Use of a simple plantation productivity model to study the profitability of irrigated Eucalyptus globulus
Sands, PJ and Rawlins, W and Battaglia, M, Use of a simple plantation productivity model to study the profitability of irrigated Eucalyptus globulus, Ecological modelling, 117, (1) pp. 125-141. ISSN 0304-3800 (1999) [Refereed Article]
There is considerable interest in growing eucalypt species under irrigation in the Semiarid Murray-Darling basin of New South Wales, Australia. The motivation for this stems from a desire to increase the Australian plantation estate, in part for commercial forestry and in part as a net sink of CO2, and as a possible tool to lower the water table in regions prone to salinity. Thus, the plan to increase the plantation estate is driven by ecological concerns. PROMOD, a simple model for predicting plantation productivity following canopy closure, was used to examine the economics of growing Eucalyptus globulus under irrigation at Deniliquin, a site with mean annual rainfall 385 mm and mean total pan evaporation 1765 mm. It was assumed that nutrients were not limiting. Productivity increases with the total annual irrigation I (M1 ha-1 year-1) and attains a maximum at some I determined by frequency of application. Maximum productivity (42 m3 ha-1 year-1) requires I ≥ 12 M1 ha-1 year-1 as weekly applications in amounts proportional to the current pan evaporation. Profitability was defined as the annual increase in wood value due to volume growth less irrigation costs, and was used to rank irrigation strategies. In general, profitability has a well-defined maximum and depends on frequency of application. Maximum profitability ($640 ha-1 year-1) requires I ≃ 12 M1 ha-1 year-1 as weekly applications in proportion to pan evaporation. Reduction of I or of frequency of application results in a major loss of productivity and profitability. Output from PROMOD was used as the basis for a more detailed discounted cash-flow analysis of a sawlog regime which confirms the better economics of high total-irrigation, high irrigation-frequency options. Available soil water capacity does not affect the total annual irrigation required for maximum profitability, but shallow soils do require more frequent, smaller applications of water. Reducing irrigation efficiency (which takes into account losses inherent in the irrigation method) from 1 to 0.5, doubles the total irrigation required for maximum profitability and reduces profitability by 70%. The results of this study have major implications. Serious consideration should be given to frequent irrigation of plantations in dry regions. The establishment and analysis of irrigated trials in regions of high VPD will have important benefits in understanding tree growth under such conditions and will provide data for validation of models under the extreme conditions that prevail in regions which might be economically exploited in the future.