A review of groundwater–surface water interactions in arid/semi‐arid wetlands and the consequences of salinity for wetland ecology

Abstract

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: 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. 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. 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. Secondary salinity can impact on wetland biota through changes in both salinity and water regime, which result from the hydrological and hydrogeological changes associated with secondary salinity. Whilst there have been some detailed studies of these interactions for some Australian riparian tree species, the combined effects on aquatic biodiversity are only just beginning to be elucidated, and are therefore, a future research need. Rainfall/flow‐pulses, which are a well‐recognized control on ecological function in arid/semi‐arid areas, also play an important, though indirect, role through their impact on wetland salinity. Freshwater pulses can be the primary means by which salt stored in both the water column and the underlying sediments are flushed from wetlands. Conversely, increased runoff is also a commonly observed consequence of secondary salinity, and so, wetlands can experience increased surface water inflows that are higher in salinity than under natural conditions. Moreover, changes in rainfall/flow‐pulse regimes can have a significant impact on wetland GW–SW interactions. It is possible that in some instances groundwater inflow to a wetland may become so heavy that it could become a major component of the water balance, and hence, mask the role of natural pulsing regimes. However, if the groundwater is low in salinity, this may provide an ecological benefit in arid/semi‐arid areas by assisting in maintaining water in wetlands that become aquatic refugia between flow‐pulses. There has been almost no modelling of GW–SW interactions in arid/semi‐arid wetlands with respect to water fluxes, let alone salinity or ecology. There is a clear need to develop modelling capabilities for the movement of salt to, from, and within wetlands to provide temporal predictions of wetland salinity which can be used to assess ecosystem outcomes. There has been a concerted effort in Australia to collect and collate data on the salinity tolerance/sensitivity of freshwater aquatic biota and riparian vegetation. There are many shortcomings and knowledge gaps in these data, a fact recognized by many of the authors of this work. Particularly notable is that there is very little time‐series data, which is a serious issue because wetland salinities are often highly temporally variable. There is also a concern that many of the data are from very controlled laboratory experiments, which may not represent the highly variable and unpredictable conditions experienced in the field. In light of these, and many other shortcomings identified, our view is that the data currently available are a useful guide but must be used with some caution. Copyright © 2008 John Wiley & Sons, Ltd.