Southern Murray-Darling Basin Research Articles

Changes in Weather Systems during Anomalously Wet and Dry Years in Southeastern Australia

Abstract This study focuses on the rainfall-producing weather systems in the southern Murray–Darling Basin (MDB), Australia. These weather systems are divided into objects: cyclones, fronts, anticyclones, warm conveyor belt (WCB) inflows, WCB ascents, potential vorticity (PV) streamers, and cutoff lows. We investigate the changes in the frequency, amplitude, and relative position of these objects as the daily and seasonal rainfall change. Days on which the rainfall is heavy, especially in winter, are characterized by more PV streamers, cutoff lows, cyclones, fronts, and WCBs in the region. In contrast, dry days are characterized by more anticyclones over southeastern Australia in winter and summer. The effect of upper-level weather objects (PV streamers and cutoff lows) on lower-level objects, and their importance in producing rainfall, is quantified using the quasigeostrophic ω equation and separating the vertical motion into that induced by the upper and lower levels. On heavy rainfall days in winter, PV streamers and cutoff lows force strong upward motion in the lower troposphere, promoting cyclogenesis at lower levels, forcing ascent in the WCBs, and producing rain downstream of the southern MDB. Lower-level ascent forced by upper-level objects is important for the development of heavy rainfall in both seasons, although particularly in winter. Rainfall is attributed to individual objects. PV streamers and WCBs contribute most to the winter and summer rainfall, respectively. The difference in rainfall between anomalously wet and dry years can be explained in winter by the changes in the rainfall associated with PV streamers, whereas in summer it is mostly due to a reduction in the rainfall associated with WCBs. Significance Statement The aim of the present study is to better understand how synoptic-scale weather systems differ in southeastern Australia in dry and wet periods, by considering a wide range of weather systems. We found weather systems are more closely aligned in the vertical on heavy rainfall days, and the majority of rainfall in this region is associated with warm conveyor belts. These results point to warm conveyor belts being an important, but not well recognized, contributor to rainfall in this region. Future work may investigate the roles of the various modes of variability and climate change in modulating warm conveyor belts and hence the regional rainfall variability in Australia.

An environmental flow to an ephemeral creek increases the input of carbon and nutrients to a downstream receiving river

Although intermittent and ephemeral rivers lack surface flow for part of the year, they provide vital refuges for biota in otherwise dry semi-arid and arid landscapes. The hydrology of many such rivers has been altered due to river regulation and climate change. Environmental flows can be delivered to address the negative impacts of regulated flows, however there is limited knowledge of how dry ephemeral ecosystems respond following environmental flows. This study examined changes in water quality of the ephemeral Thule Creek in the southern Murray-Darling Basin, Australia, following delivery of environmental water from an irrigation canal. We also examined how the environmental flow influenced water quality of Wakool River that receives inflows from Thule Creek. Six sites in Thule Creek, three in Wakool River, and one in Yarraman irrigation channel (source water) were monitored for dissolved organic carbon (DOC), nutrients and dissolved oxygen (DO) once per week over 15 weeks from October 2019 to January 2020. The environmental flow resulted in high DOC concentrations (4.4–76 mg/L). Although low DO levels at sites in Thule Creek were recorded on some dates below the threshold for fish stress (< 4 mg/L) there were no fish kills observed during the environmental flow. The carbon-rich and nutrient-rich water (DOC >10 mg/L, total phosphorus up to 94 μg/L, total nitrogen up to 1,125 μg/L) was detected in the Wakool River downstream of Thule Creek confluence compared to the Wakool River upstream of Thule Creek confluence (DOC 6.6 mg/L, total phosphorus up to 64 μg/L, total nitrogen up to 805 μg/L) during the period when the environmental flow in Thule Creek was connected with the Wakool River. This research provides an example of how irrigation canal networks can be used to deliver environmental water to an ephemeral river to maintain refuges and contribute to the productivity of a receiving river further downstream. Careful management of the timing, volume and duration of environmental flows in arid or semi-arid landscapes is needed to avoid the development of poor water quality during, or following, the delivery of environmental water.

Challenges of classifying and mapping perennial freshwater systems within highly variable climate zones: A case study in the Murray Darling Basin, Australia

Perennial freshwater systems are valuable natural resources that provide important ecological services globally. However, in highly variable climates, such as Australia, water availability in rivers and streams can vary greatly from year to year and from decade to decade. Further, across Australia and many other regions, perennial river systems are projected to decrease because of anthropogenic climate change, placing the ecosystems they support under additional pressure. Quantifying the potential impacts of climate change on perennial freshwater systems requires robust databases of existing water features with accurate classifications. This is a challenge for rivers that display a high degree of interannual variability since the river classification can be dependent on the period of available data. In this study, we carry out a regional scale comparison of three different spatial databases commonly used in environmental and ecological assessments of perennial systems of Australia, namely Geodata, Geofabric and Water Observations from Space (WOfS). Focusing on the southern Murray Darling Basin (MDB), due to its national and international significance and its highly variable flow regimes, we show that no single spatial database is reliable by itself in terms of perennial water classification, with notable differences likely arising from variations in the periods analysed and methods used to classify the systems. Further, an analysis of high-quality gauged streamflow data (with approximately 40-year daily records) for four sub-catchments, and long-term simulation data (>100 years) for two sub-catchments in the lower MDB, confirm that flow persistence can be non-stationary through time, with some ‘perennial’ systems exhibiting sustained periods of cease to flow (i.e. becoming non-perennial) during prolonged droughts. This study demonstrates that due consideration is required in developing baseline classification of perennial freshwater systems for assessing future changes and measuring adaptive capacity.

Flexible forecast value metric suitable for a wide range of decisions: application using probabilistic subseasonal streamflow forecasts

Abstract. Streamflow forecasts have the potential to improve water resource decision-making, but their economic value has not been widely evaluated, since current forecast value methods have critical limitations. The ubiquitous measure for forecast value, the relative economic value (REV) metric, is limited to binary decisions, the cost–loss economic model, and risk-neutral decision-makers (users). Expected utility theory can flexibly model more real-world decisions, but its application in forecasting has been limited and the findings are difficult to compare with those from REV. In this study, a new metric for evaluating forecast value, relative utility value (RUV), is developed using expected utility theory. RUV has the same interpretation as REV, which enables a systematic comparison of results, but RUV is more flexible and better represents real-world decisions because more aspects of the decision context are user-defined. In addition, when specific assumptions are imposed, it is shown that REV and RUV are equivalent, hence REV can be considered a special case of the more general RUV. The key differences and similarities between REV and RUV are highlighted, with a set of experiments performed to explore the sensitivity of RUV to different decision contexts, such as different decision types (binary, multi-categorical, and continuous-flow decisions), various levels of user risk aversion, and varying the relative expense of mitigation. These experiments use an illustrative case study of probabilistic subseasonal streamflow forecasts (with lead times up to 30 d) in a catchment in the southern Murray–Darling Basin of Australia. The key outcomes of the experiments are (i) choice of decision type has an impact on forecast value, hence it is critically important to match the decision type with the real-world decision; (ii) forecasts are typically more valuable for risk averse users, but the impact varies depending on the decision context; and (iii) risk aversion impact is mediated by how large the potential damages are for a given decision. All outcomes were found to critically depend on the relative expense of mitigation (i.e. the cost of action to mitigate damages relative to the magnitude of damages). In particular, for users with relatively high expense of mitigation, using an unrealistic binary decision to approximate a multi-categorical or continuous-flow decision gives a misleading measure of forecast value for forecasts longer than 1 week lead time. These findings highlight the importance of the flexibility of RUV, which enable evaluation of forecast value to be tailored to specific decisions/users and hence better capture real-world decision-making. RUV complements forecast verification and enables assessment of forecast systems through the lens of user impact.

A river basin spatial model to quantitively advance understanding of riverine tree response dynamics to water availability and hydrological management

Ecological condition continues to decline in arid and semi-arid river basins globally due to hydrological over-abstraction combined with changing climatic conditions. Whilst provision of water for the environment has been a primary approach to alleviate ecological decline, how to accurately monitor changes in riverine trees at fine spatial and temporal scales, remains a substantial challenge. This is further complicated by constantly changing water availability across expansive river basins with varying climatic zones. Within, we combine rare, fine-scale, high frequency temporal in-situ field collected data with machine learning and remote sensing, to provide a robust model that enables broadscale monitoring of physiological tree water stress response to environmental changes via actual evapotranspiration (ET). Physiological variation of Eucalyptus camaldulensis (River Red Gum) and E. largiflorens (Black Box) trees across 10 study locations in the southern Murray-Darling Basin, Australia, was captured instantaneously using sap flow sensors, substantially reducing tree response lags encountered by monitoring visual canopy changes. Actual ET measurement of both species was used to bias correct a national spatial ET product where a Random Forest model was trained using continuous timeseries of in-situ data of up to four years. Precise monthly AMLETT (Australia-wide Machine Learning ET for Trees) ET outputs in 30 m pixel resolution from 2012 to 2021, were derived by incorporating additional remote sensing layers such as soil moisture, land surface temperature, radiation and EVI and NDVI in the Random Forest model. Landsat and Sentinal-2 correlation results between in-situ ET and AMLETT ET returned R2 of 0.94 (RMSE 6.63 mm period−1) and 0.92 (RMSE 6.89 mm period−1), respectively. In comparison, correlation between in-situ ET and a national ET product returned R2 of 0.44 (RMSE 34.08 mm period−1) highlighting the need for bias correction to generate accurate absolute ET values. The AMLETT method presented here, enhances environmental management in river basins worldwide. Such robust broadscale monitoring can inform water accounting and importantly, assist decisions on where to prioritize water for the environment to restore and protect key ecological assets and preserve floodplain and riparian ecological function.

A 2D hydrodynamic model-based method for efficient flood inundation modelling

AbstractEfficient and accurate flood inundation predictions can provide useful information for flood risk mitigation and water resource management. In this paper, we propose a new modelling method, LoHy + , which can be applied to efficiently simulate the spatiotemporal evolution of flood inundation with reasonable accuracy. The method integrates a low-fidelity two-dimensional (2D) hydrodynamic model and a mapping module to estimate water depth in a catchment during floods. The performance of the proposed modelling method was evaluated using a real-world catchment of approximate 2,000 km2, in the Southern Murray–Darling Basin, Australia. The results show that there is a good agreement between flood inundation obtained from the proposed method and that simulated using a high-fidelity 2D hydrodynamic model. The proposed method is much more efficient than the high-fidelity 2D hydrodynamic model, which makes it an alternative method for applications requiring many model runs or long simulation durations. Also, the LoHy+ model has the potential to be applied in flood inundation forecast, flood risk mitigation design water resource management, etc.

Earth oven cookery and cuisines in Aboriginal Australia: Ethnographic and ethnohistoric insights from Western Cape York Peninsula and the Southern Murray Darling Basin

Earth oven cookery involves cooking food in pits using hot heating elements, typically over extended periods of time. This technique has been reported in Holocene and Late Pleistocene contexts in Australia, and is of ongoing importance to many Indigenous peoples today. Despite considerable previous work on earth ovens and related sites, few have explored earth oven cookery as a distinctive cultural phenomenon. Here, we investigate the foodways associated with earth ovens drawing on ethnohistoric and ethnographic sources from the southern Murray-Darling Basin and central Western Cape York Peninsula, Australia. While there are many commonalities in earth oven cookery, it was also a highly adaptable practice in terms of the range of foods cooked, oven construction practices, and cooking techniques. People widely used herbs and wrappings to flavour foods, added water to aid the cooking process, and made extensive use of other plant materials to impart flavour, prevent food from burning, while also keeping food free of debris. We show that earth ovens are strongly associated with culturally distinctive cuisines and foodways and an investigation of these cookery practices can enhance our understanding of past social organisation, identity, commensality and the scale of food production.

Understanding regional streamflow trend magnitudes in the Southern Murray–Darling basin, Australia

ABSTRACT Understanding long-term trends in streamflow is important for water resource management. In this study, we investigate the long-term streamflow trends at 47 gauging sites within the southern Murray–Darling basin (MDB), Australia. This study aims to estimate regional streamflow trends while understanding the impact of catchment characteristics on the spatial variation in these trends. To achieve this, we applied a Bayesian hierarchical model (BHM) to make the best use of available streamflow records from multiple sites and catchment characteristics such as climate, terrain, geology, land use and vegetation. The results show that streamflow trends from tested sites are consistently negative, with magnitudes of up to 2.7% per year relative to the annual average flow. We also find that spatial variability in trends can be best linked to differences in average climatic and terrain conditions. This finding can be used to inform future water planning for consumptive and environmental uses in the MDB.

Expanders, diversifiers or downsizers? Identifying clusters of irrigators’ water trade and farm management strategies in Australia

Australia, in particular the southern Murray-Darling Basin, has the most advanced water market in the world. However, there is still much to be understood as to how and why irrigators use the water market, and in what combination with other farm management strategies. This study used principal component analysis to identify five clusters of southern Murray-Darling Basin irrigators from a set of twenty possible farm and water strategies during 2015–16 (n = 977). Multinomial logit regression was then used to identify influences associated with each cluster. The five clusters of irrigators include those: expanding the farm (includes buying temporary water); expanding and diversifying (includes buying permanent water); downsizing (selling both temporary and permanent water); transitioning (switching away from irrigation to dryland); and saving (using carryover – water saved in storages for the following year). Around a third of irrigators can be classified as trying to expand the farm (the Expanders and the Expanders & Diversifiers); another third, of typically older irrigators, have lower levels of debt and excess water (the Savers); while a final third, who also tend to be older but also face financial and/or psychological stress, are placing more emphasis on dryland production (away from irrigation) or decreasing agricultural production altogether (the Transitioners and Downsizers). Higher long-term temperature of a farm area’s location increased the probability of the irrigator being a downsizer, indicating a future trend of irrigation farm exit, given the predicted rise in temperature from climate change in the Murray-Darling Basin.

The spatial distribution and determinants of irrigators’ price choices for water entitlement trading

ABSTRACT Although many studies have examined irrigators’ water trading behaviour, little is known about how irrigators value their water, especially for their water entitlements (permanent water rights). This article’s aim is to assess the determinants of irrigators’ values for their water (i.e. price choices for selling and buying of water entitlements). Specifically, we focus on spatial determinants and how irrigators’ price choices vary spatially. We used stated preferences data from an irrigator survey in the southern Murray-Darling Basin (MDB) in Australia at the end of the Millennium drought (2011). It was found that (spatial) influences affect the price choices of the selling and buying decision differently depending e.g. on irrigators’ location in the southern MDB, i.e. with regards to rural areas, lower resource areas and the regional socio-economic index. Furthermore, irrigators’ valued their water differently if they owned it compared to if they were going to own it, which may relate to the ‘endowment effect’.

From the 1990s climate change has decreased cool season catchment precipitation reducing river heights in Australia\u2019s southern Murray-Darling Basin

The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receives most of its annual catchment runoff during the cool season (April–September). Focusing on the Murrumbidgee River measurements at Wagga Wagga and further downstream at Hay, cool season river heights are available year to year. The 27-year period April–September Hay and Wagga Wagga river heights exhibit decreases between 1965 and 1991 and 1992–2018 not matched by declining April-September catchment rainfall. However, permutation tests of means and variances of late autumn (April–May) dam catchment precipitation and net inflows, produced p-values indicating a highly significant decline since the early 1990s. Consequently, dry catchments in late autumn, even with average cool season rainfall, have reduced dam inflows and decreased river heights downstream from Wagga Wagga, before water extraction for irrigation. It is concluded that lower April–September mean river heights at Wagga Wagga and decreased river height variability at Hay, since the mid-1990s, are due to combined lower April–May catchment precipitation and increased mean temperatures. Machine learning attribute detection revealed the southern MDB drivers as the southern annular mode (SAM), inter-decadal Pacific oscillation (IPO), Indian Ocean dipole (IOD) and global sea-surface temperature (GlobalSST). Continued catchment drying and warming will drastically reduce future water availability.

How stable are Australian farmers’ climate change risk perceptions? New evidence of the feedback loop between risk perceptions and behaviour

The exact relationship between people’s climate change attitudes and behaviour is a topic that engages policy-makers and researchers worldwide. Do climate change attitudes influence behaviour or is it possible that behaviour can change attitudes? This study uses a unique repeated survey dataset of 275 farmers (irrigators) in the southern Murray-Darling Basin from 2010–11 to 2015–16, to explore the dynamic relationship between climate change risk perceptions and farm adaptation behaviour. Farmers who had an increased risk exposure (expressed through higher debt, larger irrigated areas, greater share of permanent crops, and located in areas with higher temperatures and less rainfall) were more likely to agree climate change posed a risk. Whilst farmers became more accepting towards climate change over the time-period, a significant percentage of these attitudes were unstable. One reason suggested for this instability is the presence of a feedback loop between risk perceptions and behaviour. Namely, new evidence was found that farmers who agreed climate change was a risk in 2010–11, were more likely to undertake farm production decisions to reduce that risk (e.g. changing crop mix, reducing irrigated area and consequently selling water entitlements) – which had the impact of negatively feeding back and reducing their stated climate change risk perceptions in 2015–16. Conversely, farmers who were originally deniers were more likely to undertake somewhat riskier farm production decisions (e.g. increasing water utilisation rates and irrigation areas) – which consequently had the impact of positively increasing their climate change risk perceptions in 2015–16.

Explaining permanent and temporary water market trade patterns within local areas in the southern Murray–Darling Basin

The southern Murray–Darling Basin of Australia has the world’s largest and most sophisticated water markets. However, there has been little study on the drivers of permanent and temporary water market movements within local areas, or the substitution effects between groundwater and surface‐water extraction over time. This topic is important as it is often claimed that water market trade patterns (especially selling large amounts of permanent water) are associated with rural decline. This study uses random‐effects tobit panel models to investigate the association of regional and spatial socio‐economic characteristics with temporary and permanent southern Murray–Darling Basin water market trade, using a broker panel database at postcode level from 2010/11 to 2013/14. Overall, results suggest there is no statistical significant evidence that more disadvantaged communities sold larger amounts of permanent water. Permanent water selling was statistically more likely to be associated with other spatial and land productivity characteristics, while temporary water market trade volumes were more related to water scarcity factors. In addition, there was evidence to suggest a substitution effect between rural areas selling higher volumes of permanent water and using higher volumes of groundwater.

The drivers associated with Murray-Darling Basin irrigators’ future farm adaptation strategies

Irrigators in the Murray-Darling Basin will need to adapt to future uncertainty, because of changes in markets, industry structures and climate. Such adaptation can be classified as expansive, accommodating or contractive strategies. Expansive adaptation strategies expand irrigation, accommodating strategies modify existing processes or crops without changing the size of the irrigation component of the farm, whereas contractive strategies reduce irrigation. Using data from a 2015-16 survey of 1,000 southern Murray-Darling Basin irrigators, 19 distinct planned future adaptation strategies are aggregated into expansive, accommodating and contractive adaptation indexes. Seemingly Unrelated Regression was used to model influences associated with irrigators’ future adaptation. While 90% of all irrigators were planning for at least one form of farm adaptation, there is some evidence that they prefer expansive adaptation strategies over accommodating and contractive adaptation strategies. It was found that succession planning and past adaptation experience have a statistically significant influence on all planned adaptation indexes. The influence of financial, human, natural, physical and social capital varies between adaptation types, with financial capital variables the strongest statistically significant driver for accommodating adaptation. Expansive and contractive adaptation are more strongly impacted by human and social capital variables.

Farm Level Effects of On‐Farm Irrigation Infrastructure Programs in the Southern Murray–Darling Basin

AbstractOn‐farm irrigation infrastructure programs have become an important method of recovering water for the environment in the Australian Murray–Darling Basin (MDB). These programs offer farmers funding to upgrade infrastructure in return for a portion of their water rights. This study measures the effects of Australian Government on‐farm infrastructure programs in the southern MDB between 2009–2010 and 2016–2017, particularly the On‐Farm Irrigation Efficiency Program. A novel dataset is constructed combining program administrative data with farm survey data. This data is used to derive econometric estimates on the effects of these programs on various measures of farm productivity, profitability and water demand. On‐farm programs are found to have positive effects for participants in terms of higher farm productivity and profitability. However, the study also finds a Jevon's paradox outcome, where farm demand for water is significantly higher post‐upgrade.

Size, growth and mortality of riverine golden perch (Macquaria ambigua) across a latitudinal gradient

Effective fisheries management requires fish size, growth and mortality information representative of the population and location of interest. Golden perch Macquaria ambigua is long lived, potamodromous and widespread in the Murray–Darling Basin (MDB), Australia. Using a sample spanning 13 river systems and 10° of latitude, we examined whether the maximum size of golden perch differed by latitude and whether growth and mortality varied between northern and southern MDB regions. The length, weight and age ranges of golden perch sampled (n=873) were 52–559mm, 2–3201g and 0+ to 26+ years respectively, and maximum length and weight were unaffected by latitude. Length and age–length distributions represented by age–length keys varied by region, with greater variability in age-at-length and a larger proportion of smaller individuals in northern MDB rivers, which generally exhibit greater variability in discharge. Growth and mortality rates were similar between regions, and an MDB-wide von Bertalanffy growth model (L∞=447, k=0.32 and t0=–0.51) and instantaneous mortality rate (Z=0.20) best described the data. An MDB-wide length–weight equation also provided the best fit (W=6.76×10–6 L3.12). Our data suggest that the MDB can be treated as one management unit in terms of golden perch maximum size, growth and mortality parameters.

Engineered landscapes of the southern Murray–Darling Basin: Anthropocene archaeology in Australia

Human activities over the past 200 years have fundamentally transformed the shape of Australia’s southern Murray–Darling Basin. The arrival of British colonists in the 19th century disrupted millennia of human management of the region and brought widespread changes to biota and soils. The subsequent development of mining, transport and irrigation infrastructure re-engineered the region’s landscapes to meet human objectives and ambitions. This article offers an integrated regional history of anthropogenic change across the southern Murray–Darling Basin, identifying historical processes driving complex ongoing interactions between human activities and the natural environment. We examine three broad domains of engineering and geo-disturbance in the region, including the development of transport corridors, micro- and macro-scale water management and landforms remade by erosion and sedimentation. We use the archaeology of the recent past to integrate insights drawn from physical geography, fluvial geomorphology and related research into the enduring landscape changes of modern Australia’s food bowl.

Mapping the maximum inundation extent of lowland intermittent riverine wetland depressions using LiDAR

Accurate high-resolution maps of maximum wetland inundation extents are valuable inventorial resources, but mapping such boundaries can be difficult, time consuming and involve a level of subjectivity from the surveyor. A novel, objective and efficient method is presented for delineating the maximum inundation extents of lowland intermittent riverine wetland depressions using a high resolution LiDAR-derived digital terrain model (DTM). The method was specifically developed for wetland depressions associated with the lower reaches of the Murrumbidgee River in the southern Murray Darling Basin, Australia, but is suitable for any similar intermittent wetland depressions where the natural inundation extent is otherwise unclear. The method is based on the premise that immediately after rising water reaches a threshold level, much larger volumes of water are required to inundate not just the discrete wetland depression but also its surrounding local area; that threshold level can be considered the wetland fill level. Using a series of water levels and a corresponding series of the volume between each water level and a DTM of a wetland area, the maximum rate of increasing acceleration in volume with respect to water level, i.e. the wetland fill level, can be identified. The fill levels of eight wetland areas, ranging in size from 44 to 384 ha, were produced by this method and subsequently used to map their maximum inundation extents. Corresponding maps of the inundation frequency gradient were independently determined from a time series of Landsat derived inundation maps. The level of separation in inundation frequency of map cells inside the maximum inundation extents compared to those outside, determined by the Kolmogorov Smirnov statistic, validated the technique, while identifying that a minimum level of variation in the elevation of the wetland area may be necessary for it to be successfully applied. The technique has potential to be applied more widely as an objective and relatively inexpensive procedure to identify the potential maximum inundation extent of intermittent floodplain wetlands when a good quality high resolution DTM, acquired during a dry phase, is available.

Restoring Murray River floodplain wetlands: Does the sediment record inform on watering regime?

AbstractThe floodplain wetlands of the southern Murray Darling Basin (MDB) have been subject to the impacts of catchment and water resource development for more than a century. Their current degraded state is attributed to the regulation of the rivers and abstraction of water volume for irrigation. The MDB Plan is to return at least 2,750 Gl of mean annual flow to the system to restore the condition of waterways. Considerable recent investment in infrastructure enables water to be released into the basin's floodplain wetlands. The proposed watering regime is underpinned by modelling that suggests that, before regulation, overbank flows would have occurred regularly as discharge peaked in winter and spring. Sediment cores have been extracted from over 50 floodplain wetlands of the southern Murray Basin. Those from several, large meander wavelength billabongs extend for 1,000–5,000 years suggesting that these sites were permanently inundated over that time. Others extend to ~200 years and are presumed not to have accumulated sediment until more recently. The records of most wetlands, however, only extend to the onset of river regulation in the 1920s, suggesting that before then they were not inundated for sufficient duration for net accumulation to occur. Preserved diatoms suggest that the shallow, plant‐dominated wetlands of the past have transitioned to deep, turbid water systems today. As rivers are identified as a source of sediment to wetlands, less regular inundation, rather than more, is a viable option in restoring the ecological function of these floodplain wetlands and in slowing sediment infill.

Do neighbors influence irrigators’ permanent water selling decisions in Australia?

There has been increasing emphasis by scholars in trying to understand how neighbors influence farmers’ decision-making. In Australia, historically there has always been strong anecdotal evidence of peer pressure on irrigators’ decisions to not sell permanent water entitlements. This local pressure increased with the advent of the federal government into the Murray-Darling Basin water market to purchase permanent water for the environment from voluntary sellers from 2008 onwards. Selling permanent water entitlements is associated with perceptions about rural community decline. This article formally tests whether a neighborhood effect can be detected in permanent water entitlement selling decisions, using farm survey data across a number of years from the southern Murray-Darling Basin (n = 1462). We hypothesize that the more that an irrigator’s neighbors sell permanant water in a given area, the more likely that an individual irrigator will also sell permanant water given a decrease in social pressure. Irrigators’ locations were geocoded and locational characteristics (regional socio-economic characteristics, land and water quality) were linked to the survey data covering farm, farmer and water trade characteristics. There was significant evidence of a farmer neighborhood effect, with higher likelihood of permanent water sales occurring in areas where more neighbors had sold permanent water, holding other locational and spatial influences constant.