As in arid lands of the world, many semi-arid landscapes in Australia have plant and animal growth and reproduction, hence survival, severely limited by available water. For example, Acacia anuera (mulga) grove-intergrove landscapes are source-sink systems where water flows from low ridges and stony slopes (inter-groves) into flat areas (groves). Water can be lost from these systems, to lakes and rivers. This occurs if the water retention (filtering and storage) capacity of the sinks is too low (perhaps due to landscape degradation) or if the total area of sink is too small. A flow-filter landscape model was developed to determine the area of sink (relative to the total area) that will maximize resource (water) conservation and plant production under conditions of low rainfall. The model was also used to examine the effect of having landscape resource sinks with low and high filtering capacities. Simulation results indicate that when rainfall is low (160 mm) the area of sink needed to conserve all available water within the landscape was 40 per cent of the total landscape area when sinks had high resource-filtering capacities; this area increased to 60 per cent when sinks had a low filtering capacity as the case with landscape degradation. The flow-filter landscape model can provide land managers with guidelines on rehabilitating degraded landscapes by reconstruction of sink areas. To conserve the limited amounts of rainfall within a semi-arid landscape a minimal area of sink has to be reconstructed; the flow-filter model estimates this minimal area, thus reducing rehabilitation costs.
Various Australian rainfall records have been subjected to filter-analysis. The results represent an extension of the findings of Currie and Vines (1996) in their analyses of more than 300 annual rainfall-series gathered from weather stations widely spaced across the continent. Further evidence is presented for the existence of "cyclic" variations in precipitation with periods of 16?20 years and 10?11 years. Links are suggested with the luni-solar cycle of 18.6 years and the sunspot cycle of nominal period 10?12 years. A shorter "cycle" of 6?7 years is also postulated. Similar analyses of yearly data for the Southern Oscillation yield further suggestions of "cycles" that correspond closely with those obtained from the rainfall records. Explanations are proposed which appear to account for a substantial proportion of the interannual variability of rainfall, particularly in eastern Australia - with connections being made between EI Ni�o/Southern Oscillation events and the incidence of drought in various parts of the continent. A major implication is that a variety of climatic effects in different parts of the world can be largely ascribed to the influence of the (18.6 year) luni-solar cycle.
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