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

Tree growth, water use, climate, soil salinity and groundwater conditions w ere monitored over a 3-year period in an unirrigated 5-8-year-old plantatio n 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 ra pidly as Eucalyptus camaldulensis, in plots with initially equal mean diame ter. C. cunninghamiana had greater sapwood area but lower sap flux density, so that the daily and annual water use did not differ significantly betwee n species. Average single tree water use determined by the heat pulse metho d varied from less than 101 day(-1) in winter to over 301 day(-1) in summer . Stand water use averaged 0.9-1.0 mm day(-1) over 2 years, and was evident ly 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 t o sapwood area as 3815 m(2) m(-2). This ratio increased with sapwood area i n small trees. Hydraulic properties of the soil profile were inferred from measurements of moisture content and matric potential at 30 cm depth interv als to 390 cm, indicating the presence of a medium clay layer with heavy cl ay above and below. Salinity and chloride distribution were also determined , indicating slight-to-moderate salinity below 30 cm depth reaching a maxim um (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 wa ter balance model to predict the uptake of water from unsaturated soil and shallow groundwater based on measured daily water use and rainfall, and acc ounting for the influence of groundwater pumping. Modelled results indicate d 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 occur red annually, but discharge was predominant. Intensive monitoring of soil s alinity at 100-400 cm depth using buried sensors demonstrated substantial s easonal 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). T hese values cannot be reconciled with the observed water use and salinity b elow the maximum watertable depth, and may indicate lateral inflow of more saline perched groundwater within the medium clay layer. The results are ex pected to contribute toward the development and validation of models for pr edicting the sustainability and environmental benefit of tree plantations i n areas with shallow saline groundwater. (C) 1999 Elsevier Science B.V. All rights reserved.