigate changes in water table position and salinity stratification of a shallow fluctuating water table under furrow irrigated fields. Water samples were taken from the upper 20 cm of the shallow groundwater (the water table zone, WTZ) as it rose and fell following irrigation events at three farms in the Murrumbidgee Irrigation Area, in south-western New South Wales, Australia. The water table was within 2 m of the ground surface at all sites and irrigation events resulted in large fluctuations in water table position over the irrigation season. Salinity stratification of the shallow groundwater was found to exist with low salinity water overlying more saline groundwater and a zone of variable salinity extending up to 2 m. The salinity generally increased with depth. In addition to changes in shallow groundwater salinity with depth, salinity fluctuated at specific depths over the irrigation season. The salinity of the WTZ also varied with changes in water table position. Following an irrigation event, a rising water table is associated with lower salinity at the WTZ. At each depth interval sampled, the salinity increased between water table rise and fall. The water table position and salinity are important determinants of salinity risks since they are the foundation for capillary upflow. Consequently, short-term variations in the position and salinity of shallow water tables under field conditions will affect capillary upflow and salt movement into the root zone. Therefore, where variations in these parameters exist at the field scale, detailed measurements should be taken to accurately predict crop water uptake and assess potential soil salinisation hazards.