Addressing uncertainties in water accounting

A precursor to effective water management is the availability of useful information. Water accounts are being developed to meet these needs. However, there are considerable problems in quantifying all of the reported elements. Even where direct measurements are available, they are often prone to measurement uncertainties. The volumes of other elements are difficult to measure and often can only be quantified based on a series of assumptions. The presence of uncertainties in water accounts poses two problems. Firstly, the decisions made based on information presented in the accounts may change if the associated uncertainties were disclosed to those making decisions. Secondly, due to the uncertainties associated with each element, the accounts rarely balance. There has been surprisingly little research into understanding and reducing the uncertainty in water accounts, and at present, it is not systematically captured and reported in accounts. This paper addresses the uncertainties associated with water accounts by: • identifying and quantifying the major sources of uncertainty associated with each element in the water accounts; and, • presenting a method to constrain the uncertainty associated with each component of the water accounts by only including those values that can create a balanced set of accounts. The Werribee River catchment (Victoria, Australia) is used as a case study and accounts are prepared for 2005/06. The largest inflows to the catchment are from catchment runoff. Other sources of inflows include groundwater, recycled water, return flows and water supplied from an adjoining catchment. During 2005/06 the total inflows are estimated to be 28,002 ML. The uncertainty associated with these inflows is equivalent to ±21% of the best estimate, mainly due to the uncertainties associated with estimating ungauged catchment runoff. The outflows from the catchment consist of water use, net evaporation from reservoirs, surface and groundwater interactions and runoff into the bay. In 2005/06 the combined outflows were 40,552 ML. The associated uncertainty is equivalent to ±6% of the estimated outflows, and is predominantly due to the uncertainty in surface and groundwater interactions. Finally, during 2005/06 the drawdown from storage volumes across the catchment was 12,765 ML, with an uncertainty equivalent to ±13%. The initial water accounts (i.e. without consideration of uncertainty) for the Werribee River catchment did not balance. The combined inflows and drawdown in storage volume exceed the estimated outflows by 245 ML. This study identifies combinations of inflows and outflows, each selected from within their uncertainty range, that provide a balanced set of accounts. New estimates are adopted from the combination which has the highest likelihood of occurring. The combination with the maximum likelihood is selected and replaces the initial values used in the accounts. Finally, the uncertainty associated with each element of the accounts is reduced by excluding values that are unlikely to combine with other elements to produce a balanced set of accounts. Disclosure of the uncertainty associated with the information presented in the water accounts will improve water management decisions. Furthermore, the results highlight those elements in the water accounts that have the greatest influence on the overall uncertainty and should be the focus of further research.