In arid/semi-arid environments, where rainfall is seasonal, highly variable and significantly less than the evaporation rate, groundwater discharge can be a major component of the water and salt balance of a wetland, and hence a major determinant of wetland ecology. Under natural conditions, wetlands in arid/semi-arid zones occasionally experience periods of higher salinity as a consequence of the high evaporative conditions and the variability of inflows which provide dilution and flushing of the stored salt. However, due to the impacts of human population pressure and the associated changes in land use, surface water regulation, and water resource depletion, wetlands in arid/semi-arid environments are now often experiencing extended periods of high salinity. This article reviews the current knowledge of the role that groundwater-surface water (GW-SW) interactions play in the ecology of arid/semi-arid wetlands. The key findings of the review are as follows:1. GW-SW interactions in wetlands are highly dynamic, both temporally and spatially. Groundwater that is low in salinity has a beneficial impact on wetland ecology which can be diminished in dry periods when groundwater levels, and hence, inflows to wetlands are reduced or even cease. Conversely, if groundwater is saline, and inflows increase due to raised groundwater levels caused by factors such as land use change and river regulation, then this may have a detrimental impact on the ecology of a wetland and its surrounding areas. 2. GW-SW interactions in wetlands are mostly controlled by factors such as differences in head between the wetland surface water and groundwater, the local geomorphology of the wetland (in particular, the texture and chemistry of the wetland bed and banks), and the wetland and groundwater flow geometry. The GW-SW regime can be broadly classified into three types of flow regimes: (i) recharge-wetland loses surface water to the underlying aquifer; (ii) discharge-wetland gains water from the underlying aquifer; or (iii) flow-through-wetland gains water from the groundwater in some locations and loses it in others. However, it is important to note that individual wetlands may temporally change from one type to another depending on how the surface water levels in the wetland and the underlying groundwater levels change over time in response to climate, land use, and management. 3. The salinity in wetlands of arid/semi-arid environments will vary naturally due to high evaporative conditions, sporadic rainfall, groundwater inflows, and freshening after rains or floods. However, wetlands are often at particular risk of secondary salinity because their generally lower elevation in the landscape exposes them to increased saline groundwater inflows caused by rising water tables. Terminal wetlands are potentially at higher risk than flow-through systems as there is no salt removal mechanism. 4. Secondary salinity can impact on wetland biota through changes in both salinity and water regime, which result from the hydrological and hydrogeo...

There is a need to generalize water use behavior of eucalypts to facilitate bioengineering and landscape remediation programs in a wide range of Australian environments. A critical question can be stated as a null hypothesis: tree water use per unit leaf area (leaf efficiency) is independent of eucalypt species. This is implicitly equivalent to the hydrological equilibrium hypothesis that states that leaf area is a function of climate, at least in cases where transpiration and growth are limited by soil water. Failure to reject this null hypothesis simplifies (a) the selection of tree species for water balance management, (b) the generation of regional-scale expectations of leaf area index, and (c) the estimation (monitoring) of the effectiveness of plantations in controlling site water balance. The hypothesis was tested with tree water use data collected in natural multi-species stands across Australia, including sites in the wet-dry season tropical woodlands of the Northern Territory, the Mediterranean climate forests of Western Australia, and a woodland system in southern New South Wales receiving an even distribution of rainfall throughout the year. We also tested the hypothesis in a multi-species tree plantation growing on a saline gradient. In each case, we could not reject the hypothesis of constant leaf efficiency among eucalypts. In every case there was a common, strong, linear relationship among tree leaf area and mean daily water use by all tree species in a sample. Single factor (species) analysis of variance did not detect significant differences between leaf water efficiencies of species. For the jarrah forest (Eucalyptus marginata J. Donn ex Sm., E. calophylla R. Br. ex Lindl.), the null hypothesis held in both spring (wet) and autumn (dry) conditions. The null hypothesis held in the mixed species woodland of New South Wales (E. macrorhynca F.J. Muell. ex Benth., E. blakelyi Maiden., E. polyanthemos Schauer.) under summer and autumn conditions, and across five species in the wet-dry tropical woodland (E. miniata A. Cunn. ex Schauer, E. tetrodonta F.J. Muell., E. porecta S.T. Blake, Erythrophleum chlorostachys F.J. Muell., and Terminalia ferdinandiana Exell.). The null hypothesis also held for a plantation of E. occidentalis Endl. and provenances of E. camaldulensis Dehnh. growing on a shallow saline gradient; i.e., leaf water efficiency remained constant across species and varieties despite obvious effects of salinity on the size of individual canopies. We conclude that there is little evidence for rejecting the hypothesis that leaf efficiency does not vary significantly among sympatric eucalypt species in rainfall-limited (soil-water-limited) systems. These findings open the way for useful bioengineering generalities about the hydrological role of trees in the Australian landscape.

Abstract:This study investigated the surface water-groundwater interactions of three semi-arid floodplain wetlands of the lower River Murray (SE Australia) using a combination of hydrometric, natural tracer and geophysical methods. The current management objective for these wetlands is to mimic the natural surface water flow regime by engineering wetting and drying cycles for the benefit of the biota. However, the impact this will have on groundwater processes and wetland salinization is unknown. This study found that when inundated, two of the wetlands were groundwater recharge features, whereas the other was a groundwater throughflow system. After these wetlands were dried, there was a reversal of the hydraulic gradients and all three wetlands became groundwater discharge features. The transformation of these wetlands to groundwater discharge features after the removal of surface water means that there is an increased risk of salinization when wetting and drying cycles are reintroduced. In arid/semi-arid regions of the world where wetlands are in direct contact with saline groundwater, extreme caution should be applied when altering the management of the surface hydrology of the wetlands because increases in salinity can impact upon biodiversity.

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Abstract:The transport of saline groundwater from local and regional aquifers to the lower River Murray in South Australia is thought to be greatly influenced by the incised lagoons and wetlands that are present in the adjacent floodplain. Interactions between a saline lagoon and semi-confined aquifer at a floodplain on the River Murray were studied over a 1-year period using hydrogeological techniques and environmental tracers (Cl , υ 2 H and υ 18 O). Piezometric surface monitoring showed that the lagoon acted as a flow-through system intercepting local and regional groundwater flow. A chloride mass balance determined that approximately 70% of the lagoon winter volume was lost by evaporation. A stable isotope mass balance estimated leakage from the lagoon to the underlying aquifer. Around 0-38% of the total groundwater inflow into the lagoon was lost to leakage compared to 62-100% of groundwater inflow lost to evaporation. Overall, floodplain wetlands of the type studied here behave as groundwater flow-through systems. They intercept groundwater discharge, concentrate it and eventually recharge more saline water to the floodplain aquifer. Understanding groundwater-surface water interactions in floodplain wetlands will benefit the effective management of salinity in semi-arid rivers.