Water resource development and hydrological change in a large

Journal of Hydrology 228 (2000) 10–21 www.elsevier.com/locate/jhydrol

Water resource development and hydrological change in a large dryland river: the Barwon–Darling River, Australia M.C. Thoms a,*, F. Sheldon a,b a

Co-operative Research Centre for Freshwater Ecology, University of Canberra, Canberra, Australia b Department of Environmental Biology, University of Adelaide, Adelaide, Australia Received 25 May 1999; accepted 12 November 1999

Abstract Water resource development has had a major impact on the hydrology of the Barwon–Darling River, a large dryland river in southeast Australia. Flows are highly modified through the presence of nine headwater dams, 15 main channel weirs and 267 licensed water extractors. Median annual runoff has been reduced by 42% over a 60-year period. Small flood events (e.g. Average Recurrence Interval of ,2 years) have suffered the greatest impact with reductions in magnitude of between 35 and 70%. At a number of stations, the seasonality of flows has also been affected with a distinct shift in seasonal flow peaks relating to irrigation diversions. Overall, flows show a marked increase in predictability and consistency (sensu Colwell R.K. 1974. Predictability, constancy and contingency of periodic phenomena, Ecology 55, 1148–1153). There has also been a change in the shape of the hydrograph. Both long- and short-term hydrological changes in the Barwon–Darling, associated with water resource development, may prove to be critical for the ecological health of the system. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrology; Variability; Water resource development; Dryland river; Australia

1. Introduction 1.1. Hydrological variability in dryland rivers Hydrological variability is a feature of dryland rivers. This is commonly described using statistics from flood–frequency curves, flow duration curves and time series of annual discharge (McMahon et al., 1992). In dryland regions, hydrological variability is often associated with highly variable ‘effective’ rainfall and low rainfall–runoff ratios. This is particularly so for the low latitude Southern Hemisphere land masses in which high precipitation variability is a * Corresponding author. E-mail address: [email protected] (M.C. Thoms).

characteristic (Alexander, 1985; Simpson et al., 1993). Increasing evaporation towards the interior of these regions further reduces the percentage rainfall converted to runoff and amplifies its sporadic nature. For example, Finlayson and McMahon (1988), using a global database, demonstrate that dryland rivers are more variable than those of humid regions in terms of monthly discharge. Puckridge et al. (1998) in a multivariate analysis of the hydrographs of 52 rivers with similar catchment areas, showed Australian dryland rivers to be amongst the most hydrologically variable in the world. The average coefficient of variation (CV) for annual runoff in dryland regions is 0.99 in comparison with more humid regions in North America (0.3), Europe (0.2) and Asia (0.2) (Finlayson and McMahon, 1988). Key hydrological features of dryland rivers

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M.C. Thoms, F. Sheldon / Journal of Hydrology 228 (2000) 10–21

include (McMahon, 1979): a non linear temporal response of runoff to rainfall and basin size and highly variable seasonal flow characteristics. This variability may be further amplified by climatic conditions such as El Nino-Southern Oscillation (ENSO) events as the discharges of rivers in south-eastern Australia, including the Darling River, correlate significantly with the Southern Oscillation Index (SOI) (Simpson et al., 1993). In areas of low and highly variable rainfall, the water resources provided by rivers are subject to high levels of exploitation (Braune, 1985). Water resource development, through the construction of dams, weirs and levees and the extraction of water for irrigation purposes can significantly alter river hydrology (e.g. Maheshwari et al., 1995). Although the demand on the water resources of rivers in dryland areas is often high, there are limited data describing their hydrological response to development. Most examples are from humid or temperate regions (e.g. Petts, 1984). Extrapolating information from more temperate regions to the rivers of dryland areas is not feasible and unlikely to result in sustainable management. Dryland rivers respond differently, both physically and biologically, to hydrological change in comparison with temperate rivers (Thoms and Walker, 1993; Davies et al., 1994). This response may differ across a range of spatial and temporal scales (Walker et al., 1995). 1.2. Study area The Barwon–Darling River drains the northwesterly portion of the Murray–Darling Basin (MDB) in southeast Australia; it has a catchment of 650,000 km 2 (Fig. 1). Most of its tributaries (the Condamine–Balonne, Macintyre, Gwydir, Namoi, Castlereagh and Macquarie Rivers) drain the western margins of the Great Dividing Range in northern New South Wales and southern Queensland and thereby contribute relatively high runoff. Others, notably the Warrego and Paroo Rivers, have their headwaters in the more arid west and are intermittent contributors, only providing significant runoff during intense rainfall and help to increase the duration of high flows in the lower Darling (Thoms et al., 1996). The catchment is characterised by extreme climatic variability with low rainfall a general feature. Annual median rainfall

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tends to decrease westward from .1200 mm in the headwaters to ,200 mm at Broken Hill (Thoms et al., 1996). Flows in the Barwon–Darling show some seasonality, higher flows usually occur during the summer months (December–April). There are nine major headwater dams with a combined storage capacity of 4415 Gl. The degree of flow regulation, expressed as the ratio of storage capacity to mean annual flow, varies from 6.54 to 0.98. There are also 15 main channel weirs, constructed to assist in providing water for urban, stock and domestic purposes. Numerous small weirs (.1000) exist on tributaries and anabranch channels. Flows in the system are also modified by large-scale water abstractions for irrigation which have increased markedly since the 1960s. In 1994 there were 267 licenced water abstractors between Mungindi and Menindee, compared with only 20 in 1960. Approximately 50% of these licences abstract water from the river between Walgett and Bourke. The scale of these diversions can be seen if you compare the 97/98 recorded diversion volume from the Darling catchment (2074 Gl; MDBC, 1998) with the long-term annual mean flow at Wilcannia (2370 Gl; Table 1). In 1997/98 diversions from the river accounted for 87% of the long-term mean annual flow. Flowing through the dryland regions of the western part of the MDB the Barwon–Darling has been subjected to a rapid rate of water resource development over the past 30 years. This development was implicated as a major cause of the 1991 blue–green algal bloom that stretched for over 1000 km (Bowling and Baker, 1996) and the decline in abundance and diversity of native fish (Gehrke et al., 1995). This paper examines the impact of water resource development on the hydrological regime of the Barwon– Darling River. Changes are investigated at a variety of temporal scales ranging from decades to months. Predictions are made regarding the implications of hydrological change to the functioning of riverine ecological processes.

2. Methods Although the record for the Mungindi, Walgett, Bourke and Wilcannia gauging stations (see Fig. 1) spans almost 100 years (Table 1), a rapid rate of water

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Table 1 Summary statistics of historical daily flow record (Ml/day) for four stations on the Barwon–Darling River

Period of record N Basic statistics Mean Median Median as percentage of the Mean Zero flows (%) Minimum Maximum Measure of the distribution Skew Measures of variability Interquartile CV

Mungindi

Walgett

Bourke

Wilcannia

1889–1995 39,113

1886–1995 27,886

1945–1995 18,629

1913–1995 30,316

1705.9 423.3 25 11 0 180,758

6749.1 951.2 14 7.3 0 471,000

10798.8 2500.4 23 3.7 0 499,297

6493.1 2020.7 31 4.9 0 68,444

12.6

9.4

7.6

2.5

1523.5 2.6

4227.7 3.2

8393.8 2.7

7405.8 1.6

resource development, combined with the naturally variable flow, makes it difficult to evaluate the impact of development on the hydrological regime using only historical data. Thus, simulated daily discharge data from the New South Wales Department of Land and Water Conservation (DLWC) Integrated Quantity Quality Model (IQQM) were also used. Simulated ‘natural’ flows were compared with simulated ‘current’ flows for the period 1936–1996 for the four stations. A full description of the model and its reliability are detailed in Black et al. (1997). The ‘natural’ output is simulated with a zero setting for flow regulating structures, abstractions of water and catchment development, utilising long-term mean climatic conditions, whereas the ‘current’ simulated output uses water and catchment development conditions present in 1993–1994 combined with long-term mean climatic conditions. Comparisons of these simulated outputs were made for annual and monthly volumes and flood discharges. Annual series flood frequency analyses were performed on both simulated data sets for each station using a Log Pearson 3 distribution (Pilgram, 1987). Development has not impacted all flood flows equally, therefore, discharges were partitioned according to their Average Recurrence Interval (ARI) in years and compared. The Colwell indices of predictability, constancy

and contingency (Colwell, 1974) were calculated for the simulated monthly ‘natural’ and ‘current’ flow data. These two scenario outputs are of the same length, data type and have roughly the same range of values. The use of Colwell indices in this instance can be justified (cf. Gan et al., 1991) and allow the direction of hydrological change attributable to water resource development to be highlighted. Predictability is a measure of the relative certainty of knowing a state at a given time and is the sum of consistency and contingency. These two latter measures indicate the degree to which a state stays the same and how closely different states correspond to different time periods, respectively. The influence of water resource development on the character of individual floods was examined by comparing an observed event at Bourke in February–March 1996 with the simulated ‘natural’ event. Historical stage data for the Bourke gauging station have also been used to assess the impact of development on the stability of water levels (the number of days taken for the river level to undergo a total stage change of 10, 20, 30, 40, 50 and 60 cm). A limit of 60 cm was chosen as it represents the average maximum depth of light penetration, approximately three times the measured Secchi depth (cf. Cole, 1983). The data were divided into the ‘Periods of Regulation’ (cf.

Fig. 1. Study area in south-eastern Australia. (a) The position of the Murray–Darling Basin; (b) detail of the Murray–Darling Basin in south-east Australia; and (c) detail of the Barwon–Darling and associated tributaries in north-western New South Wales. The gauging stations, from which flow data were used, are shown.

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Fig. 2. Historical flow data for the Barwon–Darling River at Bourke (1913–1997) with corresponding values for the Southern Oscillation Index (SOI).

Maheshwari et al., 1995) and the average number of days taken for the river to undergo a given stage change plotted against that change in stage. The Periods of Regulation used were: Period 1—preconstruction of Keepit Dam (,1960); Period 2—

from the construction of Keepit Dam to construction of Pindari Dam (1961–1969); Period 3—Pindari Dam to Three Dams (1970–1976); Period 4—Three Dams to Split Rock (1977–1987); Period 5—Split Rock to present.

M.C. Thoms, F. Sheldon / Journal of Hydrology 228 (2000) 10–21 Table 2 Mean and median flows (Ml/day) for the four stations calculated using the stimulated ‘current’ hydrological data and their percentage change from ‘natural’ conditions

Mean Median

Mungindi

Walgett

Bourke

Wilcannia

1499 (231) 341 (233)

3987 (236) 120 (224)

6975 (227) 1342 (254)

5824 (229) 688 (273)

3. Results 3.1. Historical flows The discharge of the Barwon–Darling is highly skewed (Table 1) with a large proportion of average flows occurring in very wet years and during major floods (Fig. 2). Median annual flows comprise less than 30% of mean annual flows. Flows (both annual volumes and flood peaks) generally increase downstream towards Bourke. However, downstream of Bourke flows decrease due to a lack of tributary contributions and the increased levels of evaporation, a characteristic of inland Australia. There have been secular changes in the hydrological regime of the Barwon–Darling over the last 100 years. Years prior to the 1900s and since the mid-1940s have been wetter with greater runoff and flood activity than the period 1900–1945 (Riley, 1988). This perhaps reflects the shift in the geographical pattern of correlation between precipitation and the SOI for the years before 1950s, compared with the years since the 1950s (Simpson et al., 1993). 3.2. IQQM simulated data The IQQM simulation data (1936–1996) suggests that water resource development has had a measurable impact on annual flows. The ‘current’ development scenario suggested an average reduction of 32.5 and

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44.5% in mean and median annual volumes of water, respectively, with the largest reductions at Walgett. Median flows have suffered a greater impact than mean flows. The median daily flow at Wilcannia has been reduced by 73% (Table 2). When the ratio of ‘current’ to ‘natural’ flows was calculated for four different flows duration percentiles the period of low flow has increased at all stations (Table 3). The percentage of time spent under higher flow conditions (i.e. greater than 25th percentile) has decreased for all stations. When flood flows were grouped according to their ARI, the impact of water resource development varied for flows of different ARI (Table 4). At each station, flows with an ARI ,2 years experienced the greatest impact being reduced from 40 to 61%. Floods with an average recurrence interval greater than 2 years have also been affected with reductions ranging from 7 to 40%. Although flows in the Barwon–Darling River are highly variable, higher flows tend to occur in the summer months. Under ‘natural’ conditions maximum monthly flows occurred during February at Mungindi and Walgett, March at Bourke and between March and April at Wilcannia. The volume of water diverted from the river, however, reflects a seasonal demand: largest volumes were removed between October and April corresponding to the summer irrigation season. Monthly volumes of water diverted from the river ranged from 1.13 to 161.40 Gl corresponding to monthly flow reductions between 21 and 125%. This has influenced the current seasonal distribution of downstream flows. A greater proportion of the flow was removed during the summer months and as a consequence the annual hydrograph has been somewhat flattened, summer flow peaks have been depressed. The predictability of flows in the Barwon–Darling, measured by Colwell Indices, was greater under

Table 3 The ratio of stimulated (‘current’: ‘natural’) flow events exceeding various flow percentiles Station

80th percentile

50th percentile

25th percentile

10th percentile

Mungindi Walgett Bourke Wilcannia

2.13 1.47 1.29 1.40

1.88 0.88 1.57 0.60

0.63 0.63 0.78 0.71

0.71 0.72 0.97 0.81

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Fig. 3. Colwell indices of predictability, consistency and constancy for the Barwon–Darling River.

Fig. 4. Average number of days taken for river levels to change 10, 20, 30, 40, 50 and 60 cm for the Darling River at Bourke. Periods of regulation are described in the text.


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Fig. 5. (a) Water abstractions (Ml/day), (b) discharge (Ml/day) and (c) percent of discharge abstracted for the March 1996 flood at Bourke.

‘current’ conditions at all stations (i.e. a decrease in flow variability ranging from 13 to 25%). The increase in predictability was mainly due to a marked increase in consistency (Fig. 3). River levels examined at the smaller scale of ‘weeks’ and ‘days’ appear to have become more stable. For example, at Bourke there was an increase in the number of days river levels take to undergo changes of 20 cm, or greater (Fig. 4). Overall, there was a steady increase in river level stability from the first period of development, the construction of Keepit

Table 4 Simulated ‘current’ flood discharges (Ml/day) and their percentage change from ‘natural’ conditions for four stations. Average Recurrence Interval ARI ARI

Mungindi

Walgett

Bourke

Wilcannia

1.01 1.11 2 5 10 25

437 (241) 1154 (251) 2120 (240) 3022 (221) 1758 (27) 3574 (29)

1166 (256) 2537 (255) 5299 (240) 10741 (226) 19598 (221) 41167 (225)

916 (242) 2392 (248) 4737 (234) 9914 (222) 18188 (217) 32610 (217)

2322 (261) 4234 (261) 4487 (234) 5270 (218) 10041 (217) 28863 (236)

Dam with the greatest stability occurring after the construction of Split Rock Dam. The comparison of an observed flood, from January–March 1996, with the simulated ‘natural’ event highlights the impact water resource development has on individual hydrographs. This flood, which originated upstream of Mungindi, peaked at Bourke with a discharge of 95,900 Ml/day on 26th Feburary 1996. During the flood 74,310 Ml of water was extracted, with a maximum daily extraction of 2230 Ml on March 11th (Fig. 5a). The impact of extraction on the hydrograph is most apparent for the recession limb where up to 65% of the daily flow was abstracted (Fig. 5c). A sensitivity analysis of the modelled hydrograph was undertaken by comparing the simulated ‘actual’ with the observed flood, errors of less than 2% of the daily discharge were obtained. Water extractions changed the shape of this flood event, steepening the falling limb of the hydrograph (Fig. 5b). When the modelled ‘natural’ data were compared with the observed data for this event, recession rates were found to have increased between 22 to 954 Ml/day. Actual rates of recession over a five day period were 2052, 2882, 4621 and 3560 Ml/day for

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Mungindi, Walgett, Bourke and Wilcannia, respectively, these are the steepest rates of recession in the historical record at Bourke and Wilcannia.

4. Discussion Rivers are nested hierarchical ecosystems and hydrological change through water resource development may influence all or some of the various hierarchies. Following the scalar approaches of Schumm (1988) for the investigation of geomorphic processes and Salo (1990) for biological communities, ecological processes in dryland rivers may respond to three scales of hydrological behaviour. The flow regime (long term, statistical generalisation of flow behaviour—macro scale influences that extend over hundreds of years); flow history (the sequence of floods or droughts—meso scale influences between 1 and 100 years); and, the flood pulse (a flood event—micro-scale influences that generally extend less than one year). It is apparent from this study that water resource development in the Barwon–Darling catchment has had a marked but variable impact on all three hydrological scales. The flow regime of the Barwon–Darling at Wilcannia shows long-term hydrological variability (Fig. 2). For the Barwon–Darling there is a relationship between discharge and the sea surface temperature anomalies that underlie ENSO (Simpson et al., 1993). Although the ENSO influence on discharge is strong, it does not account for all the observed variation, much of which is attributable to other atmospheric circulation phenomena (McMahon et al., 1992). Typical measures of flow variability (e.g. CVs and Colwell indices) tend to be higher in the Barwon– Darling than ‘expected’ in comparison to other similar systems. Finlayson and McMahon (1988) note the CVs for annual flows to be of the order of 1.12 for similar dryland rivers. Although Walker et al. (1995) report high values of skew for the hydrological record of Cooper Creek, northeast South Australia and Puckridge et al. (1998) demonstrate the high levels of variability inherent in Australian dryland rivers. The CVs of ‘natural’ annual flows for Mungindi, Walgett, Bourke and Wilcannia range from 1.59 to 3. However, it is apparent that development has resulted in flows

becoming more predictable, as indicated by increases in the Colwell predictability index of between 15 and 34%. The comparison of the simulated ‘natural’ with ‘current’ development data also suggests an impact on various aspects of both the flow regime and flow history. Overall, the median flows at Wilcannia have been reduced by 73%. Flows with an ARI of 1.01 years have been reduced by between 40 and 61%. Even flows with an ARI of 25 years have experienced reductions of between 9 and 36%. These results are consistent with other estimations of the impact of water resource development within the Barwon– Darling catchment. Between 1988 and 1994 there was a 32% increase in the flow diverted from the upper Darling system combined with a 187.2% increase from the Queensland portion of the Border Rivers, 38.2% increase from the New South Wales portion and a 63.5% increase for the Condamine– Balonne system (MDBMC, 1995). Australia is not alone in diverting a large proportion of the flow from its dryland rivers. Approximately 50% of all the flows in South Africa are held within storage dams, with an unknown percentage diverted. Such hydrological changes have had drastic impacts on the aquatic ecology of the southern African rivers (Allanson et al., 1990). Less than 1% of the natural flow of the Colorado River, in the American southwest, now reaches its mouth (Petts, 1984); seasonality has changed with high spring flows reduced and summer flows vastly increased (Carlson and Muth, 1989). The vast amount of water resource development in the Colorado catchment has changed the ecological character of the river, endemic fauna are threatened and salinity levels are increasing rapidly (Stanford and Ward, 1986). The most striking example of the consequence of diverting water from a dryland river system, however, must be that of the Aral Sea in Uzebekistan and Kazakhstan where water from the incoming Amu- and Syr-Darya Rivers has been diverted for irrigation. There has been a change in lake volume from 1090 km 3 in 1960 to 310 km 3 in the 1990s (Aladin and Williams, 1993). As a result, 83% of the fish fauna and 96% of the macroinvertebrate fauna of the Aral Sea are extinct, with only 3.6% of the vast reedbeds remaining (Micklen, 1988). Water resource development may also impact the


seasonality of flow (Petts, 1984). In the Barwon– Darling, the greatest impact has been on summer flows (October–March), associated with increased demand from upstream irrigation industries. Monthly flows have been reduced by up to 56% (medians: 31– 39%) in summer months compared to a maximum of 36% (medians: 24–32%) during winter months. These reductions in seasonality can impact the portion of time rivers spend at low or drought flow levels. In comparison, development in the Murrumbidgee River in the southern part of the MDB has resulted in median outflows equivalent to 25% of natural flows with worst case drought scenarios expected in 57% of years (MDBC, 1995). A similar situation is developing in the Darling River with drought scenarios now expected in 27% of years compared with 10% under natural conditions (MDBMC, 1995). The impact of water resource development in the Barwon–Darling catchment has therefore followed the same pattern as in other dryland areas. A large proportion of the flow is diverted or held in storage, with a result the seasonality has been dampened and the flow regime has become more predictable. Increases in predictability are also evident for daily to weekly changes in river levels, with increasing river level stability corresponding to increasing water resource development for historical stage data at Bourke. A similar trend was seen for stage data from the lower River Murray, South Australia, with a general trend of an increase in the number of days taken for river levels to rise and fall within the channel with increasing impact of flow regulation (Sheldon, 1994). An increase in river level stability may impact micro-level processes within the river. For example, if a stable photic depth is maintained benthic algae are likely to dominate littoral biofilms with implications for food availability for many taxa (Sheldon and Walker, 1997). Ecological processes in dryland rivers are driven by flow variability at a range of scales (Walker et al., 1995; Puckridge et al., 1998). For fish, flow variability parameters influence length of breeding season, spawning periodicity, length of life cycles, age at maturity, colonisation ability, species richness and major variations in assemblage structure (Puckridge et al., 1998). Thus, the hydrological changes described here for the Barwon–Darling system, in association with water resource development, are

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likely to have had a large impact on ecological processes within the system and may be implicated in the change in fish assemblage structure (see Gehrke, 1995; Driver et al., 1997). It is often difficult to assess the impact of water resource development on the physical morphology of a river system, as changes often lag development. The examination of a real flood event in the Barwon– Darling during the summer of 1996 highlighted some of the physical changes that may have occurred as a direct result of water resource development. Bank erosion is a common feature along the river especially following sustained rapid falls in water level. During the summer 1996 flood, Thoms (2000) recorded average daily falls in water level ranging from 0.76 to 1.9 m, over a five-day period, between Walgett and Wilcannia. These falls corresponded to periods of major water extraction (Fig. 5) and were associated with areas of bank slumping (Thoms, 2000). Water extractions doubled the average rate of fall over this five-day period and over 91,000 tonnes of sediment were eroded from the river banks between Walgett and Wilcannia. Most of the bank slumping was associated with erosion of inset benches and hence modification to the complexity of the in-channel environment. In many dryland rivers, it is the river flow and its associated variability that maintains the complexity of the in-channel environment (Graf, 1988; Thoms and Walker, 1993; Thoms and Sheldon 1997). Using historic flow and channel survey data that pre-date significant water resource development in the Barwon–Darling River, Thoms and Sheldon (1997) illustrated that the cross-sectional morphology of the unregulated river was complex being characterised by a series of flat surfaces or ‘benches’. We suggest that these benches provide aquatic habitats during highflow events and are sites for the accumulation and temporary storage of organic matter. Therefore, the greater the physical complexity of the channel, the greater the surface area available for organic matter accumulation. Organic matter entering and accumulating in river channels from the floodplain and riparian zone has been suggested as the principal source of particulate matter driving the food webs of large rivers (Cuffney, 1988; Hedges et al., 1994; Thorp and Delong, 1994). In the Barwon–Darling River hydrological change, through a reduction in flow volumes and the frequency of flooding, has the potential to

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reduce the availability and functionality of the inchannel habitat. In summary, water resource development in the Barwon–Darling catchment has altered the hydrological regime of the Barwon–Darling River. The median annual discharge has been reduced, flows have become more predictable and the summer flow peaks have been dampened. These hydrological changes will impact both physical and biological processes within the river. Acknowledgements This work was supported through the Land and Water Resources Research and Development Cooporation (LWRRDC) project UOC8. The discharge and stage data were provided by the New South Wales Department of Land and Water Conservation. Comments on an earlier draft by Profs. T. McMahon and K.F. Walker were appreciated. References Aladin, N.V., Williams, W.D., 1993. Recent changes in the biota of the Aral Sea, Central Asia. Verh. Internat. Verein. Limnol. 25, 790–792. Allanson, B.R., Hart, R.C., O’Keefe, J.H., Robarts, R.D., 1990. In: Dumont, H.J., Werger, M.J.A. (Eds.), Inland Waters of Southern Africa: An Ecological Perspective, Monograph Biologicae, 64. Kluwer Academic, Dordrecht, 458 pp. Alexander, W.J.R., 1985. Hydrology of low latitude Southern Hemisphere land masses. Hydrobiologia 125, 75–83. Black, D., Sharma, P., Podger, G., 1997. Simulation modelling for the Barwon–Darling river system for management planning. In: Thoms, M.C., Gordon, A., Tatnell, W. (Eds.), Researching the Barwon Darling, CRC for Freshwater Ecology, Canberra, pp. 34–43. Bowling, L.C., Baker, P.D., 1996. Major cyanobacterial blooms in the Barwon–Darling River, Australia, in and underlying limnological conditions. Mar. Freshwater Res. 47, 643–657. Braune, E., 1985. Aridity and hydrological characteristics: chairman’s summary. Hydrobiologia 125, 131–136. Carlson, C.A., Muth, R.T., 1989. The Colorado River: lifeline of the American Southwest. In: Dodge, D.P. (Ed.). Proceedings of the International Large River Symposium Can. Spec. Publ. Fish. Aquat. Sci., 106, pp. 220–239. Cole, G.A., 1983. Textbook of Limnology. Waveland Press, Illinois 401pp. Colwell, R.K., 1974. Predictability, constancy and contingency of periodic phenomena. Ecology 55, 1148–1153. Cuffney, T.F., 1988. Input, movement and exchange of organic

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