Analysing the risk of climate change using an irrigation demand model

Due to the high degree of uncertainty accompanying projections of greenhouse-induced climate change, specific impacts cannot be predicted with any accuracy. At best, a range of projected climate change bounded by its high and low extremes can be used to produce a range of impacts, results that are often too broad to be of practical use in planning for adaptation and mitigation. However, by addressing outcomes in the initial stages of an impact assessment through the construction of user-defined thresholds, it is possible to identify outcomes that should either be avoided, in the case of a negative impact, or aimed for, in the case of a positive impact. By quantifying these thresholds as functions of key climatic variables, and creating projections for these variables that take account of a comprehensive range of quantifiable uncertainties, the risk of threshold exceedance can be analysed. This information can then be used in a risk assessment to identify windows for adaptation, describing the timing and degree of adaptation needed to prevent 'dangerous' climate change occurring for a particular activity. This procedure is illustrated through the use of an irrigation demand model for perennial pasture, based on data collected from a farm in northern Victoria, Australia. Seasonal water-use is used to estimate an annual farm cap of 12 M1 ha(-1) based on the annual water right. The exceedance of this farm cap in 50% of years is taken to represent a critical threshold beyond which the farmer cannot adapt. The method of risk analysis utilises projected ranges of regional rainfall and temperature change, combined with a sensitivity analysis, to construct risk response surfaces. Monte Carlo sampling is used to scale 100 yr of weather-generated data to calculate the probability of the annual farm cap being exceeded across ranges of temperature and rainfall change projected at 10 yr intervals from 2000 to 2100. Based on the model projections of changing water demand, some degree of adaptation is indicated by 2030, although the theoretical critical threshold is not approached until 2050. This procedure represents a considerable advance in 'bottom up' studies where the impact on a specific activity is being addressed. It provides a basis for the planning of adaptation measures and can potentially contribute to the assessment of dangerous climate change as required by the UN Framework Convention for Climate Change.