A computer based simulation model for evaluating farm scale irrigation development in hill and high country, New Zealand

Abstract

The basic objective of this study was to develop a simulation model that would enable physical and economic evaluation of alternative farm irrigation schemes in the hill and high country.

The model used the output of the engineering systems model, "HAKA" to provide a management and economic evaluation of each engineering option.

The proposal of the study was to build a farm scale grazing model capable of simulating most forms of herbage production, animal types and management systems employed in the hill and high country. The model had to incorporate weather induced variability and irrigation responses over a 25 year period.

The model required a framework that would allow further components to be added to it. Initially it was designed to investigate the Hakataramea but was flexible enough to study other areas with simple modification.

Comprehensive experimentation to demonstrate the model was limited to one farm in the Hakataramea valley which has invested in irrigation. Dryland experimentation consisted of feeding hay to ewes pre lambing and at tupping, growing lucerne hay for sale with reduced stock numbers and increasing stock numbers. Irrigation experimentation consisted of firstly simulating unlimited water supply to get an approximation of the property’s potential and then interacting with the "HAKA" engineering systems model. Two dam sizes irrigating two areas were evaluated plus the impact of three different sets of river diversion rules for filling the large size dam. The model indicated that irrigating the large area from the large dam was financially the best option. It reduced variation in feed supply -between years and halved the incidence of feed shortfalls even with increased stock numbers. In severe drought years dam filling was inadequate and severe financial penalties were incurred in feeding stock. This would appear to indicate that the unstated assumption, that all farmers have equal and necessary management skills, must be examined carefully if irrigation water is in limited supply.

The objective of this report has been to demonstrate the model constructed rather than the effectiveness or economics of hill and high country irrigation. If the results presented here are to be used as an indicator of the economics of irrigation then they can be considered to be at the worst end of the spectrum because variation in engineering costs was not considered, nor were farm management practises, and the farmers individual taxation position.