This research has shown that the profitability of both agriculture on land and aquaculture in the estuary is affected by changing freshwater flows. To assess the value of water to different users across a catchment we developed a generic water accounting framework and populated it with available data from the Little Swanport catchment as an example. We also developed an estuarine ecosystem model which we used along with field observations and nutrient budgeting to assess the value of freshwater flows to oyster production in the estuary.

During this study the catchment moved into a severe drought. This necessitated some revision to our research methods and we used the drought conditions to estimate the value of water to the different users across the catchment from the loss in production during drought years compared to normal rainfall years. This provided estimates of the economic value of water at two extreme points on a continuum.

Across the catchment the loss of income from wool production, fat lamb sales and beef production when rainfall was approximately 60% of a normal year was estimated to be $3,36 million, or approximately one-third of its normal state (cash crops were not included as there were insufficient data). This value was determined from the sum of preventative expenditure, replacement costs and loss of production incurred due to the drought. In the estuary the nutrient budget and ecosystem model predicted that the drought years of 2006 and 2007 would have led to a decrease in the nutrients in the estuary, and a subsequent decline in the productivity of phytoplankton, oysters and benthic microalgae. By comparison, in the two wet years (2004 and 2005) nitrogen budgeting indicated that the increase in oyster harvest was ~43 kg N or a 12% increase relative to the drought years 2006-07. This equated to a loss of approximately $500,000 in a severe drought year.

The loss in production in the estuary during the drought was largely due to a lowering of the growth rate of the oysters, and as a consequence they took longer to reach market size and condition. On land, however, many farmers were forced to destock and only keep essential breeding animals. Crops either failed or produced less than normal and were not sown due to lack of water storage. Thus, the recovery time after the drought is likely to be greater on agricultural farms, taking several years to improve grazing land and to restock, whereas in the estuary the recovery time is in the order of months. Recovery time also depends on the stocking density before the drought and whether the farmers were stocked to full capacity for good growing conditions or whether they maintained a lower stocking level which would provide a buffer during droughts. 

In relation to environmental flows to the estuary, it is important to note that maintaining the low flows is most important. Ecosystem model simulations at different levels of base flows predicted that phytoplankton biomass, and consequently oyster growth, initially increases rapidly with base flow before the rate of increase slows to a steadier rate at higher flows. Therefore, there are greater benefits to the estuary per ML of river flow at low flow than at high flows. At low river flows primary producers have more time to take up the additional nutrient inputs from the river because the time to pass through the estuary is longer. In contrast, at higher flows, there is less time for biological uptake as the flushing time is shorter, and so the benefits are smaller per ML of river flow. The results of this study therefore support the cease to take requirements for low flows in the Water Management Plan for the catchment. However, the modelling predicted that the greatest benefits from river flow are achieved over the summer months because higher water temperatures significantly enhance the growth rates of phytoplankton and oysters. 

An assessment of the implications of increased water that could be allocated for stock, domestic and irrigation purposes in the Water Management Plan (2006) from 3882 to 6084 ML per year was shown by modelling to be unlikely to have a significant impact on the estuary for average and dry years, but in very dry years, as recently experienced in 2007, there was a detectable effect of the full allocation, most notably in summer. However, given the uncertainty inherent in model simulations, the result should be treated with caution. The important message is that harvesting water during a very dry year is more likely to affect the estuary, especially during summer. 

Although this research has centred on the Little Swanport catchment, the techniques developed are of relevance to many catchments across southern Australia. The biogeochemical model can be applied in other estuaries where there are sufficient local data, particularly on hydrodynamics. The nutrient budget process can also be used in other estuaries with relevant local nutrient data available. The water evaluation framework developed for the catchment provides a generic template for catchments to assess the value of water to different users across a catchment. Data requirements, survey methods and types of analyses, along with likely issues and potential difficulties to water accounting are discussed. 

Keywords: Water management, catchments, environmental flows, estuarine health, oyster aquaculture