Estimating unmetered stock and domestic water use

Water accounts present information on the stores and movement of water, including the volume of water diverted for consumptive use. Water extracted directly by water users to meet their stock and domestic water requirements is not metered in Australia and as a result, estimates are used to quantify this component in the water accounts and for other purposes. Two approaches to estimate the water extracted for stock and domestic purposes are identified: 1. An estimate of the number of households and stock that rely on self extracted water is multiplied by their expected water requirements. 2. The number and volume of stock and domestic licenses issued by the water authorities to divert water from waterways or groundwater is combined with information on the use of farm dams to estimate the stock and domestic water use. This paper compares directly estimates of stock and domestic water use generated by each of these two approaches for the Werribee River catchment (Victoria, Australia) for 2005/06. The two approaches are not expected to generate identical estimates due to the uncertainties associated with each of the estimates. Therefore, the uncertainty surrounding each of the estimates is also quantified and represented with a probability distribution. Ideally these two probability distributions would overlap and thereby show that the two approaches provide consistent results. Data collected by the Australian Bureau of Statistics is used to estimate the number of households (2006 Population census) and stock (2005-06 Agricultural Census) in the catchment. These data are reported by the ABS according to statistical boundaries that do not correspond to the Werribee River catchment, and so had to be adjusted for use in the study, which provides an additional level of uncertainty. The uncertainty associated with expected water requirements of individual households and stock is also considered. The best estimate of the total stock and domestic water use using this first approach is 690 ML, and the 95% confidence interval is equivalent to ±63% of the best estimate. The source of uncertainty that has the largest influence on the results is the number of stock in the catchment, followed by the estimate of the expected water use per head of stock. Using the second approach, extractions from waterways and groundwater sources are assumed to be related to license conditions and be within the limits set by these conditions. As extractions from farm dams are not licensed, estimates are based on simulation modelling. The estimate of stock and domestic water use generated by this second approach is higher, and is 810 ML. The 95% confidence interval surrounding this estimate is equivalent to ±55% of the best estimate. The uncertainty surrounding the volume of groundwater extractions has the largest influence on the overall uncertainty. The estimates of stock and domestic water use generated by the two approaches diverge by 119 ML (or 15% of the upper value), but there is substantial overlap of the 95% confidence intervals associated with each estimate. Which is the better estimate of D&S water use is unclear as the uncertainty surrounding each of the two estimates is similar. As the data needed for the first approach are available for all of Australia on an on- going basis from the ABS, then the demand approach is probably the most practical approach at present for deriving standardised and comparable estimates of domestic and stock water use across Australia.