Ecosystems in shallow micro-tidal lagoons are particularly sensitive to hydrologic changes. Lagoons are complex transitional ecosystems between land and sea, and the signals of direct human disturbance can be confounded by variability of the climate system, but from an effective estuary management perspective, the effects of climate versus direct human engineering interventions need to be identified separately. This study developed a 3D finite-volume hydrodynamic model to assess changes in hydrodynamics of the Peel–Harvey Estuary, a large shallow lagoon with restricted connection with ocean; this was done by considering how attributes such as water retention time, salinity and stratification have responded to a range of factors, focusing on the drying climate trend and the opening of a large artificial channel over the period from 1970 to 2016, and how they will evolve under current climate projections. The results show that the introduction of the artificial channel has fundamentally modified the flushing and mixing within the lagoon, and the drying climate has changed the hydrology by comparable magnitudes to that of the opening of the artificial channel. The results also highlight the complexity of their interacting impacts. Firstly, the artificial channel successfully improved the estuary flushing by reducing average water ages by 20–110 d, while in contrast the reduced precipitation and catchment inflow had a gradual opposite effect on the water ages; during the wet season this has almost counteracted the reduction brought about by the channel. Secondly, the drying climate caused an increase in the salinity of the lagoon by 10–30 PSU (Practical Salinity Unit); whilst the artificial channel increased the salinity during the wet season, it has reduced the likelihood of hypersalinity (
Hydrologic features such as water circulation and retention, as well as the pattern of saline water intrusion, are critical in shaping estuarine ecosystems. The interactions between freshwater runoff pulses with ocean water can create complex hydrodynamics that subsequently structure coastal biogeochemical processes, including the distributions of sediment and nutrients, and areas favourable for primary productivity (e.g. Legović et al., 1994; Kasai et al., 2010; Watanabe et al., 2014; Cloern et al., 2017). Whilst nutrient loads are the primary determinant affecting the long-term trophic state of coastal waters (Howarth and Marino, 2006; Williamson et al., 2017), the timescales associated with water retention and mixing are critical in mediating the relationship between nutrient inputs and the ensuing water quality response, including the likelihood of nuisance algal blooms or hypoxia (e.g. Knoppers et al., 1991; Ferreira et al., 2005; Paerl et al., 2006; Zhu et al., 2017). The retention of water and hydrodynamic patterns that emerge in any given site are largely dependent upon local geomorphological features, though increasingly coastal engineering and changes in river hydrology disturb natural patterns of water exchange (Knoppers et al., 1991; Kjerfve et al., 1996; Dufour et al., 2001; Gong et al., 2008; Odebrecht et al., 2015; Almroth-Rosell et al., 2016). Understanding and predicting these hydrologic changes are critical to underpin adaptive approaches to estuary water quality management and ecological restoration.
Coastal lagoons and embayments with low rates of ocean exchange are particularly sensitive relative to other estuary forms. The typical low flushing rates lead to high rates of deposition of sediment and particulate matter and accumulation of nutrients (e.g. Newton et al., 2014, 2018; Paerl et al., 2014). They are also productive ecosystems and often experience conflicting interests between the ecosystem services they provide and the pressures from urban development and agricultural expansion (Petersen et al., 2008; Zaldívar et al., 2008; Pérez-Ruzafa et al., 2011; Basset et al., 2013; Newton et al., 2014). In most cases, salt intrusion mediates lagoon salinity and drives a difference between the surface and bottom salinity (salinity stratification). In highly seasonal systems this effect leads to notable oxygen depletion and establishes hypoxia in the bottom boundary layer (Bruce et al., 2014; Cottingham et al., 2014; Huang et al., 2018). In Mediterranean climate regions, further concerns of hypersalinity through evaporation during the long dry summer and autumn months also exist (Potter et al., 2010). As a result, the hydrodynamics of coastal lagoons are frequently modified by the creation of artificial channels built to enhance hydrologic connectivity to the ocean and increase nutrient export (e.g. Breardley, 2005; Manda et al., 2014; Prestrelo and Monteiro-Neto, 2016), or they are indirectly modified by engineering projects associated with dredging and coastal management (e.g. Ghezzo et al., 2010; Sahu et al., 2014).
Changes in lagoon hydrology result from variability in river flows and meteorological and ocean conditions, alongside (sporadic) human interventions associated with coastal engineering developments. Globally, many studies have shown that coastal lagoon systems are vulnerable to climate change, including the factors from reduced flow and/or sea level rise (Nicholls and Hoozemans, 1996; Nicholls et al., 1999; Scavia et al., 2002; Chapman, 2012; Newton et al., 2014; Umgiesser et al., 2014). In particular, shallow coastal lagoons respond quickly to the ocean and catchment inputs, whilst their geomorphological characteristics (bathymetry and especially the configuration of their inlets with the open sea) affect their hydrodynamics, including circulation patterns, flushing time and water mixing (e.g. Smith, 1994; Spaulding, 1994; Koutitonsky, 2005; Umgiesser et al., 2014). This attribute has meant that these systems therefore amplify the salinity and temperature changes expected from climate change relative to the open sea, and they can serve as sentinel systems for global change studies (Ferrarin et al., 2014). On the other hand, anthropogenic activities introduce hydrological modifications associated with water resource management (e.g. Hollis, 1990; Kingsford et al., 2006; Gong et al., 2008) and engineering modifications (Ghezzo et al., 2010; García-Oliva et al., 2018). Among which, the opening of artificial channels in lagoon systems has been a popular measure to enhance ocean connectivity, but this activity has the effect to fundamentally alter the hydrology and aquatic communities (e.g. Lord, 1998; Manda et al., 2014; Prestrelo and Monteiro-Neto, 2016; García-Oliva et al., 2018). Changes in the connection of restricted lagoons with the ocean can exhibit a marked change in the salinity pattern or the extent of hypersaline conditions (Kjerfve, 1994; Gamito et al., 2005) and subsequently influence the ecosystem within these lagoons (Gamito, 2006; García-Oliva et al., 2018). These changes motivate us to assess lagoon hydrology in order to predict impacts and implement mitigation measures that also account for climate influence. However, tracking the long-term changes in hydrology remains an ongoing challenge since the signals of human disturbance are often confounded by variability of the climate system and lost in the dynamic estuarine conditions (Feyrer et al., 2015; Cloern et al., 2016). With the interacting effects of human interventions in conjunction with climate change, trends are not necessarily easy to predict. For example, the opening of an artificial channel and a drying climate can both introduce more ocean water into an estuary. On the other hand, the drying climate enhances water residence time, which may cancel out flushing benefits from the artificial channel. The combined effects are further complicated in large lagoon-type estuaries with complex morphology, where complicated patterns of water retention and stratification can develop (e.g. Ferrarin et al., 2013). From a lagoon management perspective, it is necessary to attribute the impacts from climate and human activity factors to better plan the necessary estuary and catchment management activities, including adaptation strategies associated with nutrient load targets and, in some cases, environmental water provision.
Here we explore these ideas through reconstruction of the long-term hydrologic evolution of a large estuarine lagoon in Western Australia: the Peel–Harvey Estuary (PHE). The PHE system has been subject to both a notable drying climate trend and substantial coastal modification in the form of the opening of a large artificial channel, coastal development and dredging. The artificial channel, termed the “Dawesville Cut” (hereafter referred to as “the Cut”), was built in 1994 with the purpose of increasing flushing and reducing nutrient concentrations. In parallel, the impact on inland water resources of recent climate trends has been particularly acute in the PHE catchment, which was acknowledged by the IPCC AR4 (IPCC Fourth Assessment Report), identifying this region as one amongst those that have experienced the greatest impact on divertible water resources in the world (Izrael et al., 2007; Bates et al., 2008). From the 1970s, rainfall has decreased by 16 % and streamflows have declined by more than 50 %, a trend which has appeared to accelerate since the 2000s (Silberstein et al., 2012). Whilst the nutrient and phytoplankton concentrations have been successfully reduced by the construction of the channel (Brearley, 2005), the long-term river flows have shown a clear trend of decreasing inputs to the estuary with concerns for the conditions of the tidal riverine portions of the system (Gillanders et al., 2011; Hallett et al., 2018). A series of water quality improvement plans (e.g. Environmental Protection Authority, 2008; Rogers et al., 2010) continue to be developed to promote estuary health; however, ongoing concerns about the current and future water quality and ecologic conditions of the system (Valesini et al., 2019) require knowledge of spatio-temporal changes in water retention, stratification and salinization to support adaptation efforts.
It is therefore the aim of this study to develop a methodology to disentangle drivers of change of the PHE system, over the period from 1970 to 2016, and outline the expected future trajectory of lagoon conditions. To this end, we employ a three-dimensional finite-volume hydrodynamic model for analysis of environmental drivers on estuarine hydrology by comparing current and counter-factual modelling scenarios to enable attribution of the drivers of change. To enable the long-term reconstruction of the model simulations for periods before the instrument record (and for future conditions), we drive the model with a hybrid set of weather and hydrological boundary condition data from observations and supporting models. The results of simulations are presented to analyse the sensitivity of water retention time, salinity and stratification within the lagoon to selected factors. By untangling the effect of the drying climate versus the Cut opening (through time and space), we explore the results through the lens of nutrient load reduction targets and biodiversity management implications. We anticipate that the approach adopted here can be useful to assist in the climate change adaptation efforts for other estuarine lagoons in mid-latitude regions.
PHE is a large shallow coastal estuary-lagoon system located approximately
75 km south of Perth in Western Australia (Fig. 1), which is listed under
the Ramsar convention for wetlands of international significance. The
estuary has a complex morphometry and comprises two shallow lagoons: one is
the Peel Inlet, a circular inlet to the north, and the other is the Harvey
estuary, an oblong lagoon attached to the Peel Inlet at its northeastern
edge, with a combined area of approximately 133 km
The estuary experiences a micro-tidal regime, with a range
Principal tidal constituents for Fremantle tide record from 1970 to 2017.
The TUFLOW-FV (BMT WBM, 2013) hydrodynamic model was adopted, using a flexible-mesh (finite-volume) approach to resolve the variations in water level, horizontal salinity distribution and vertical density stratification in response to tides, inflows and surface thermodynamics. The mesh consists of triangular and quadrilateral elements of different sizes that are suited to simulating areas of complex estuarine morphometry. To meet accuracy requirements, a fine grid resolution (mean mesh area
Surface momentum exchange and heat dynamics are solved internally within
TUFLOW-FV. In the current application, turbulent mixing of momentum and
scalars has been calculated using the Smagorinsky scheme in the horizontal
plane and through coupling with the General Ocean Turbulence Model (GOTM) for vertical mixing. The bottom shear stress was calculated using a roughness–length relationship assuming a rough-turbulent logarithmic velocity profile in the lowest model layer. The roughness length,
Zonal characteristics and roughness length setting (
Multiple concepts of hydrodynamic time parameters (flushing time, residence
time, water age, export time, etc.) have been used in coastal hydrology
research, and each of these parameters are different in their definition and
application (e.g. Monsen et al., 2002; Jouon et al., 2006; Sheldon and Alber, 2006). This study employed a few hydrodynamic time parameters to serve for different study purposes. The first time parameter was the “water age”, which was defined as the time the water had spent since entering the estuary through the boundaries (either the ocean or rivers), and it was computed in each computational cell as a conservative tracer subject to a constant increase with time (“ageing”) and mixing (Li et al., 2019) as
Secondly, the hydrodynamic time parameters of water flushing time (WFT) and
water renewal time (WRT) were used to investigate the changes in the mixing
efficiency (ME) due to the changes in the ocean connectivity by the opening
of the artificial channel and the changes in catchment inflows. WFT is a
bulk basin-wide water flushing timescale, defined as
After WFT and WRT are calculated, the ME can be obtained as the ratio
between WFT and WRT. ME ranges between 0 and 1. In the theoretical case of
ME
The model was calibrated with a structured hierarchical approach, similar to
those described in Muleta and Nicklow (2004) and Hipsey et al. (2020). This
approach first identified the key parameters of importance to the hydrology
in the current study based on literature review and prior expert knowledge.
In this stage, the key parameters were identified to be the bottom drag
coefficient, the bulk aerodynamic coefficients and the mixing scheme
options associated with the vertical turbulence model (in this case this is
parameterized through the GOTM plugin), as well as the bulk transfer coefficient
for latent heat flux. In the second stage, a matrix of simulations, each
with predetermined parameter vectors and model options, was assessed against the observed salinity and temperature data at six stations within the estuary (Fig. 1, at both surface and bottom levels), and the water elevation at the centre of the Peel Inlet in year 1998, which presented a year with median rainfall and catchment inflows. The capability of the model to reproduce the salinity stratification (magnitude of difference between the surface and bottom salinity) created by the interaction of ocean intrusion and freshwater runoff during the wet season was also considered in the model calibration. Based on the calibration results, a
The current study focused on the impact of reduced inflow, due to the drying
climate and the Cut, on the estuary hydrology. However, the perturbations of environmental factors such as air temperature, tidal elevation and benthic vegetation could also affect the local hydrology, and so their influence on the modelling results was explored. To evaluate the effects of these factors, the sensitivity of
Historical observations of nearby precipitation and the gauged data of the major Murray River inflow have shown a decreasing trend from 1970 to the present (Fig. 2), though variability from year to year is noticeable. The average annual precipitation has declined by 15 % when comparing the period 1994–2016 relative to 1970–1993, and this led to a dramatic decrease in annual inflow volumes, which is most notable in the past decade.
Historical record of
Years with inflow rates close to the 10-year moving average were selected
for hydrologic modelling simulations to explore in more detail the hydrologic changes occurring within these years (depicted relative to the trend in Fig. 2b). Due to the concern that the drying climate will continue into the 21st century (Silberstein et al., 2012; Smith and Power, 2014), we also undertook model simulations to investigate potential hydrologic changes under future conditions representative of 2040 and 2060 by considering reduced streamflow and rising sea levels. The runoff declines were based on the mean projection by Smith and Power (2014) that suggested the total runoff to the rivers and estuaries within the southwest Western Australia region will drop by about 0.96 % per year, corresponding to the projected reduction in precipitation of 0.27 % per year, on average. Sea level rise was also included in the future scenarios, estimated from the long-term (1897–2000) tide gauge observations at the Fremantle tide gauge station that shows a sea level trend of 1.50 mm yr
For each selected year, the modelling simulation started from 1 September of the previous year, giving a 4-month spin-up period, and the results from 1 January to the end of the selected year were used for analysis. The initial conditions of water temperature and salinity were interpolated from the field data when they were available (years 1985–2016), except the year 1970 when no field data were available and 1978 when field data at sites PH31 and PH58 were missing, so the same initial conditions as for 1985 were adopted. For the future scenarios, the same initial conditions as in 2016 were used.
For the modelling years after 1994, when the artificial channel was constructed, we also ran “no-Cut” counter-factual scenarios, which assumed the Dawesville Cut engineering intervention was not constructed, in order to separate the impact of the artificial channel on hydrology relative to the “with-Cut” scenarios (Table 3).
Summary of simulated scenarios and their annual precipitation, catchment inflow volumes, mean sea level and Cut-opening information.
Gauged flow rate data for the Murray River, Serpentine River and Harvey
River data were applied to the hydrodynamic model whenever they are available.
Gauged flow rate data for Murray River were available from 1970 to the present,
while for Serpentine River and Harvey River were available from 1982 to
present. For the missing periods in the gauged flows and the ungauged
drains, the output from the SOURCE (eWater®) catchment modelling platform (Kelsey et al., 2011; Welsh et al., 2013), operated by the Western Australia Department of Water and Environmental Regulation, was used to estimate flows by carefully comparing the measured and modelled flow data. Groundwater inputs were previously estimated to represent only
Various data sources were used to set up meteorological inputs due to the study period spanning back to 1970, when meteorological observations were not routinely available across the modelling domain at hourly frequencies. The first data source was the local Mandurah weather station located beside the natural channel of the estuary (Fig. 1). This dataset provided hourly records since 2001. The hourly fields over the period 1981–2000 were obtained from regional climate model simulations for Southwest Australia at a 5 km resolution (Andrys et al., 2015; Kala et al., 2015), which were carried out using the Weather Research and Forecasting (WRF) model, one of the most widely used regional climate models. Andrys et al. (2015) showed that the WRF model was able to adequately simulate the climate of southwestern Australia, and these simulations have also been used to assess the impacts of current and future climate on temperature and precipitation (Andrys et al., 2016, 2017) as well as climate indices relevant to viticulture for southwestern Australia (Firth et al., 2017). The WRF simulations of Andrys et al. (2015) have also been benchmarked against other regional climate model simulations across the Australian continent and have shown to perform well in simulating both temperature and precipitation (Di Virgilio et al., 2019) as well as heat-wave events (Hirsch et al., 2019). For the years before 1981, the weather conditions measured at the nearby Halls Head weather station (4.2 km away from the Mandurah station) were used. Although various sources of climate data were used, the wind regimes of these data sources showed a similar distribution in wind magnitudes and directions (Fig. 3). The winds in the Mandurah station record are relatively smaller when compared to the other two sources; however, this difference may be due to the natural variation in the climate and are not expected to change the main hydrological features in the lagoon.
Rose plot of wind condition in years of
A summary of all historical simulations and future scenarios is provided in Table 3. The total inflow into the estuary of the chosen simulation years shows a general decrease from past to future, except for the year 1978 when the total inflow rate was less than that in 1985 and 1990. This was due to an exceptionally low inflow rate within the Harvey River, produced from the catchment model output, which had an effect mostly on the Harvey Estuary. We still include this year to show the historical evolution during the past decades.
The impact of the changes in ocean connectivity due to the construction of the artificial channel on the estuary hydrology was first investigated by analysing the observed and modelled salinity and temperature at the centre of the two lagoons, as well as the surface elevation within the Peel Inlet, in the years of 1990 (representing a “pre-Cut” year) and 1998 (representing a “post-Cut” year). These 2 years were compared because they have similar annual precipitation and catchment inflow rates (Table 3) and tidal forcing characteristics in terms of the annual mean sea level and tidal range (Table 4, Fig. 4). Therefore, the comparison provided a valuable insight into the impacts of the artificial channel on the estuary environment.
Comparison of principal tidal constituents in years 1990 and 1998.
Annual variation in 1990 (left column,
The monitored and modelled salinity and temperature at the centres of the
two lagoons demonstrate the changes in the seasonal cycle in response to the
catchment inflows and the Cut opening, as well as the model's capability to capture
these changes (Fig. 5). In summer and early autumn the flow rates were
low, followed by high salinity and weak salinity stratification in the two
lagoons. In contrast, there were large inflows to the estuary in winter and
early spring. The peaks of the inflows occurred in winter (July–September), followed by a significant drop in the salinity in the estuary due to the freshwater flushing. However, differences in the salinity response to freshwater flushing can be observed between the pre-Cut year (1990, left column of Fig. 5a) and the post-Cut year (1998, right column of Fig. 5b). In 1990 when the estuary had limited connection without the opening of the Cut, the salinity stratification was small in the Harvey Estuary. The salinity dropped to below 5 PSU, indicating that the hydrology of Harvey Estuary was mainly dominated by the Harvey River. Whilst during 1998, with greater ocean connection due to the opening of the Cut, stronger salinity stratification was observed in the Harvey estuary, and the minimum salinity was lifted to over 10 PSU due to more seawater intrusion from the Cut. The water temperature also showed a clear seasonal signal, ranging from about 10
The opening of the Cut also affected the surface elevations of the estuary
(Fig. 6). The estuary surface elevation in 1998 had a much wider range of
Modelled vs. measured surface elevation in the centre of Peel Inlet in
Spatial distribution of modelled WRT in four selected scenarios.
The impacts of the changes in ocean connectivity and catchment flows on the
estuary mixing is then explored with results of WRT, WFT and ME in four
short theoretical scenarios (constructed to represent a matrix of open/close status
of the artificial channel and wet/dry catchment inflows).
Scenario 1: cut open, with inflow conditions in February 1998 (dry month with mean inflow rate of 5.60 m Scenario 2: cut open, with inflow conditions in August 1998 (wet month with mean inflow rate of 40.23 m Scenario 3: cut closed, with inflow conditions in February 1998 (dry month with mean inflow rate of 5.60 m Scenario 4: cut closed, with inflow conditions in August 1998 (wet month with mean inflow rate of 40.23 m
The results of fluxes at the inlets, fraction of lagoon water volume exchanged daily with the sea (FVE), and average WRT, WFT, and ME are summarized in Table 5. The results highlight the hydrodynamic variability with the changes in catchment runoff and ocean connectivity. The bulk WRT ranges from 154.80 d with small catchment flows in the dry seasons and with only the natural channel (scenario 3) to 13.92 d with higher catchment flows in wet seasons and with both the natural and artificial channels (scenario 2). The exchange flux through the inlets between the lagoon and ocean increased by
Model simulation results for the average water flux through the channels (Flux), fraction of lagoon volume exchanged daily with the ocean (FVE), WRT, WFT and ME in selected scenarios.
The spatial distributions of WRT corresponding to the changes in the inflow condition and the opening of the artificial channel are shown in Fig. 7. These maps clearly identify areas where waters are either well flushed or poorly flushed and show the Peel–Harvey system exhibiting a highly heterogeneous spatial distribution of the WRT. In all scenarios, WRT is mainly dependent on the relative distance from the inlets and on the presence of the channel. The areas connected to these channels are directly influenced by the sea and consequently their water renewal times are lower. In the wet season, the river runoff also plays a role in determining the water renewal heterogeneity. The Harvey Estuary (defined in Fig. 1) is shown to have the highest WRT than other parts of the lagoon, indicating the poor flushing in this area.
Mean water retention time,
Spatial distribution of season-averaged water age in 1990 (top panels) and 1998 (bottom panels).
Water retention is highly dynamic depending on seasonal flows, tidal
conditions and in different regions of the estuary. The evolution of water
age,
The spatial difference in water age is further illustrated in Fig. 9, which shows a plan view of the seasonally averaged water ages in years 1990 and 1998. The spatial distribution pattern of water age is similar to the one of WRT (Fig. 7), showing that the water age in the areas adjacent to the Cut entry point has been largely reduced by the Cut opening, yet the South Harvey Estuary and some parts of the East Peel Inlet still showed high water retention.
Similar to
Changes of mean salinity in PHE in simulated years and future scenarios. As for Fig. 8, the changes are categorized into four zones and four seasons.
Hypersalinity was often observed in the summer and autumn seasons in the
Peel Inlet from both the with-Cut and no-Cut scenarios. The Harvey
Estuary shows an increasing salinity with the drying climate in summer and
becomes hypersaline after 2011. High salinity with values over 50 PSU was
observed in autumn in South Harvey Estuary in the no-Cut scenarios, whilst the Cut opening reduced the hypersalinity risks in autumn in the Harvey Estuary. The relationships between the hypersalinity and the catchment inflows are further investigated with monitoring data at six regular monitoring sites (Fig. 11), which highlights that the maximum salinity recorded in autumn has increased with reduced inflows, especially in the period before the Cut opening. Opening of the Cut reduced the maximum salinity at the sites
near the Cut (sites PH2 and PH58) under an annual flow threshold of about
Maximum salinity recorded in March/April and the annual inflow in the hydrologic year (March to March) at six monitoring sites (see the site
locations in Fig. 1). The darker symbols indicate the years with accidental summer rainfall events, during which the catchment inflows were higher than
The magnitude of salinity stratification (salinity difference between the
bottom and surface water) in winter and spring has shown a declining trend
with the drying climate, while the variations were small in summer and autumn (Fig. 12). The opening of the Cut has enhanced the rate of ocean water intrusion, which creates stronger salt stratification during the wet season when it interacts with the freshwater inflows. The salt stratification was reduced to mostly
Changes of mean salinity difference between surface and bottom waters in PHE in the simulated years and future scenarios. As for Fig. 8, the changes are categorized into four zones and four seasons.
According to Kjerve and Magill (1989), coastal lagoons can be conveniently subdivided into choked, restricted and leaky systems based on the water exchange between the lagoon and the ocean. Umgiesser et al. (2014) compared 10 Mediterranean lagoons and classified the lagoon types based on the WRT and the fraction of lagoon water volume exchanged daily with the open sea. Based on the numbers in their comparison, the Peel–Harvey Estuary can be classified as a restricted type before the opening of the artificial channel and as a moderately leaky type after the opening of the artificial channel.
The reduction in the catchment runoff led to a smaller fraction of lagoon water volume exchanged daily with the open sea, but the magnitude is much smaller than that introduced by the opening of the artificial channel. However, the catchment runoff is shown to increase the mixing efficiency. During the dry season, the Harvey Estuary lagoon, especially the southern area, received relatively lower rates of ocean flushing that effectively lowered the ME. Higher values of ME can be found in wet seasons that enhanced the mixing of the lagoon. The increased ocean connectivity by the opening of the artificial channel, whilst enhancing exchange fluxes, is shown to lower the ME. This is similar to the findings in Umgiesser et al. (2014), where they found the exchanges with the open seas were low in the more restricted lagoon type such that the wind has more time to mix the basins well.
The impact of the artificial channel on the water transportation was further explored by the residual currents (Fig. 13), calculated as the mean currents over the period of selected scenarios described above. The results suggested that the opening of the Dawesville Cut had a strong impact on the residual currents. In the scenarios with the Cut open, strong residual currents around the Dawesville Cut are observed, and during the wet months the catchment runoff from the Serpentine River and Murray River flows to either the Mandurah channel or the Dawesville Cut. Whilst in the scenarios with the Cut closed, the surface residual currents in the Harvey Estuary were mostly moving northward, and during the wet months the catchment runoff from the Serpentine River and Murray River formed a shortcut to the Mandurah channel via the Peel Inlet; the West Peel Inlet received relatively less flushing. The results also indicated that the surface residual current speeds in the shallow water of the basins, such as the southeastern area of the Peel Inlet, were relatively lower than that in the deeper water, indicating less flushing in these areas that is coincident with the spatial distribution of WRT (Fig. 7) and water age (Fig. 9).
Plan views of residual currents in selected scenarios. The black–white colour gradient indicates the total current speed; the blue arrows indicate the current vectors.
The results of the long-term changes in the water age and salinity clearly
showed that the hydrology in PHE was profoundly changed, corresponding to the
reduced precipitation and catchment inflow as well as to the opening of the
artificial channel, although other factors such as changes in air temperature, sea level rise and benthic roughness also affected the
hydrology over much smaller scales. The results have highlighted magnitudes
of hydrologic changes introduced by the drying climate and the complexity
of the interacting impacts from climate and the artificial channel in time
and space. Firstly, the artificial channel successfully improved the estuary
flushing by reducing average water age by 20–110 d; in contrast the reduced precipitation and inflow had a gradual opposite effect on the water age, and during the wet season this has almost counteracted the reduction brought about by the channel. Secondly, the drying climate caused an increase in the salinity by 10–30 PSU; whilst the artificial channel increased the salinity during the wet season, it has reduced the likelihood of hypersalinity (
The climate factor had not been considered in previous reports evaluating or predicting the consequence of the Cut opening when it was originally designed (Lord, 1998; Manda et al., 2014; Prestrelo and Monteiro-Neto, 2016), as the focus was on the flushing benefit to reduce the accumulation of nutrients and algal biomass. The findings from this study suggest that climate change has been taking effect over the period when the Cut was implemented, and from the view point of particular metrics, it is now overtaking the effect of the Cut in its significance. The lessons from this case study highlights the need to look more broadly at environmental impacts when designing or operating large-scale engineering projects on coastal lagoons, due to the potential for long-term non-stationarity in contributing river flows.
Of relevance to management, the impacts also varied spatially in this large lagoon. The water age and salinity have showed distinct responses to the climate change and Cut opening with various connections with the rivers and ocean (Figs. 8 and 10). The southern Harvey Estuary, which has the least connection with the ocean through the natural channel, is most sensitive to climate change and the opening of the artificial channel. The bulk flushing time also showed significant reduction corresponding to the Cut opening; however, it was less sensitive to the drying climate. The results of water age distribution indicated that incomplete mixing had led to area-specific retention of water. In this case, the concept of bulk flushing time therefore needs to be used with caution in such a large choked-type lagoon, because it only gives an average estimation of water retention for the whole estuary and fails to consider the strong gradients in lagoon hydrodynamics. Understanding these patterns can be important to help understand local effects on lagoon ecology (e.g. crab larval recruitment) and processes related to nutrient deposition and retention within the sediment.
Longitudinal gradient in annual salinity variability in four selected scenarios (1970, 1998 without the Cut opening, 1998 with the Cut
opening, and a future scenario 2060 with assumptions of reduced flow and sea
level rise) moving upstream along the
Aside from changes in flushing and the mean salinity fields within the lagoon, the changes in the climate and ocean flushing also altered the hydrology in the tidal reaches of the rivers connecting to the PHE. The annual variability of salinity along the rivers (Fig. 14) indicated that there is an increasing risk of hypersalinity in the Serpentine River (connecting to the PHE from the north) and an upward movement of the salt wedge in the Murray River (the major inflow connecting PHE from the east). For example, the mean salinity at the Serpentine River mouth was about 20 PSU in 1970, then increased to 24 PSU in 1998 and is projected to increase to over 30 PSU in 2060. In the upstream areas of the Serpentine River, the mean salinity increased from about 15 PSU in 1970 to near 35 PSU in 2060. While there is less hypersalinity risk in the Murray River due to larger volumes of freshwater flushing, there is also a trend of increasing salinity along the river with the drying climate. The differences between the Cut-closed and Cut-open scenarios in year 1998 are much smaller than those caused by the drying climate, which indicates that the drying climate is the major cause of the salinity changes in the rivers.
In the current study, we assumed that the morphological change over the study period (except through the construction of the Dawesville Cut) were not significant to the overall hydrology. The morphology data we used for the model were the latest morphology dataset from the Western Australia Department of Water, obtained in 2016 (integrated DEM at 2 m resolution). There were no historical topography data available for each of the selected simulated years; therefore, the 2016 morphology was applied to the study period. The changes in the morphology during this long-term period could potentially affect the hydrology and the interpretation of the results.
The estuary morphology over the study period may have been modified by (1) changes to the net sedimentation of particles and (2) dredging activities related to estuary management such as marina and navigation channel developments. The net sedimentation rates in the Peel–Harvey Estuary had been investigated by a few early studies. Gabrielson and Lukatelich (1985) estimated a net sedimentation rate of about 0.4–1.5 mm yr
The development of canal estates and navigation channels would have further
changed the local morphology, but this is expected to only slightly modify the
estuary hydrology at the regional scale. For example, the Yunderup navigation channel, located at the east side of the Peel Inlet, is one of the more significant dredging projects in the Peel–Harvey Estuary in the past decades. The Yunderup channel has a length of
Another concern is that the catchment runoff will not only be affected by the effects of reduced rainfall, but also by land-use change, urban development and water diversion. The impacts from the catchment development
on the flow conditions have been extensively discussed in the Peel–Harvey
catchment modelling report (Kelsey et al., 2011), which showed different catchment developments had a combined effect on the flows. For example, land
clearing is expected to increase the streamflow, while local drainage
changes have led to water diversions and reductions of the inflows. The
catchment development was estimated to lead to a net increase in annual flow
of about
Changes in the mean nutrient concentration of
This study has investigated the hydrologic changes under projected future drying climate; however, the drying climate was idealized based on trends from a combination of climate models (Smith and Power, 2014), and the applied annual perturbations used to generate the future climate remains the subject of uncertainty. Our future climate projections for weather and flow change were based on the average trend reported from more detailed studies using an ensemble of climate models (Silberstein et al., 2012; Smith and Power, 2014). The Peel–Harvey region has experienced a widely reported decline in rainfall over the last several decades (CSIRO and BoM, 2007; IPCC, 2007; CSIRO, 2009; Hope and Ganter, 2010). The trend in rainfall decline is expected to continue, based on the climate projections from general circulation model (GCM) results (CSIRO, 2009; Smith and Power, 2014). Given that the nature of our research questions was to extrapolate the mean trend that we reported from the hindcast simulations, we focus the future scenarios on the changes of hydrology under the projected average reduction in the flow from the ensemble models (Smith and Power, 2014), with an assumed mean rate of sea level rise (Kuhn et al., 2011), to highlight the general trend and allow for prioritization of adaptation strategies such as environmental water allocation policies. This approach is somewhat simplistic in that it assumes no seasonal change in hydrologic trends, and there has been recent evidence that increasing summer floods are occurring and the winter peak flows are decreasing as a fraction of the annual total (McFarlane et al., 2020). As shown in Cloern et al. (2016), the hydrology of lagoons has been changing at a faster pace in the past decade from a combination of human activity and climate variability. The sea level of the ocean adjacent to the PHE has been rising at faster speeds in the past decades (Kuhn et al., 2011). The PHE catchment is also undergoing fast development due to the increasing population and agricultural expansion (Kelsey et al., 2011). Intensification of human activities, such as water consumption and diversion, will further affect the lagoon's hydrology and associated ecosystem, but how these factors will change in the future remains unclear. Therefore, our results related to the future prediction are simply to indicate the possible changes of hydrology under the projected drying climate. Continuous monitoring of the hydrology and water quality of the lagoon and its catchment must therefore be prioritized to closely observe further hydrologic change in order to provide prompt actions for management.
The Cut had an obvious and dramatic effect on increasing the export of
nutrients that would have otherwise been retained (Fig. 15). Since the Cut
opening in 1994, the main monitoring stations have shown that the total nitrogen (TN) concentration has been stable around 0.5 mg L
The hydrologic changes led not only to changes in the nutrient concentrations but also to the mean salinity, with potential ramifications for the ecological community. In particular, the phytoplankton biomass dropped dramatically since the Cut opening (Fig. 2) due to the improvement of ocean connectivity and flushing but also due to a less desirable salinity regime in summertime for the toxic cyanobacteria
This study has sought to analyse the hydrologic changes in the Peel–Harvey Estuary to a range of drivers, and it focused on the effects of the recent climate change trend on the hydrologic evolution in the Peel–Harvey Estuary, relative to the changes brought about by construction of the Dawesville Cut. Our results suggested that climate change in the past decades has a remarkable effect on the hydrology with the same magnitude as that caused by the opening of the artificial channel and also highlighted the complexity of their interactions. The artificial channel was effective in reducing the water retention time, especially in areas close to the channel, while the drying climate trend has acted to increase the water retention time. The artificial channel enhanced the ocean intrusion, which had a mutual effect with the drying climate to increase the estuary salinity during the wet season, but it had the opposite effect of reducing the hypersalinity during the dry season. The artificial channel increased the seawater fluxes and the salinity stratification, mostly in the Harvey Estuary, while the drying climate reduced the salinity stratification in the main body of the estuary. The changes in nutrient levels and habitat of pelagic and benthic communities related to hydrology were also discussed, which showed that the communities are sensitive to the hydrologic changes. Consideration of the projected drying trend is essential in designing management plans associated with planning for environmental water provision and setting water quality loading targets.
The datasets generated during the current study are available from the corresponding author on request.
The supplement related to this article is available online at:
All the authors contributed to the design of the study. PH carried the hydrology modelling work and prepared the first draft of the article. KH provided the catchment outputs of the inflow rates and nutrient concentrations. JK and JA provided the WRF weather data. MRH was the project leader and provided technical and financial support. JK and MRH helped to interpret the model data and write the article.
The authors declare that they have no conflict of interest.
The authors would like to thank Fiona Valesini, Christopher Hallett and three anonymous reviewers for their valuable and constructive comments.
This research has been supported by the Australian Research Council (grant no. LP150100451).
This paper was edited by Hubert H. G. Savenije and reviewed by three anonymous referees.