Part 3 - Natural versus Artificial watering of floodplains and wetlands - M/BUS/248
MDFRC Publication 10/2011
1 of 4 reports associated with project see (Ecosystem Services and Productive Base for the Basin Plan), (Clarification of Definitions in the Water Act 2007) and (Feasibility Assessment of Ecological Outcomes (indicators) proposed for the Basin Plan Monitoring and Evaluation Program).
At a global scale, society's desire to control water for a range of purposes (e.g. irrigation, industry, stock and domestic supply, flood mitigation) has led to the regulation of a significant proportion of the world's rivers. Alteration of flow regimes is regarded as the most serious and continuing threat to ecological sustainability of rivers and their associated floodplain wetlands. Long-term drying has severely altered the ecology of many freshwater ecosystems, causing unprecedented, long-term or potentially irreversible damage (i.e. species extinctions). It is considered that much of the natural capacity (both resistance and resilience) of aquatic ecosystems to cope with drought has been lost. Re-establishment of natural flow regimes represents a neat theoretical objective. However, the reality is that this is impractical as the demands of society preclude returning our rivers to natural flow. The existing impacts of regulation combined with future impacts of climate change imply that in many river systems, overbank flows may no longer occur frequently enough to maintain ecological processes, and many wetlands and floodplains will become increasingly reliant on targeted environmental water allocations (EWA). New approaches to management will be essential in order to maintain a larger active floodplain than possible under the current water sharing arrangements. However, in order for managers to be successful in achieving the stated ecological objectives of river restoration and ecological management programs, it is necessary to have an appreciation of the role of flow in natural systems and the limitations of methods of delivering EWA. Within this synthesis we: 1. Briefly summarise the role of flow in unregulated floodplain ecosystems; 2. Define key state variables that characterise the flow regime of a floodplain system; 3. Discuss the major types of EWA currently in use; 4. Summarise key ecological processes and the impact of method of EWA delivery; 5. Outline the prevailing management paradigm; and 6. Identify management considerations for progress towards sustainable river systems. Flow is regarded as the key driver regulating processes and diversity in river systems and can be regarded as the master variable. The processes which are influenced by flow and floodplain inundation include hydrodynamics, biogeochemistry and primary productivity. Higher order organisms respond to these habitat and primary productivity drivers. It is not just the presence of water that is important for maintenance of ecosystem function; the provision of water is a critical link in the ecology of wetland and floodplain systems but that does not automatically imply that the link is functional. Flow magnitude, frequency, timing, duration, variability, rate of change and sequence all hold major ecological significance. It is important to note that the quality of water (i.e. chemical and thermal properties) is equally as important as the quantity of water or the temporal patterns of flow. In this context, the method of maintaining inundation (i.e. ponded flood versus flowing flood) and the resultant dilution and downstream dispersal of carbon and nutrients will have a significant impact on water quality via biogeochemically mediated processes. In unmodified catchments natural flooding regimes that are completely unaltered represent the reference condition. However, due to the extent of regulation and development throughout the MDB, there are very few sub-catchments that experience an unimpeded, natural flood. In modified catchments the closest approximation is an uncontrolled flow where the effects of storages and in-stream structures have largely been nullified. River management has skewed river channels and floodplains in opposite directions; towards an anti-drought an engineered drought scenario respectively. Regulated river systems are therefore likely to be in an extreme state of precariousness. Management needs to focus on reinstating resilience as the most pragmatic and effective way of managing ecosystems in order to withstand future droughts and provide ecosystem services. The concept of downsizing rivers has some merit but in reality it is a process of reinstating the small floods that river regulation has removed. It also overlooks the role of the interface between the aquatic (regularly inundated) and terrestrial (never inundated) zones in subsidising terrestrial food webs. Abandonment of large sections of floodplain may create an extremely dysfunctional and potential hostile zone or 'no-man's land' that is neither aquatic or terrestrial, generating a new barrier to energy flux. Enacted as an emergency measure, pumping water to targeted wetlands pumping water into individual sites has been highly successful in achieving a limited set of objectives. There is an emerging risk that construction and operation of new, large infrastructure specifically designed, constructed and operated for environmental outcomes is seen as an alternative to unregulated overbank floods to maintain ecosystems. It is essential to recognise that there there are a number of critical limitations associated with this approach; primarily related to spatial, connectivity and water quality issues. The expectation that fragmented sites will function as refuges that serve as the major sources of propagules and colonists for other areas and lead to improvement of the Murray-Darling Basin is unproven. Furthermore, it is critical to recognise that using a regulator to inundate large floodplains under low flow conditions has not been used as a restoration technique anywhere in the world. Consequently there is no precedence for this management activity and actual responses may differ from those expected. Releases of large volumes of water from storages may lead to the provision of flow-associated cues and conditions otherwise absent during base flows. However, water released from an upstream storage and transferred as an EWA into an individual site during periods of in-channel flow may restrict the ecological outcomes as the productivity gains from upstream flooding are not available to be transported into the managed site. The "missing pieces" are likely to include plant and invertebrate propagules dispersed from upstream sites, increased carbon and nutrient concentrations and other chemical cues resulting from inundation of floodplain soils and plant material, eggs and larvae of fish and other organisms spawned at upstream sites. We propose that there is a hierarchical time scale relationship between inundation events and ecological responses that is associated with all inundations. This relationship can be described as follows; Instantaneous (occur within minutes-hours of inundation), Fast (occur within hours-weeks of inundation), Slow (occur weeks-months after inundation), Delayed (processes that occur within months-years after inundation), and Cumulative (responses that may only occur/be realised after a series of events). We consider that the influence of any EWA delivery method will be related to the rate at which different processes occur. For example, chemically mediated processes occur very quickly (instantaneous) and are therefore unlikely to be affected by the method of delivery of EWA. In contrast, many biogeochemically mediated and biotic processes occur over longer time scales and are more likely to be influenced by the method of EWA delivery. This will be driven by the lag phase in ecological response providing opportunities for differences in responses/processes between natural and managed floods to cascade across multiple levels and manifest into large differences in the quality of outcomes. Methods of delivering environmental water that do not maximise (i) connectivity (i) the provision of appropriate habitat; and (ii) the development of appropriate food resources will deliver minimal benefits and compromise the ability of the EWA to achieve positive ecological outcomes. It must be recognised that the use of EWA's is fundamentally a large-scale manipulative experiment. We currently lack sufficient ecological knowledge to predict how floodplains in different conditions will respond. This represents a major hurdle for managers as volumes of environmental water are limited and resilience is an ecosystem property that can be either created or destroyed. Investment in recovering water and construction of infrastructure for delivery of EWA's needs to be underpinned by investment in research to inform adaptive management to ensure that critical ecological processes and functions are reinstated. If this is not undertaken, there is no way that EWA's will be able to reinstate resilience. The most appropriate method for delivery of an EWA to any site will vary accordingly with a range of factors including but not limited to; availability of water, connectivity of site to water source, and management targets. Environmental water allocations cannot replace the function of natural overbank flows and there is no ‘Silver Bullet' for repairing water-dependant ecosystems deprived of a natural flooding regime. Consequently pragmatic solutions are required to ensure environmental watering at intervals sufficient to enable system preservation and recovery. Reinstating flows and reoperation of existing infrastructure should be actively used during wet and median conditions to build resilience at the system scale. Delivery of EWA to isolated sites should be relegated to use during dry and extreme dry conditions to avoid long-term or irreversible damage and maintain refugia. The use of these techniques as the primary tool for the long-term management of floodplains and wetlands is not recommended.