The purpose for undertaking a sensitivity analysis is to identify whether or not the model results change, in response to the uncertainty associated with key model parameters. The results from the sensitivity analysis are presented in Table 5.11. The model is insensitive to the uncertainty associated with fruit energy content, energy content in diesel, fruit yield, the embodied energy in organic materials and CO2 sequestered per individual kiwifruit vine.
Sensitivity analyses suggest that the model results will change, only if the fruit energy content lowers from 2.5 to 1.7 MJ/kg. This result means that insofar as the fruit energy content remains above 1.7 MJ/kg, energy use of the model organic kiwifruit will be efficient.
The growers expressed the possibility of variation in yield up to 5 t/ha/yr. Sensitivity analyses suggests that the model results will change only when the yield
reduces by 7.5 t/ha/yr or more. Thus, a variation in yield by 5 t/ha/yr will not have any effect on the model results.
The embodied energy in organic materials was assumed to be 5 MJ/kg in this research, following Helsel (1993). Sensitivity analyses suggest that the model response changes only when the embodied energy content of the organic materials increases from 5 to 60 MJ/kg.
The energy content in diesel is considered to be 46.7 MJ/L, following Wells (2001). The model response changes only when the energy content in diesel increases to 145 MJ/L. Insofar as the energy content in diesel does not increase from 46.7 to 145 MJ/L or above, the results remain the same.
An average kiwifruit systems sequesters about 13.7 t CO2/ha/yr (Greer et al., 2004). Sensitivity analyses suggests that the system is carbon neutral only when the sequestration of the individual kiwifruit vine is as low as 15 t CO2/ha/yr (which is 7.5 t CO2/ha/yr with 500 vine/ha). Hence, insofar as the CO2 sequestered in an individual vine does not lower to 15 kg, the model results will not change.
The only parameter, to which the model results are sensitive, is the variation in CO2 emission coefficient from the decomposition of organic matter. This parameter value was taken from Grogan and Matthews (2002) and it was 82% relative to the soil carbon input. If this parameter value increases from 82% to 95%, then the CO2 ratio lowers below one and the organic orchard system is not a sink of CO2. This means that insofar as less than 95% of the carbon that enters the orchard soil is lost, the organic orchard system remains CO2 sink.
Table 5.11 Model responses to variation in parameters in the model organic kiwifruit system
Model parameter Original value* New value to change the model response** Uncertainty or the range in variation % variation from the original value Sensitive/ insensitive Fruit energy content MJ/kg 2.5 1.7 - 32 insensitive Yield*** t/ha/yr 21 13. 5 5 t/ha/yr 36 Insensitive Embodied energy in organic materials MJ/kg 5 60 - 110 0 insensitive Energy content in diesel MJ/kg 46.7 14 5 ±15% 210 insensitive CO2 sequestered per kiwifruit vine kg/yr 27.4 6 15 - 45 insensitive CO2 emission coefficient from soil organic matter decomposition % 82 95 - 16 sensitive
* The value taken from published literature, as described in the methodology chapter ** Hypothetical value of the respective parameter to change the sustainability result *** The growers estimate an average variation in yield of 5 t/ha/yr
- Uncertainty unknown
5.6 Summary
In this chapter, a sustainability assessment of the organic kiwifruit system was undertaken. The first step was to derive a model organic kiwifruit system, which was typical of the studied orchards. This description of the model organic kiwifruit system was then used in conjunction with the data from the published literature, in order to estimate sustainability indicators using computer modelling tools. Management scenario analysis and sensitivity analysis were carried out, in order to
see the effect of variation in management practices and model parameters on sustainability indicators, respectively.
In summary, the sustainability indicators suggested that the model organic kiwifruit system was efficient in the conversion of input energy into fruit energy output. All the CO2-equivalent emissions were offset, without any release into the atmosphere and hence the model organic kiwifruit system was a net sink of CO2. The amount of carbon sequestered to the orchard soil, through the application of prunings and compost, was 846 kg/ha/yr. The nutrient balances were positive. However, N- leaching level from the orchard system could be a potential threat of eutrophication. Orchard understory was a significant factor to cause N surplus and N-leaching. However, maintaining the orchard without this understory meant that N was not adequately supplied to meet crop requirements.
The orchard, in which irrigation and frost fighting were used, lowered the energy ratio and CO2 ratio but it managed to remain energy efficient and a net sink of CO2. The CO2 ratio was higher when compost was prepared in the orchard. However, the amount of carbon added to the system was lower than when the compost was purchased. The sensitivity analysis suggested that the model was insensitive to the uncertainties, in most of the parameters under investigation, except for the CO2- emission coefficient from the decomposition of soil organic matter.