Water Journal : Water Journal April 2012
catchment management water APRIL 2012 103 Fallout radionuclides (137Cs and 210Pb) have been widely used to determine the relative contribution of hillslope and channel erosion to stream sediments (Olley et al., 1993). As both fallout radionuclides are concentrated in the surface soil, sediments derived from hillslope erosion will have high concentrations of both nuclides, while sediment eroded from gullies or channels have little or no fallout nuclides present. By measuring the concentration in suspended sediments moving down the river, and comparing them with concentrations in sediments produced by the different erosion processes, the relative contributions of each process can be determined. Results from seven water supply catchments so far investigated are consistent with channel erosion being the dominant source of sediment (Table 1). Information from such analyses provide critical information for management and are key to the development of complex models to further interpret catchment processes. Models are excellent tools to take such information discussed above to further interpret catchment processes and scenario-test changes in the catchment. Models such as the Environmental Management Support System (EMSS) (Chiew et al., 2001) or more recently the E2/WaterCAST/Source Catchment Modelllng (Stewart, 2009) have had much success in identifying broad-scale regional processes; however, they lack the required predictive capability for fine- scale individual sub-catchment analysis. Seqwater has begun developing an empirically based predictive model to identify potential sources of sediment and subsequent sediment delivery throughout the landscape at the catchment outlet. This SEQ GeoDynamic model (based on the OzMUSLE approach) will in time facilitate identification of sediment- associated nutrients and other pollutants that are likely to be generated at the annual, seasonal and event temporal scales. Hence, strategies to reduce or completely alleviate the risks of sediments and associated contaminants in storages can be prepared and scenario-tested well ahead of such occurrences. An example of such applications is provided in Figure 4. Management Approach Under Seqwater's Asset Management Framework, assets include water treatment plants, weirs, dams, borefields, buffer land holdings and the greater catchment. This is despite the fact that, as previously discussed, Seqwater has limited ownership within the broader catchment. However, Seqwater recognised the value of the natural features of the catchments as assets and has adopted this philosophy into its strategic planning and policies. There is currently a stark imbalance between investment in natural assets and investment in built assets, driven primarily by a short-term investment strategy to consolidate processes and performance under the recently formed organisation. Seqwater has recognised that the current investment pattern over ensuing decades may lead to sub-optimal outcomes and unsustainable practices, and was not consistent with the adopted catchment- to-supply approach. To drive this process, Seqwater is developing a Catchment Investment Efficiency Measures (CIEM) Framework which aims to address a decision-making gap and provide an economic rationale for investment optimisation and integration between built and natural assets over five-, 10-, 20-, 30-, 50- and 100-year timeframes. Seqwater has committed to developing a set of metrics and tools that will provide the basis of an economic framework (cost optimisation model) that informs long-term comparative analysis of investment options in the catchments and their impact on future built asset treatment costs. Information from the aforementioned tools is necessary if Seqwater is to implement targeted catchment management strategies that are effective at reducing the loss of soil and nutrients from the land to waterways and to reduce pathogen loads where they may cause water quality problems. Furthermore, this information will provide the biophysical data on catchment processes required to develop accurate socio-economic spatial optimisation models, and aid in catchment investment decision making, which will more appropriately inform effective watershed management and investment strategies that will optimise the treatment capacity of the catchments in the mid- to long-term. Due to the limited ownership and enforcement powers that Seqwater has within the catchments, implementing identified catchment management strategies and improvements is also a major challenge. Seqwater is currently Figure 4. Scenarios of the spatial variation of soil erosion in a defined catchment with changing frequency and magnitude of rainfall and runoff events (1-in-1yr, 1-in-10yr and 1-in-50yr events) as modelled using OzMUSLE.
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