Water use and salt accumulation by Eucalyptus camaldulensis and Casuarina cunninghamiana on a site with shallow saline groundwater

Abstract

Tree growth, water use, climate, soil salinity and groundwater conditions were monitored over a 3-year period in an unirrigated 5–8-year-old plantation in northern Victoria, within an area where groundwater pumping is in use for control of shallow watertables and salinisation of irrigated farmland. Diameter and sapwood area of Casuarina cunninghamiana increased twice as rapidly as Eucalyptus camaldulensis, in plots with initially equal mean diameter. C. cunninghamiana had greater sapwood area but lower sap flux density, so that the daily and annual water use did not differ significantly between species. Average single tree water use determined by the heat pulse method varied from less than 10 l day −1 in winter to over 30 l day −1 in summer. Stand water use averaged 0.9–1.0 mm day −1 over 2 years, and was evidently limited by soil water availability. The leaf area index (LAI) of the E. camaldulensis stand was estimated as 2.07 and the mean ratio of leaf area to sapwood area as 3815 m 2 m −2. This ratio increased with sapwood area in small trees. Hydraulic properties of the soil profile were inferred from measurements of moisture content and matric potential at 30 cm depth intervals to 390 cm, indicating the presence of a medium clay layer with heavy clay above and below. Salinity and chloride distribution were also determined, indicating slight-to-moderate salinity below 30 cm depth reaching a maximum (saturation extract electrical conductivity 8 dS m −1) at around 200 cm. Saturated hydraulic conductivity of the heavy clay was estimated as 3 mm day −1. Daily watertable depth observations were used to parameterise a water balance model to predict the uptake of water from unsaturated soil and shallow groundwater based on measured daily water use and rainfall, and accounting for the influence of groundwater pumping. Modelled results indicated that more than half the tree water uptake (170–220 mm year −1) was drawn from the groundwater. Both recharge and discharge of the groundwater occurred annually, but discharge was predominant. Intensive monitoring of soil salinity at 100–400 cm depth using buried sensors demonstrated substantial seasonal variation, apparently related to watertable movements. The average soil solution salinity rose by over 5 dS m −1 in 32 months of monitoring, estimated to correspond to chloride accumulation of more than 4 kg m −2. These values cannot be reconciled with the observed water use and salinity below the maximum watertable depth, and may indicate lateral inflow of more saline perched groundwater within the medium clay layer. The results are expected to contribute toward the development and validation of models for predicting the sustainability and environmental benefit of tree plantations in areas with shallow saline groundwater.