The total catchment area is 11,378 km2, and can be divided into three basins (Figure 2): that of the Harvey River and associated drains which enter the southern end of Harvey Estuary; and those of the Serpentine and Murray Rivers which enter the eastern side of Peel Inlet.
The Harvey and Serpentine catchments are on sandy coastal plain soils cleared for grazing by cattle, sheep and horses. There are also potentially significant point sources of nutrient in these two basins, consisting of stock holding yards used in connection with live sheep export to the Middle East, and piggeries of which the largest, in the Serpentine Catchment, has some 20,000 animals. Both the Harvey and Serpentine rivers have been dammed, and their truncated catchments consist only, of the coastal plain sections of what were larger catchments with forested uplands. Water from the Harvey Reservoir is used in part for channel irrigation on the coastal plain, but excess water from this irrigated land passes directly to the ocean through a drainage canal and does not enter Harvey Estuary. Despite the construction of the dams, it is estimated that the total volume of water entering the estuary from the Harvey River has not been greatly affected by dam construction; what was lost from the upper catchment has been approximately replaced by flow, from agricultural drains on the coastal plain.1
The large catchment of the Murray River (the main channel of which is undammed) ranges from high to low rainfall zones. The higher rainfall area is covered by Eucalyptus forest, mainly Eucalyptus marginata (jarrah) and Eucalyptus calophylla (marri). This forested catchment has a low water yield by world standards (less than 10% of precipitation may, be accounted for by streamflow, but the water is of high quality and rainfall is relatively reliable. Further inland the catchments have been cleared. and this leads to increased salinity (up to 3 ppt is recorded during peak flow of the Murray River where it crosses the escarpment as well as increased nutrient loads. The clearing has taken place for orchards, wheat and sheep farming, depending approximately on rainfall. The lower rainfall sections of the catchment have less reliable rainfall than do the catchments further west.
River flow is highly seasonal, more than two thirds in the 5 months of June to October, often with half of the flow in July and August; on the other hand there is little or no flow from December to April inclusive. There is also large year-to-year variation; over the period 1977-1986 mean streamflow from the Murray River was 595 m3 x 106 with a standard deviation of 185 (71% of the mean). Over the same period, flow from the Harvey catchment, which is nearer to the coast and with more reliable rainfall, had a mean flow of 221 m3 x 106 with a standard deviation 35% of the mean. The truncated catchments of the Serpentine and Harvey rivers show rapid changes in hydrographs in response to individual winter storm events.
Forested subcatchments in the jarrah forest are very effective at trapping nutrients, and the loads of nitrogen and phosphorus leaving microcatchments can be less than those which arrive in rainfall. The clearing of forested subcatchments in the Murray River basin has been accompanied by increasing concentrations of nitrogen in streamflow; there is a linear relationship between flow-weighted mean nitrogen concentration and percentage of catchment cleared, (for a range of land uses, including orchards, sheep farms and even a piggery). Most of the increased nitrogen in streamflow can be attributed to increased fertiliser usage and the fixation of atmospheric nitrogen by pasture legumes. Much of the nitrogen in the Murray River is in the form of nitrate, which can reach a concentration of 6 mg L-1. The situation is very different for phosphorus, which is retained in the gravely and loamy soils of the upper Murray catchments.
In contrast, soils of the coastal plain show little ability to retain phosphorus. The natural soils are highly leached and phosphorus deficient, and the native vegetation is adapted to retain and recycle what little is available of this scarce resource. The establishment of agriculture has only been possible through the use of phosphatic and other fertilisers and the development of strains of legumes efficient in nitrogen fixation under the particular climatic conditions of the region. The use of phosphatic fertilisers on soils with a low phosphorus-retaining capacity has led to increases in phosphorus in streamflow. The increase has been exacerbated because it is the practice to spread superphosphate onto paddocks before the winter rains, when the pastures are dry and plants dormant; it is not convenient to spread the fertiliser later, because machinery is readily bogged down in the wet soil. Thus the rains occur when there is little capacity for plants to retain phosphate, and it is estimated from experimental catchments on pasture that some 30% of phosphorus lost from catchments on deep grey sands and 15% of the loss from sands over clays derive directly from fertiliser application, because of poor retention of the high content of water soluble phosphorus in superphosphate.1,7
Mean flows and nutrient loads from the three catchments are presented in Table 1. While there are large differences between years, because of variability in rainfall (especially on the inland section of the Murray Catchment), in an "average" year 43% of the flow is contributed by the Murray, but only 13%, of the phosphorus; in contrast the Harvey and its associated drains contribute 36% of the flow but 57% of the phosphorus.8,9. In years when there is little flow, from the Murray River, the Harvey may contribute up to 75% of the phosphorus. The Serpentine Catchment, which has a much lower drainage density than the Harvey, is somewhat intermediate in behaviour. For these reasons attempts to control phosphorus concentrations in drainage have concentrated on the sandy, coastal-plain catchments.