S. Theiveyanathan

Impacts of tree plantations on groundwater in south-eastern Australia

In some regions dependent on groundwater, such as the lower south-east of South Australia in the Green Triangle, deep-rooted, woody vegetation might have undesirable hydrological impacts by competing for finite, good-quality groundwater resources. In other regions, such as the Riverina in south-central New South Wales, where rising watertables and associated salinisation is threatening the viability of agriculture, woody vegetation might have beneficial hydrological impacts. In response to a growing need to better understand the impacts of tree plantations on groundwater, annual evapotranspiration and transpiration were measured at 21 plantation sites in the Green Triangle and the Riverina. Sources of tree water uptake from rainfall and groundwater were determined by measurements of evapotranspiration and soil water over periods of 2-5 years. In the Green Triangle, under a combination of permeable soil over groundwater of low salinity (<2000 mg L −1 ) at 6-m depth or less, in a highly transmissive aquifer, annual evapotranspiration at eight research sites in Pinus radiata D.Don and Eucalyptus globulus Labill. plantations averaged 1090 mm year −1 (range 847-1343 mm year −1 ), compared with mean annual precipitation of 630 mm year −1 . These plantation sites used groundwater at a mean annual rate of 435 mm year −1 (range 108-670 mm year −1 ). At eight other plantation sites that had greater depth to the watertable or a root- impeding layer, annual evapotranspiration was equal to, or slightly less than, annual rainfall (mean 623 mm year −1 , range 540-795 mm year −1 ). In the Riverina, where groundwater was always present within 3 m of the surface, Eucalyptus grandis Hill ex Maiden trees at three sites with medium or heavy clay, alkaline, sodic, saline subsoils used little or no groundwater, whereas E. grandis and Corymbia maculata (Hook.) K.D.Hill and L.A.S.Johnson trees at a site with a neutral sandy soil and groundwater of low salinity used 380 and 730 mm year −1 of groundwater (respectively 41 and 53% of total annual evapotranspiration). We conclude that commonly grown Eucalyptus species and P. radiata are able to use groundwater under a combination of light- or medium-textured soil and shallow depth to a low-salinity watertable.

Modelling the water balance of effluent-irrigated trees

Irrigation of effluent is an increasingly popular treatment option due to concern about nutrient additions to rivers and coastal waters. Since some studies have shown that irrigation with waste water can lead to contamination of groundwater resources, there is need for a model to predict the fate of irrigated water, salt, and nitrogen that can be applied to a variety of different soils, climates, and crops. We present the development of the water balance part of such a model, APSIM for Effluent, and carry out a comparison against data obtained from an effluent-irrigated plantation of Eucalyptus grandis. Over 10 months, modelled tree water use was within 1.5% of that obtained by sap-flux measurements. When compared over 5 years of the experiment, modelled drainage lay above that estimated by a water balance technique, which was known a priori to underestimate drainage, and was close to that estimated by the chloride mass balance technique. Simulated chloride accumulated in the soil was within the scatter of the observations, although it was consistently at the lower end of the range of the data. There was good agreement between the model predictions and measured chloride concentration distribution with depth in the soil. A considerable amount of water was lost as deep drainage, even for the treatment that aimed to add only enough effluent to replace that lost by evaporation. During 5 years, of the 3370 mm rainfall and 4480 mm effluent received by that treatment, 6710 mm was lost by the various evaporative routes, and 1080 mm was lost by deep drainage.

Models for Estimating Evapotranspiration of Irrigated Eucalypt Plantations

Evapotranspiration, a major component of water balance and net primary productivity in plant-based terrestrial production systems at local and regional scale, is difficult to measure. In order to better understand tree growth and water-use relationships, and to design plantations and optimize their irrigation schedules, it is important to estimate the climatically induced evapotranspiration demand of tree crops. This demand, considered as the maximum evapotranspiration (ETm), is regulated by the resistances imposed by canopy surfaces during the process of evapotranspiration. This chapter describes several simple methods that have been proposed previously to estimate ETm and compares various process-based estimates of ETm with water-use rates determined from a water balance study. The observations from the study conducted at Forest Hill near Wagga Wagga, NSW, Australia, show that ETm can be estimated from standard meteorological parameters as a one-step approach using the Penman-Monteith equation. In the absence of required climatic data, ETm can be estimated from the radiation using Priestley-Taylor technique. For irrigation scheduling, however, ETm may be estimated from pan evaporation data using an estimated pan factor. This factor is site specific and varies with the season and the age of the plantations. For purposes of design and scheduling of irrigation, monthly pan factors can also be determined from climatic data using the Penman-Monteith equation.