Modelling the economic trade-offs of irrigation pipeline investments for improved energy management
The main objective of this research is to develop an integrated non-linear programming model that unifies the interrelated linkages between mainline pipe diameter choice and the timing of irrigation events in conjunction with electricity tariff choice to facilitate better evaluation of the economic trade-offs of irrigation pipe investments for improved energy management. The Soil Water Irrigation Planning and Energy Management (SWIP-E) programming model was developed to address the main objective of the research. The model includes an irrigation mainline design component, soil water budget calculations and an energy accounting component to model the interaction between irrigation system design, irrigation management and time-of-use electricity tariff structures. The SWIP-E model was applied in Douglas to evaluate the impact of different electricity tariff structures and irrigation system designs on the optimal pipe diameter of an irrigation mainline, electricity costs and profitability. The results showed that Ruraflex is more profitable than Landrate which is a direct result of higher electricity costs associated with Landrate. The large center pivot resulted in higher net present values than the smaller center pivot and the lower delivery capacities were more profitable than higher delivery capacities. More intense management is necessary for delivery capacities lower than 12 mm/day to minimise irrigation during peak timeslots. Variable electricity costs are highly dependent on the interaction between kilowatt requirement and irrigation hours. For the large center pivot the interaction is dominated by changes in kilowatt whereas the effect of irrigation hours in relation to kilowatts is more important for smaller pivots. Landrate with relatively higher electricity tariff charges resulted in a change in the optimal pipe diameter at lower delivery capacities compared to Ruraflex. Optimal pipe diameters will increase for a breakeven percentage of between 0.6% and 0.66% for Ruraflex and between 0.4% and 0.6% for Landrate which is much lower than the design norm of 1.5%. The overall conclusion is that the SWIP-E model was successful in modelling the complex interrelated relationships between irrigation system design, management and electricity tariff choice that influence the trade-off between main pipeline investment decisions and the resulting operating costs. Electricity tariff choice has a significant impact on the results which suggest that economic principles are important and that it should be included in the design process. A shortcoming of the model is that the risk of lower irrigation system delivery capacities was not included in the model. The conclusion that lower delivery capacities are more profitable should therefore be interpreted with care. The low breakeven friction percentages optimised in this research suggest that the norm of 1.5% friction is too high and a lower norm should be considered. Future research should focus on extending the model to include a combination of irrigation systems and the inclusion of risk to evaluate the risk associated with low irrigation delivery capacities in combination with load shedding.