A portion of the Gambier Plain underlain by an unconfined aquifer with readily definable hydrologic boundaries has been divided into a number of areas within which soil types have similar hydrologic properties. Mean annual recharge has been estimated for each area using both the tritium concentration and the chloride concentration of water within the soil profile. Good agreement was obtained between the two methods with local recharge varying between 50 and 250 mm year-1. Total mean annual recharge for the area has been estimated to be 2.4 ± 0.3 x 108 m3 year-1, and this compares favourably with an estimated discharge of 2.5 ± 0.3 x 108 m3 year-1.

The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer

Mallee, a shrub-eucalypt association, once covered large areas of the cereal growing land in Western Australia. The hydrologic consequences of land development have been more than a doubling of water yield and increased deep drainage beyond the plant-root zone. The latter has led to large areas of soil salinization.



The uncleared catchment studied was covered with mallee vegetation (65%) and a heath association (35%). Over 12 years of measurement, runoff from the catchment has averaged 0.025 mm per annum with a mean annual rainfall of 376 mm. Half of the runoff resulted from two major events totalling 0.15 mm. Since there was no evidence of groundwater accession in the catchment, the rainfall was balanced by evapotranspiration. Despite virtually no runoff from the whole catchment, considerable internal redistribution of surface water occurred, with localized overland flow in some areas as high as 7.7 mm from 30.9 mm of rainfall in one day.



Rainfall penetrated rapidly under the mallee vegetation and the rate of penetration observed could not be achieved solely through the soil even if there was saturated flow.



A typical stand of native mallee trees (Eucalyptus pileata and E. eremophila) in the catchment effectively redistributed 8% of the annual rain falling on the stand with 3% lost as interception and 5% going to stemflow. However, on an individual tree basis some 15% of the rain falling on the canopy was lost as intercepted water and 25% ran down the stem. The stemflow caused saturated conditions around the bole of the mallee and dye tracing showed that the water penetrated the soil via the annular pathways of the soil-root interface.



Roots of mallee trees were found at 28 m depth and it is postulated that the mallees are adapted to the semi-arid environment by virtue of their ability to store water deep in the soil profile for use during the dry summer months.

The fate of rainfall in a mallee and heath vegetated catchment in southern Western Australia

The southern Australian landscape is characterised by large climatic variability and an old, weathered surface with generally low hydraulic gradients. Many of these landscapes are prone to salinisation. The original ecosystems have strategies to cope with this environment in ways which minimise runoff and groundwater recharge. Replacement of these ecosystems with conventional agriculture has resulted in large-scale disruption of the natural water cycle with severe environmental, economic and social consequences. Sustainable agricultural systems aimed at mimicking natural hydrology must either minimise groundwater recharge over most of this landscape or intercept groundwater discharge in lower parts of the landscape prior to salinisation to restore the original hydrological balance. Empirical and theoretical analyses suggest that however vegetation is managed in the landscape, effective water balance control will be achieved only at a leaf area index approaching that of the natural state, indicating revegetation of most or all parts of the catchment with either trees or plants with similar ecohydrological characteristics as trees. There are at present few economically- attractive agricultural systems able to achieve hydrologic mimicry likely to be adopted at the necessary scale.

Towards achieving functional ecosystem mimicry with respect to water cycling in southern australian agriculture

Rain and dry deposition were sampled together for 28-day periods throughout 1974-75 at 27 stations, all but three being in the Murray-Darling Drainage Division (1.06 x 106 km2). Samples were analysed for calcium, magnesium, potassium, sodium, chloride and sulfate. Similar procedures continued during 1975-77 for six stations used previously and a new one. Rainfall records and the mean ionic concentrations, including estimates of HCO3 + NO3, indicate that the mean aggregate concentration for eight ions in 1974-75 ranges from 3 to 35 mgL-1 for 24 stations in the drainage division and from 14 to 23 mgL-1 for three stations near the South Australian coast. Higher concentrations in the drainage division refer principally to stations in a semi-arid district and are associated with relative abundance of calcium and HCO3 derived from the calcareous soils there. Terrestrial and oceanic sources of the ions are considered. Oceanic input of chloride is estimated using mean annual concentrations to give a maximum value and concentrations for the most rainy periods to give a minimum value. These estimates are compared with discharge of chloride by the Murray and Darling Rivers, with the conclusion that in 1974-75 the oceanic input of chloride to the Darling catchment was mostly retained there, but amounts equal to the accessions to the Murray catchment were probably all flushed out. Details are given in an appendix of NO3 concentrations determined by others at two stations in frequent samples of rain.

Salinity of atmospheric precipitation in the Murray-Darling drainage division, Australia

The wheatbelt region of Western Australia has been extensively cleared of indigenous vegetation for agriculture and is now severely affected by dryland salinity. Wetlands that were once freshwater are now saline and others are under threat, as are the animals and plants that inhabit them. Rising groundwater is also affecting the many naturally saline playas. To provide a framework for setting conservation priorities in this region a biological survey was undertaken, including sampling of aquatic invertebrates at 230 wetlands. In this paper, we have used data from the survey to summarise occurrence of species in relation to salinity. Total species richness at a wetland showed no response to salinity below 4.1 g l−1 and then declined dramatically as salinity increased. When halophilic species were excluded from consideration, species richness was found to decline from 2.6 g l−1. These patterns are compared to previous studies of richness-salinity relationships. There is some evidence that the freshwater invertebrate fauna of the wheatbelt may be comparatively salt tolerant, with 46% of freshwater species collected at salinities above 3 g l−1 and 17% above 10 g l−1, though these proportions differed between various invertebrate groups. While this tolerance will provide a buffer against the effects of mild salinisation, many species are at risk of regional extinction as salinisation becomes more widespread.

Occurrence of aquatic invertebrates of the wheatbelt region of Western Australia in relation to salinity

Loss of chloride in streamflow (salt flow) from forested catchments in southwestern Australia is only slightly greater than the total annual input (salt fall) from rain and dust. However, salt flow from catchments in which a significant area of the forest vegetation has been cleared for farming is much larger and exceeds salt fall by up to 690 kg/ha yr, or a factor 21. The net loss of Cl is interpreted to result from decreased storage in catchment soil water. Removal of the forest vegetation is estimated to have increased groundwater discharge of whole catchments by amounts ranging from about 1 to 13 cm3/cm2yr. From present rates of groundwater discharge, characteristic times for equilibration of Cl input and loss on farmed catchments are estimated to range from 30 to 400 years. In equilibrium the salty streams draining farmed catchments would yield water with Cl concentrations acceptable for drinking.

D. H. Hurle

Papers

Chloride balance of some farmed and forested catchments in southwestern Australia