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Modelling water and salt balances in a deep,groundwater-throughflow lake—Lake Purrumbete,southeastern AustraliaYohannes Yihdegoab, John Webba & Paul Leahyc

a Environmental Geoscience, La Trobe University, Melbourne, Australiab Snowy Mountains Engineering Corporation (SMEC), Sydney, Australiac EPA Victoria, Applied Sciences Group, Macleod, Victoria AustraliaAccepted author version posted online: 16 Oct 2014.

To cite this article: Yohannes Yihdego, John Webb & Paul Leahy (2014): Modelling water and salt balances in adeep, groundwater-throughflow lake—Lake Purrumbete, southeastern Australia, Hydrological Sciences Journal, DOI:10.1080/02626667.2014.975132

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Publisher: Taylor & Francis & IAHS Press

Journal: Hydrological Sciences Journal

DOI: 10.1080/02626667.2014.975132

Modelling water and salt balances in a deep, groundwater-throughflow lake - Lake Purrumbete, southeastern Australia  Yohannes Yihdego1,2*, John Webb1, Paul Leahy3

1Environmental Geoscience, La Trobe University, Melbourne, Victoria 3086, Australia 2Snowy Mountains Engineering Corporation (SMEC), Sydney, New South Wales 2060, Australia 3EPA Victoria, Applied Sciences Group, Macleod, Victoria 3085, Australia *Corresponding author. E-mail: yohannesyihdego@gmail.com Tel: +61-04-13539214 Fax: +61 3 9479 1272

Abstract

Lake Purrumbete is a deep volcanic freshwater lake, cylindrical in shape, located within an extensive basalt

plain in southeastern Australia. A modified difference water budget method for lakes, that estimates net

groundwater flux through the difference between the level of the lake and the watertable, along with the specific

yield and area of the aquifer, successfully modelled the lake level fluctuations. The major influences are

evaporation and direct precipitation; however groundwater fluxes are significant (~17%). The salt balance

modelling shows that the salinity in Lake Purrumbete is largely controlled by groundwater and surface water

fluxes. Lake Purrumbete has a greater interaction with the surrounding groundwater system than other nearby

volcanic lakes due to its significant depth and the presence of highly permeable basalts along the upgradient

shoreline. Its low salinity reflects the substantial salt export in the groundwater and surface outflow, as well as

the lake’s relatively small evaporation due to the low surface area/volume ratio. The fluctuations in lake level

and salinity for Lake Purrumbete are much less than for other lakes in the region, even during the 1997-2010

drought, due partly to the large, relatively constant groundwater flux. Low salinity, deep water and estimated

depth of stratification from this study which leads to a distinct limnology, implies that the creatures need to

adapt to a different habitat to survive in vicinity of Lake Purrumbete. Thus Lake Purrumbete will be an

important aquatic ecosystem refugium if climate change causes frequent seasonal drying of other lakes in the

plain and it is important to develop management strategies to maintain its water resources and quality.

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Key words water balance; salt balance; surface-groundwater interaction; deep/fresh lake; Australia

 

Introduction

Conceptual models of groundwater flow into lakes, based primarily on numerical simulations and near-shore

measurements, have been carried out mostly for lakes with high surface area/volume ratios, low gradient beds

and sandy shorelines (e.g. Barwell and Lee 1981, Cherkauer and Zager 1989, Genereux and Bandopadhyay

2001, Lee and Barry 1980, Munter and Anderson 1981, Nakayama and Watanabe 2008, Sacks et al. 1992,

Townley and Davidson 1988, Winter 1976, Winter 1978, Winter 1981, Yao et al. 2009). These models predict

that groundwater inflow and outflow is usually only a small proportion of the overall lake water budget, and that

most occurs through the shallow near shore zone, due mainly to the relatively high hydraulic conductivity of the

sandier littoral sediments compared to the muddier sediments covering the majority of the lake bed. However,

deeper lakes with low surface area/volume ratios may have higher permeability, steeply inclined, rocky margins,

resulting in a greater contribution of groundwater flow to the water budget, but there have been few studies (e.g.

Harvey et al. 2000, Kirillin et al. 2013) that have attempted to quantify this, because of the difficulties in

obtaining the data necessary to compute a groundwater flux.

Furthermore, water balance modelling of lakes generally estimates the groundwater flux either from the

difference between recorded lake levels and those simulated using the precipitation–evaporation balance at the

surface (difference method; e.g. Kirillin et al. 2013), or from Darcy’s law, using near-shore hydraulic gradient

and hydraulic conductivity (flow-net method; e.g. LaBaugh et al. 1997). If groundwater fluxes are small, or the

hydraulic gradient and hydraulic conductivity cannot be accurately measured, these methods can give poor

results (Lee 1996; Winter 1978; Yihdego and Becht 2013).

Generalised water balance modelling of lakes on the volcanic plain, including the integration of satellite

imagery with monitoring data, found that for over 30 lakes in the region of western Victoria, southeastern

Australia, climatic factors (precipitation and evaporation) are the dominant influences on lake level, area and

salinity, with concomitant impacts on groundwater-lake interactions (Dewi et al. 2009, Kirono et al. 2012, Jones

et al. 2001, Tibby and Tiller 2007, Tweed et al. 2009). The groundwater component of the lake water balances

(in those studies which considered groundwater) was quantified by the difference method, but Barton et al.

(2007) and Cox et al. (2007) used stable isotopes and Cl/Br ratios to determine groundwater input for 28 lakes,

and found that that eight of them were groundwater through-flow lakes of these, Lake Purrumbete is unusual

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because of its depth (~45 m) and low salinity (median ~460 mg/L Total Dissolved Solids); it has a distinctive

limnology and is a popular trout fishing spot and a wildlife sanctuary (Timms 1976).

The present study constructs a detailed hydrogeological model for Lake Purrumbete, in order to quantify the

groundwater component of the water budget in a lake of this type. In this study, previously developed model

(Yihdego and Webb 2012) has been applied to a lake which has different characteristics from the previous

applications. The modified difference water budget method (Yihdego and Webb 2012) improves the

groundwater flux estimate by using the difference between the level of the lake and groundwater, along with the

specific yield and area of the aquifer to overcome the problems inherent in previous approaches to calculating

groundwater flux to lakes,

The results can be used to develop management strategies for Lake Purrumbete, which is the deepest natural

freshwater lake in western Victoria and therefore important for water sustainability and aquatic ecosystem

preservation, as other lakes in this area are projected to be seasonally dry by the end of the 21st century (Leahy

et al. 2010). The paper will assist the development of better management strategies for the lake, in line with the

need to consider the full impact of climate and anthropogenic change on the regional water resources and

ecosystem health.

Regional setting

Lake Purrumbete is located in western Victoria, southeastern Australia (Figure 1). This region has a

temperate climate with hot summers and mild winters. Evaporation exceeds precipitation except from June to

September. Average annual rainfall at Cobden, ~13 km southwest of Lake Purrumbete, is 839 mm, and average

annual evaporation exceeds 1000 mm.

Lake Purrumbete is located near the southeastern margin of an extensive, gently undulating plain occupying

much of western Victoria, which is composed of Plio-Pleistocene basaltic lava flows (Newer Volcanics; Figure

2 and Figure 3) erupted from ~400 scattered scoria cones and lava shields (Joyce et al. 2003). The basalts are

10-130 m thick and form a major unconfined to semi-confined regional aquifer system. The older flows (1-6

Ma) are generally covered by thick, clay-rich soils up to 10 m thick, and the original vegetation covering them

has been extensively cleared for agriculture. The more recent basalts (300,000 to 8,000 years old) are dominated

by rocky outcrop with little soil development ("stony rises"), and are often covered with native eucalypt

woodland; they frequently form local aquifers within extensive lava aprons spreading radially around individual

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volcanoes. Concentrated along the southern edge of the volcanic plains are ~40 maars, volcanic craters each

surrounded by a ring of volcanic ash deposits, and formed by explosive eruptions that probably occurred in the

Late Pleistocene (Paine et al. 2004).

The basalts overlie the Upper Miocene to Pliocene Loxton Formation (formerly called Dorodong Sand and

Moorabool Viaduct Formation in this area), which comprises marine and fluvial quartz-rich sands and clayey-

sands usually <10 m thick (Holdgate and Gallagher 2003, Paine et al. 2004). Beneath the Loxton Sand are the

Upper Eocene to Oligocene Port Campbell Limestone and underlying Gellibrand Marl (Figure 3). The limestone

is composed of dominantly sand-sized bioclastic grains with a high intergranular porosity (~40%) and

permeability (150-400 m2/day); it forms a major regional aquifer underlain by the aquiclude of the Gellibrand

Marl (Shugg 1998).

The surface of the volcanic plain has a poorly developed surface drainage network, with over 1500 widely

dispersed wetlands and lakes, many of which are connected to the water table and drain internally. In general

they are shallow (0.5–6 m deep) and small (<1 km2), and range from freshwater (<0.5 mS/cm) to saline brines

(>400 mS/cm). The largest is Lake Corangamite (241 km2), an extensive complex of shallow salt lakes, and the

deepest are three crater lakes, Gnotuk, Purrumbete and Bullenmerri (15–66 m deep; De Deckker and Williams

1988).

Lake Purrumbete

Lake Purrumbete is a circular maar lake 2.5 km in diameter and 5.52 km2 in area, with a cylindrical shape

and a maximum depth of 45.7 m; it is one of the deepest lakes on the Australian mainland. It was formed by an

explosive single-point eruption (Ollier 1968, Rosengren 1994), and is surrounded by a low ring of volcanic ash,

highest on the eastern side (Figure 3). The eruption probably occurred in the Late Pleistocene, based on its

relatively young appearance and dates of ~20 ka for Tower Hill maar to the west (Joyce et al. 2003). Natural and

artificial drainage channels flow into the northern and eastern sides of Lake Purrumbete, giving it a catchment

area of ~ 30 km2. The other maar lakes in the region have relatively high crater rims and therefore smaller

catchments and greater wind protection (Timms 1976).

To the southeast of Lake Purrumbete is the volcanic dome of Mount Porndon (Figure 2(a)), surrounded by

‘stony rise’ basalt lava flows dated as 0.26-0.3 Ma (Edwards et al. 2006), with thin rocky soils and partly

covered by native woodland. These lava flows have sharp boundaries where they often rise several meters. They

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run out on to the floor of Lake Corangamite to the north, and extend around the eastern and southern sides of

Lake Purrumbete, where they are buried by volcanic ash around the crater rim (Ollier 1968, Rosengren 1994).

The ‘stony rise’ lavas overlie older flows; the total thickness of the basalts is generally 20–25 m thick in this

area (Edwards et al. 1996). Beneath them is a marine clayey sand usually < 10 m thick (Pliocene Loxton

Formation). There is generally a good hydraulic connection between basalts and underlying sand (Cox et al.

2007), based on identical bore hydrographs in nested peizometers screened across the two aquifers and similar

hydraulic conductivity values (Dahlhaus et al. 2002).

Beneath the Loxton Formation is the Upper Miocene Port Campbell Limestone, which in turn overlies the

Gellibrand Marl, a regional aquiclude. A cross-section constructed using available geological and geophysical

(radiometric, magnetic and gravity) data indicates that Lake Purrumbete intersects the Port Campbell Limestone

(Figure 3). However, the degree of hydraulic connection with this unit is probably small, because the average

groundwater salinity in the limestone (1000 mg/L; SKM 2007) is substantially greater than the lake salinity

(median ~460 mg/L). It is therefore assumed that only groundwater in the basalts and the Loxton Formation

interacts with the lake.

Potentiometric surface contours (Figure 2(a)) indicate that groundwater flows into Lake Purrumbete from the

stony rises to the southeast and away from the lake to the northeast (towards Lake Corangamite) and southwest

(towards Curdies River).

Monthly lake level and salinity data are available for Lake Purrumbete from 1978 and 1982 respectively

(lake level measurements are lacking from 1994 to 1997); this represents one of the longest records of

continuous water-quality monitoring in Victoria. A graph of cumulative deviation of rainfall from the mean for

Cobden shows a strong correlation with lake levels; peaks in the lake level generally correspond to periods of

above average rainfall. In years of below average rainfall, the lake level declines, as occurred in 1983 and during

the recent drought from 1997 to 2010. There is considerable variability in the lake salinity records; the

anomalously high values are most likely errors in measurement. Tibby and Tiller (2007) demonstrated that the

salinity of Lake Purrumbete increased in response to reduced effective moisture.

The lake is isothermal at 11-120C during winter, but is thermally stratified from October until June, when

warming by the sun overcomes wind-driven mixing (Timms 1976, Tibby and Tiller 2007). The thermocline

reaches a maximum depth of 18-22 m in March-April, and EPA Victoria (2008) recorded a difference in salinity

between the surface layer (0.1 m depth) and bottom water (40 m depth) of up to 500 mg/L during the summer

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thermal stratification. Compared to nearby maar lakes, Lake Purrumbete is warmer and the thermal stratification

is deeper, reflecting its relatively large surface area.

High lake levels formerly caused flooding of low swampy areas to the south and southeast of Lake

Purrumbete; to prevent this, an outlet ~3 m wide was dug through a low rock ridge on the southwestern shore,

flowing into Curdies River (Figure 2), and the swamps were artificially drained by channels running into Lake

Purrumbete. The outlet has been deepened several times, from an original level of 134.99 m Australian Height

Datum (AHD) to 134.79 m AHD in 1981 and 134.6 m AHD in 1992.

Water Balance Modelling

Within spreadsheet water balance models, the unknown components, usually the groundwater flux, can be

estimated by comparing recorded and simulated lake levels (the difference approach; e.g. Tweed et al. 2009,

Kirillin et al. 2013), but if groundwater fluxes are small, this method can be very inaccurate (Anderson and

Cheng 1993, Anderson and Munter 1981, Hunt and Kobelski 1996, Lee 1996, Winter 1978, Yang et al. 2014).

Determination of groundwater fluxes can also be made from Darcy’s law, using the hydraulic gradient from a

network of nearby wells and estimates of hydraulic conductivity (the flow net approach; LaBaugh et al. 1997).

However, the flow net approach is limited by the accuracy of the values for the hydraulic conductivity of the

sediments and the hydraulic gradient; the latter can be difficult to measure if it is very low (Cook et al. 2008).

The difference method (Becht and Harper 2002) estimates the net groundwater flux by using the difference

between the level of the lake and groundwater, along with the specific yield of the aquifer (from pumping test

results from wells surrounding the lakes) and the area of the aquifer (obtained from hydrogeological maps). The

present study employs the modified difference water budget method (Yihdego and Webb 2012), that improves

the groundwater flux estimate using only Darcy’s law by linking the lake to the groundwater and incorporating

the seasonal variation of the groundwater fluxes, thereby including seasonal changes to the hydrogeological and

meteorological conditions within the lake catchment.

The water balance components in this model are lake surface precipitation/evaporation, surface

inflow/outflow, abstraction and groundwater fluxes. Each of these components will now be discussed in detail.

Lake surface precipitation/evaporation

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The model used monthly precipitation data from the weather station at Cobden, the nearest weather station with

a long-term, continuous record extending to the present, and monthly pan evaporation data measured at

Hamilton Research Centre, 120 km to the northwest of Lake Purrumbete, from 1965 to 2001. The pan

evaporation data from 2001 to the present was added from measurements at White Swan Reservoir near

Ballarat, 100 km north-northeast of Lake Purrumbete; data at Hamilton Research Centre and White Swan

Reservoir are very strongly correlated. A local pan coefficient was used to adjust the evaporation data. An

optimal calibration coefficient of 0.95 was used to adjust the seasonal pan evaporation data from Hamilton

Research Centre for the best fit of the model. This takes into account the spatial variation in position, elevation

and storage effect between Hamilton Research Centre and Lake Purrumbete. Given that the current study is

dealing with a deep lake, the varying seasonal heat capacity in the lake should be modelled as it affects seasonal

lake evaporation, and there is a range of literature dealing with this issue (e.g. Hoy and Stephens 1979,

McJannet et al. 2008, Morton 1986, Wang et al. 2001).

Surface water inflow/outflow

Lake Purrumbete is located in the headwaters of Curdies River (Figure 2(a)). The monthly flow in Curdies

River was modelled from the gauging station data using the tanh cumulative surplus rainfall approach (modified

after Boughton 1966, Grayson et al. 1996, Yihdego and Webb 2013), which predicts runoff in the Curdies River

for any given monthly precipitation/evaporation (Figure 4). Using area scaling, this can be converted to a

surface inflow to Lake Purrumbete; the catchment areas for Lake Purrumbete and Curdies River at Timboon are

30 km2 and 824 km2 respectively. There could be errors in this estimation, because the swamps/wetlands that

were artificially drained into Lake Purrumbete might have a different runoff ratio to the rest of the catchment

(Nash and Sutcliffe 1970). Moreover, with Curdies River being downstream of the lake, there might be a large

groundwater component to its runoff when compared to the upland areas around the lake.

Surface water outflow was modelled using the level of the outflow channel, which varied from 134.99 m

AHD before 1981, to 134.79 m AHD from 1982 to 1992 and 134.6 m AHD after 1992.

Abstraction

The maximum licensed abstraction from Lake Purrumbete for irrigation is 510 x 103 m3/year; some of the

irrigation licenses do not allow pumping if the lake level is below 135.01 m AHD on 1st November (SRW

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2009). Stock and domestic abstraction from the lake does not require a licence, and the amount of water used for

these purposes is unknown; an average value of 2 x 103 m3/yr for each user is often assumed by water

management authorities in Victoria.

For the modelling, a constant abstraction of 532 x 103m3/yr was used if the water level was above 135.01 m

AHD, to allow for irrigation, stock and domestic use.

Groundwater inflow/outflow

For relatively small lakes like Lake Purrumbete, groundwater fluxes are inherently more important in the

water budget because of the higher ratio of lake perimeter (the site of most groundwater inflow/outflow) to

surface area (Fellows and Brezonik 1980). Furthermore, the influence of groundwater inflows becomes

increasingly important in relatively low rainfall areas like western Victoria (Jankowski and Jacobson 1989).

The calculation of the groundwater flux for the Lake Purrumbete is therefore a very important part of the

water budget. In the present model, the initial estimations of groundwater inflow/outflow were carried out using

Darcy’s Law.

Groundwater discharge/recharge to/from Lake Purrumbete, which has a cylindrical shape, occurs both

through the lake bed and the steep rocky wall around the sides of lake. A value of 9 x 10-4 m/day (typical of

clays) was chosen for the permeability of the clayey lake floor sediments; this low permeability means that,

even though the area of the lake floor is greater than that of the lake margin, the groundwater flux through the

lake floor is very small and can be neglected.

Lake Purrumbete is hydraulically connected through the side of the lake to the basalt lavas and underlying

sand of the Loxton Formation; the two formations are grouped as a single hydrogeological unit for the purposes

of this model (as discussed previously). Any interaction with the underlying Port Campbell Limestone is likely

to be small (Figure 3). Permeability in the basalt is very variable; an average hydraulic conductivity value of 8

m/day was used based on pumping tests in the area (Cox et al. 2007), and this estimate is in agreement with the

average value for the Loxton Formation in this region (4.3 m/day; Coram 1996).

The upflow and downflow horizontal hydraulic gradients, calculated from the potentiometric surface map,

are on average 0.002.

Groundwater enters the lake along its southeast perimeter (~1.8 km) and leaves from southwestern-

northeastern perimeter (~6.2 km). The cross-sectional area was calculated by multiplying the lake perimeter by

the estimated thickness of the basalt and Loxton Formation aquifers intersected by the lake (~ 40 m; Figure 3).

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However, due to aquifer heterogeneity and a lack of regularly monitored groundwater observation bores in

the immediate surroundings of Lake Purrumbete, these calculations were difficult and probably contain large

errors, mainly due to uncertainty in values of hydraulic gradient and conductivity, meaning that the groundwater

flow system is not well defined. This is important because lake levels rise and decline more quickly than

groundwater levels due to direct input from precipitation and open water evaporation, so that hydraulic gradients

from groundwater to lake will change seasonally, causing variations in the groundwater flux to the lake. For

Lake Purrumbete, groundwater hydrograph and lake level data indicate the hydraulic gradients varied in value

but not direction throughout the study period.

As a result, the initial Darcy’s Law estimations were supplemented using a bucket model, in which the net

groundwater flux between the aquifer and the lake can also be calculated from the change in groundwater

storage in the aquifer, using the storage coefficient (specific yield) of the aquifer, the area of aquifer

hydraulically connected to the lake, i.e. the area of the aquifer in which the groundwater level fluctuates due to

interaction with the lake, and the change in the groundwater level within the aquifer due to groundwater flow

to/from the lake (Yihdego and Webb 2012).

The total groundwater inflow/outflow is then calculated by combining the initial estimate of groundwater

inflow/outflow using Darcy’s Law with the net seasonal groundwater flux from the bucket model (Yihdego and

Webb 2012).

The change in lake storage was calculated for any month from the precipitation, evaporation, surface inflow,

abstraction, groundwater inflow and outflow during that month, and surface outflow during the preceding

month. So that all the water budget terms are expressed in volumetric units, the precipitation and evaporation are

multiplied by the average lake surface area during that month, calculated from the area-volume relationship for

Lake Purrumbete.

The lake volume is updated for every month with the volume from the previous month and the lake volume

change, and compared to the actual lake volume for that month, calculated from the measured water depth using

the depth-area-volume relationship for Lake Purrumbete from the available bathymetry data (Timms 1973). The

relationship between lake area and depth was also established by measuring the lake area on 12 Landsat images

taken between 1972 and 2004 and plotting this against the measured lake depth in the month when the images

were taken. The two relationships are closely comparable.

Model calibration and sensitivity analysis

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The water balance model was calibrated (for the period from 1978 to 2006), to minimize the sum of the

squared differences between observed and simulated lake levels, by optimizing (using solver algorithm) the

parameters used to estimate the net groundwater inflow/outflow (specific yield, area of aquifer hydraulically

connected to the lake), along with the local pan coefficient, without changing the static and dynamic inputs. The

modified difference method developed by Yihdego and Webb (2012), used in this study to a different lake than

the previous applications, provides flexibility to fit the data in terms of the number of parameters that can be

optimized.

The calculated value of the change in the level of the potentiometric surface is comparable with observed

borehole data, and the optimized specific yield (0.13) is similar to previous specific yield estimates for the basalt

aquifer of 0.01-0.2 (Cox et al. 2007, Dahlhaus et al. 2002, GHD 2004). The optimised aquifer area in hydraulic

contact with Lake Purrumbete (50 km2) is greater than the surface catchment area of the lake (30 km2),

indicating that the lake interacts with the groundwater beyond the surrounding drainage divide, probably due to

its significant depth (45 m).

A sensitivity analysis performed using a set of model runs showed that the model is more sensitive to the

input variables of evaporation and precipitation than surface inflow. Thus although could be errors in surface

inflow estimation due to the way it was calculated (described previously), these will not affect the water budget

modelling significantly.

Adjusting the surface outflow showed that the best possible fit was attained when the outlet level was 22%

above the actual outflow level. This is probably because the constriction of the outflow, which is only ~3 m

wide, means that the rate of surface outflow is low, and the lake height can rise well above the base of the outlet

level during times of high rates of inflow.

Water budget results

The water balance (Table 1) shows that the major overall influence on lake levels is evaporation (average

~38% of the total water budget), followed by direct precipitation (average ~25%). Surface water and

groundwater fluxes are 19% and 16% respectively. Abstraction accounts for ~ 1.4%.

In general, the lake levels predicted by the water budget show good agreement with the measured lake level

data, with an r2 value of 0.84 (Figure 6); the sum of the squared differences did not exceed 0.09 m2 (Figure 7)

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However, after 2000 the measured levels at Lake Purrumbete are as much as 70 cm lower than the modelled

levels (Figure 5). This is not due to a temporary lowering of the outlet level, which is excavated in rock, and

there is no evidence for any increase in groundwater outflow, which is in any case quite small. It is likely that

the lower levels are due to ungauged abstraction from the lake during the severe drought that lasted from 1997

to 2010.

During wetter months (Table 1), the model shows that the lake levels are primarily driven by the volumes of

direct rainfall and surface water inflow to the lake, which can individually account for up to 63% and 51% of the

lake budget respectively.

Salt Balance Modelling

The change in salt storage in the lake for any month was calculated from the mass of salt in precipitation,

evaporation (considered negligible), surface inflow/outflow, groundwater inflow/outflow, and abstraction. The

concentration (salinity) of groundwater and surface outflow in any month is given by the salinity of the lake in

the previous month.

The salt budget was calculated monthly on a mass basis from October 1982, when relatively continuous

salinity measurements began, using the volumes already calculated from the water budget, together with

available salinity data.

The salinity (TDS) of precipitation at Hamilton, ~125 km northwest of Lake Purrumbete, was 41.6 mg/L in

2004 (Bormann 2004); it is likely to be representative of rainfall at Lake Purrumbete over the study period due

to its comparable distance from the coast, and has been used in these calculations.

The salinity of groundwater inflow was estimated from the Electrical Conductivity (EC) of groundwater in

boreholes screened in the basalt aquifer up-gradient of the lake (average 500 µS/cm), converted to TDS using

the relationship: TDS = EC x 0.61, based on 230 groundwater analyses in the study area (Bennetts 2005).

The salinity of surface inflow has not been measured. Based on measured salinities in nearby rivers, it was

assumed to range from 250 to 750 µS/cm, varying inversely with surface inflow volume.

The initial salt budget modelling could not reproduce the salinity fluctuations (Figure 8(a)) until the lake was

allowed to stratify at a depth of ~ 0.6 m (Figure 8(b)). Lake Purrumbete stratifies thermally in summer, when

there may be a difference in salinity between the surface layer (0.1 m) and deeper water (40 m) of up to 500

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mg/L (EPA Victoria 2008). This corresponds with the peaks in the modelled salinity, which occur in summer

(Figure 8(b)). In winter there is minimal, if any, salinity stratification.

Salt budget results

The salt balance model indicates that the salinity in Lake Purrumbete is largely controlled by groundwater

and surface water fluxes (average 43% and 41% respectively). Direct precipitation contributes 12% on average,

and abstraction ~ 4%.

Errors in the model are probably partly due to errors carried over from the water budget, but the salt budget

sensitivity analysis shows that the model is particularly sensitive to changes in the salt concentration of

groundwater outflow and surface inflow. Furthermore, the relatively high observed TDS values (600 to 750

mg/L) in 1986, 1989, and 1990 (Figure 9) may be erroneous.

Tibby and Tiller (2007) analysed the relationship between Lake Purrumbete water quality and climate for the

period 1984-2000, and demonstrated that the salinity increased in response to reduced effective moisture,

confirming the results of the present modelling.

Discussion

Lake Purrumbete was classified by Barton et al. (2007) as a marginally surface-water-dominated,

groundwater through-flow lake, based on hydrochemical and isotopic analysis (deuterium excess and

bicarbonate/chloride ratio). However, the water budget from the current study shows that direct rainfall and

surface water inflow (together) substantially exceed groundwater inflow (~34% and ~16% of the water budget

respectively). The discrepancy may be due to hydrochemical and isotopic differences between rainfall and

surface water inflow, which are lumped together in Barton et al.’s (2007) study, and the impact of the thermal

stratification of Lake Purrumbete over summer, which means that the evaporated surface layer will have a

different stable isotope and ionic composition to the bulk of the lake water.

The modelling results show that although the fluctuations in level for Lake Purrumbete are primarily driven

by surface water inflows and direct rainfall during wetter months and evaporation during drier periods, these

variables are damped to 83% of the water budget because of the substantial lake–groundwater exchange. The

other lakes scattered across the volcanic plain in western Victoria have a much smaller groundwater flux,

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typically <5% of the water budget (Coram 1996, Dewi et al. 2009, Jones et al. 2001, Kirono et al. 2012, Tweed

et al. 2009).

Lake Purrumbete has a greater interaction with the surrounding groundwater system for two reasons: firstly

its significant depth, with high permeability, steeply inclined, rocky margins, and secondly the presence of

‘stony rise’ basalts along the upgradient shoreline. The thin soils on these basalt flows result in rapid recharge,

so that the least saline groundwaters on the basalt plain in western Victoria are present in ‘stony rise’ lavas

(Raiber et al. 2009). Furthermore, these relatively young basalts are highly permeable, and often have springs

around their margins (Joyce et al. 2003).

Lake Purrumbete is also unusual among the lakes on the volcanic plain in western Victoria because of its

low salinity. Lakes with substantial groundwater and surface water flux like Lake Purrumbete (~35% of the total

water budget) are relatively fresh because there is significant salt export in the outflow, and because the amount

of time the lake water is subject to evaporation is limited. In addition, Lake Purrumbete’s relatively low surface

area/volume ratio means that the effect of evaporation is less pronounced than for shallower lakes (although it

still contributes ~38% of the total water budget). The more saline lakes on the basalt plain are shallow, with no

surface outflow and limited groundwater outflow, so that most of the salt input in precipitation and

surface/groundwater inflow is not exported, and remains in the lake to be concentrated by evaporation.

Nevertheless, the water and salt budget for Lake Purrumbete demonstrates that the major cause of salinity here,

as for the other lakes in western Victoria, is evaporation. Solutes are derived from rainfall, both directly and

indirectly through groundwater and surface water input (Bennetts et al. 2006).

The fluctuations in lake level and surface area for Lake Purrumbete are much less than those for most other

lakes in the region, both on an absolute and relative basis. During the 1997-2010 drought, many of the other

lakes decreased in surface area by between 60% and 36%, and there was a 25% increase in the number of lakes

with evaporite deposits (Tweed et al. 2009). Lake Purrumbete showed only a minor reduction in level and

increase in salinity during the drought; this was due both to its low surface area/volume ratio and large

groundwater flux. Because the groundwater flow is relatively constant compared to the frequently episodic

nature of rainfall events and surface water flows in this area (typical of southeastern Australia; McMahon et al.

1987), it smooths out lake level fluctuations due to the latter factors. As a result, lakes with a higher percentage

of groundwater inflow/outflow like Lake Purrumbete are less affected by short-term climate fluctuations.

The results can be used to develop management strategies for Lake Purrumbete in order to maintain surface

water resources and quality under the likely decrease in effective precipitation predicted by climate change

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scenarios for this region. Because Lake Purrumbete was relatively unaffected by the 1997-2010 drought, when

many other lakes in this area became more saline and/or dried out, it served as an important refugium for aquatic

ecosystem preservation, and this will also be the case if climate change causes seasonal drying of lakes to be a

frequent occurrence by the end of the 21st century (Leahy et al. 2010). Nevertheless, Lake Purrumbete is

susceptible to fluctuations in nutrient concentrations due to the impact of surface warming on algal dynamics

(Tibby and Tiller 2007), and its limnology is more sensitive to climatic variability than the catchment vegetation

(Tibby et al. 2012). Similarly, groundwater-levels in the region are sensitive to climatic variability (Dahlhaus et

al. 2010, Yihdego and Webb 2011).

Lake Purrumbete is popular for trout fishing and also needs to be managed to maintain its amenity for this

recreation. If the lake level drops permanently below the outlet, as predicted for continuing dry conditions like

the recent drought, this will not significantly affect the lake salinity or volume, so it will still be suitable as a

trout habitat. However, the drop in level will require that the ramps allowing boat entry to the lake will have to

be extended.

Furthermore, the water budget modelling suggests that ungauged abstraction from the lake during the

drought starting in 1997 caused lake levels to drop as much as 70 cm after 2000 (Figure 5). The amount of water

pumped from the lake for irrigation, stock and domestic uses needs to be closely monitored, to ensure that it

does not increase and adversely impact the lake.

Therefore, although the lake level fluctuations at Lake Purrumbete are predominantly explained by climate

variation, they demonstrate anthropogenic influences on the hydrology. As a result, a regional plan to manage

the quality and amenity of lakes in western Victoria under the influence of not only a drying climate but also

increasing to human disturbance must develop different scenarios for lakes with different characteristics, and

even deep lakes like Lake Purrumbete are vulnerable in certain respects.

Conclusions

A modified difference water budget method for lakes, previously developed by Yihdego and Webb (2012)

that closely links the lake to the groundwater was able to successfully model the level fluctuations in Lake

Purrumbete, a deep, cylindrical lake located within an extensive basalt plain in western Victoria, which has

different characteristics from previous applications. The modified method used in this study incorporates

seasonal variation and also provides flexibility through the number of parameters and processes modelled and

hence served as an instrumental tool to bridge the gap in the context of understanding the hydrological

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components that may or may not related to the problems of increasing salinity in many western Victoria’s lakes,

considered as a valuable contribution to factual information about the hydrology of the region.

The modelling showed that the major influences are evaporation and direct precipitation; however

groundwater fluxes are significant (~16%). Lake Purrumbete is unusual among the lakes on the volcanic plain in

western Victoria because of this substantial groundwater flux, which is due to its significant depth and the

presence of highly permeable basalts along the upgradient shoreline; these basalts are rapidly recharged because

of their thin stony soils. This confirms that deeper lakes with low surface area/volume ratios may have a greater

contribution of groundwater flow to the water budget.

Lake Purrumbete’s low salinity reflects the relatively high groundwater and surface water flux (~35% of the

total water budget), so that there is significant salt export in the outflow, and the lake’s relatively low surface

area/volume ratio means that the effect of evaporation is less pronounced than for other, shallower more saline

lakes on the basalt plain.

The fluctuations in lake level and salinity for Lake Purrumbete are much less than for other lakes in the

region, even during the 1997-2010 drought, due to its cylindrical shape and the large relatively constant

groundwater flux, which damps the episodic influence of rainfall and surface inflow. Lakes with a higher

percentage of groundwater inflow/outflow like Lake Purrumbete are therefore less affected by climate

fluctuations.

Lake Purrumbete will be an important refugium for aquatic ecosystem in western Victoria if climate change

causes frequent seasonal drying of other lakes in the plain, so it is important to develop management strategies

to maintain its water resources and quality. Although the water budget modelling demonstrates that lake level

fluctuations are predominantly due to climate variation, anthropogenic influences are also evident, and in

particular pumping for farm and domestic uses needs to be closely monitored to ensure that it does not adversely

impact the lake. Thus management strategies need to consider the full impact of both climate and anthropogenic

change.

Acknowledgments

This study is a portion of the author’s doctoral research conducted while at La Trobe University, Australia.

Support was provided by the Environmental Protection Authority (EPA) Victoria, and the Glenelg–Hopkins

Catchment Management Authority. We thank two anonymous reviewers and Frances Watkins (Editorial

Manager) for helpful comments and suggestions.

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Figure 1. Location of Lake Purrumbete. Topographic DEM downloaded from Shuttle Radar Topographic Mission website; x–y resolution 15–30 m, elevation accuracy <5 m

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Figure 2. (a) Topographic setting of Lake Purrumbete, showing surface catchment boundary (dashed line) and 2010 watertable elevation contours (m AHD; modified after Tweed et al., 2009); DEM downloaded from Shuttle Radar Topographic Mission website (b) Airborne radiometric RGB KThU image showing geological setting of Lake Purrumbete; image supplied by the Geological Survey of Victoria

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Figure 3. Geological cross-sections showing aquifers that interact with Lake Purrumbete; for locations see Figure 2b

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Figure 4. Stream flow modelling for the Curdies River

Figure 5. Comparison of measured and modelled water levels for Lake Purrumbete. Maximum and minimum outlet heights are shown as a solid line

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Figure 6. Correlation between observed and calculated lake elevation

Figure 7. Temporal distribution of square difference between observed and calculated lake levels

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Figure 8. Comparison of measured and modelled salinity for Lake Purrumbete; (a) without stratification, (b) with stratification

   

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Table 1 Long-term mean monthly volumetric components of Lake Purrumbete water budget (103 m3).

Surface inflow

Surface outflow

Evaporation

Precipitation

Groundwater inflow

Groundwater outflow

Abstraction

Jan 16 189 1034 234 221 12 18

Feb 4 119 946 198 222 11 14

Mar 2 76 759 244 222 11 9

Apr 2 54 456 342 223 11 6

May 8 50 260 377 223 11 6

Jun 51 58 185 478 223 11 8

Jul 159 89 213 477 223 11 12

Aug 295 135 299 525 222 11 17

Sep 416 192 403 505 222 12 43

Oct 340 245 557 440 221 13 62

Nov 183 260 699 318 221 13 80

Dec 72 233 909 312 221 13 41

Annual 1549 1700 6720 4451 2665 140 316

 

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