Modelling the water balance of effluent-irrigated trees

irrigation of effluent is an increasingly popular treatment option due to c oncern about nutrient additions to rivers and coastal waters. Since some st udies 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 di fferent soils, climates, and crops. We present the development of the water balance part of such a model, APSIM for Effluent, and carry out a comparis on against data obtained from an effluent-irrigated plantation of Eucalyptu s 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 experi ment, modelled drainage lay above that estimated by a water balance techniq ue, which was known a priori to underestimate drainage, and was close to th at estimated by the chloride mass balance technique. Simulated chloride acc umulated in the soil was within the scatter of the observations, although i t was consistently at the lower end of the range of the data. There was goo d agreement between the model predictions and measured chloride concentrati on 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 l ost by the various evaporative routes, and 1080 mm was lost by deep drainag e. (C) 1999 Elsevier Science B.V. All rights reserved.