Modelling the water balance of effluent-irrigated trees

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Abstract

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.

Introduction

Concern about the health of inland rivers, coastal waters and groundwater is increasing. In Australia, of particular concern is the degree of eutrophication and the occurrence of toxic blue–green algal blooms in the River Murray and other surface water bodies in the Murray–Darling Basin. Gutteridge, Haskins and Davey Pty. Ltd (1991) found that the major point sources of nitrogen and phosphorus in the Murray were from the discharge of effluents from sewage treatment plants. As a result there is increasing pressure to apply treated sewage effluent onto land.

Irrigated tree plantations are an increasingly popular alternative to irrigation of agricultural land and recreation areas as a method of land treatment of effluent. In 1991 there were approximately 50 operational effluent-irrigated plantations (Stewart and Boardman, 1991). Myers et al. (1995) estimated that another 100 plantations were established between 1991 and 1995. Trees have been popular because of reported high growth and high water use rates (Stewart and Flinn, 1984; Stewart et al., 1988), as well as socio-economic reasons (Myers et al., 1994). Young, rapidly growing plantations accumulate nitrogen at a high rate. Once canopy closure is attained, however, the rate of accumulation by the trees decreases (Polglase et al., 1995), possibly leading to nitrogen contamination of groundwater. Sewage effluent is also relatively high in salt (Feigin et al., 1991; Bond et al., 1995) which may lead to unacceptable leaching of salt to groundwater. Furthermore, the need to control salt accumulation in the trees' root zone can exacerbate nitrate leaching.

Dillon et al. (1993) identified that a model useful for assessing the design of effluent irrigation systems must be responsive to a range of different conditions of soil, effluent, climate, and crops. They also indicated that such a model would have to be capable of extrapolating from limited data at a few sites to a range of unknown sites. Needs such as these lead to debates about the nature of a suitable model, including model complexity and parameter requirements (e.g. Hauhs, 1990; Beven, 1993). While simpler models may require fewer inputs and parameters, they may not be sufficiently comprehensive to express the degree of variation found under a range of conditions. Such debates generally lead to the conclusion that if the aim of the modelling is to investigate complex interactions between processes under different soils, climates, and management, comprehensive physically based models are most appropriate. By physically based, we mean models that use primary parameters or inputs that can be measured, rather than those which employ regression-based or lumped parameters.

We present the development and testing of APSIM for Effluent, a largely physically based model composed of several modules suitable for the design of effluent irrigation systems. Although the simulation environment allows the use of any of the several agriculturally important crop modules, currently the crop module of APSIM for Effluent is for Eucalyptus grandis and all testing has been done against data from an E. grandis plantation.

Section snippets

Simulation software

All computer modelling was done within the Agricultural Productions Systems Simulator (APSIM) (McCown et al., 1996). APSIM is a simulation environment that can be configured with modules suitable for the simulation of many different agricultural production systems. APSIM (Snow et al., 1997) is a particular configuration of APSIM, which includes the modules required for the simulation of effluent-irrigated plantations. The modules of importance to the water balance simulations are described in

Site, soil and experimental design

Experimental work was done on an effluent-irrigated plantation near Wagga Wagga, New South Wales, Australia. Rainfall, measured at the airport 2 km away from the experimental site, shows the long-term average to be 570 mm year−1, with a slight winter dominance. Mean annual pan evaporation is 1860 mm and mean daily temperature ranges from 3°C in mid-winter to 31°C in mid-summer.

Myers et al. (1996) and Falkiner and Smith (1997) have described in detail the site and experimental design of the Wagga

Results

Although some data were collected on all plots, the southern medium plot had more information available than any of the others. It was the only plot for which both heat pulse data were available and plot water balances were constructed. Therefore, before presenting data and simulations of all the treatments, simulations for the southern medium plot will be examined in detail.

Discussion

The comparison between the above data and simulation has been done with a relatively small amount of site-specific information and without resort to calibration. Work in progress aims to further reduce the site-specific information required, and therefore to enhance the ease of use of the model for truly predictive purposes. For example, Myers et al. (1996) found a relationship between tree growth, annual pan evaporation, and vapour pressure deficit across several irrigated and fertilised tree

Conclusions

Accurate prediction of water and solute movement through effluent-irrigated plantations is an essential pre-requisite to the development of design and operation guidelines for such plantations. The comparison of data and simulation here constitute the initial stages of testing for such a predictive model. Although all comparisons here are against data from effluent-irrigated trees, APSIM for Effluent is not limited to simulation of trees as the APSIM suite includes modules capable of simulating

Acknowledgements

The authors wish to thank Dr Randall Falkiner (CSIRO Forestry and Forest Products) for the provision of effluent chemistry and soil survey data. The software for the modelling environment and some of the modules used in this study were developed by the Agricultural Production Systems Research Unit, a collaboration of CSIRO Tropical Agriculture and Queensland Departments of Primary Industries and Natural Resources. We wish to thank Mr John Hargreaves (CSIRO Tropical Agriculture) for assistance

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