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Water Assessment Branch
Department of Primary Industries and Water
Report compiled for Water Resources Division
Technical Report No. WA 07/02
June 2007
Surface Water Hydrologyof the
South Esk River Catchment
A report supporting the development of a water management plan for thecatchment
2
Copyright Notice:
Material contained in the report provided is subject to Australian copyright law. Other than
in accordance with the Copyright Act 1968 of the Commonwealth Parliament, no part of this
report may, in any form or by any means, be reproduced, transmitted or used. This report
cannot be redistributed for any commercial purpose whatsoever, or distributed to a third
party for such purpose, without prior written permission being sought from the Department
of Primary Industries and Water, on behalf of the Crown in Right of the State of Tasmania.
Disclaimer:
Whilst DPIW has made every attempt to ensure the accuracy and reliability of the
information and data provided, it is the responsibility of the data user to make their own
decisions about the accuracy, currency, reliability and correctness of information provided.
The Department of Primary Industries and Water, its employees and agents, and the Crown
in the Right of the State of Tasmania do not accept any liability for any damage caused by, or
economic loss arising from, reliance on this information.
Preferred Citation:
DPIW (2007). Surface Water Hydrology of the South Esk River Catchment. Technical Report
No. WA 07/02. Water Assessment Branch, Department of Primary Industries and Water,
Hobart.
Cover Page Images: South Esk River Catchment.
The Department of Primary Industries and Water
The Department of Primary Industries and Water provides leadership in the sustainable
management and development of Tasmania’s resources. The Mission of the Department is to
advance Tasmania’s prosperity through the sustainable development of our natural resources
and the conservation of our natural and cultural heritage for the future.
The Water Resources Division provides a focus for water management and water
development in Tasmania through a diverse range of functions including the design of policy
and regulatory frameworks to ensure sustainable use of the surface water and groundwater
resources; monitoring, assessment and reporting on the condition of the State’s freshwater
resources; facilitation of infrastructure development projects to ensure the efficient and
sustainable supply of water; and implementation of the Water Management Act 1999, relatedlegislation and the State Water Development Plan.
3
Executive Summary
The catchment of the South Esk River upstream of Longford covers an area of approximately
3,350 km2. The catchment experiences widely varying climatic conditions with rainfall ranging
from 500 mm in the low lying areas to up to 1,500 mm in the highlands. With an annual
average rainfall of 835 mm, the total water input into the South Esk River catchment is
approximately 3,000 GL/year. The total catchment annual yield at Longford is around 900
GL/year and therefore comprises about 43% of the total annual water input. This simple water
budget indicates that the vast majority (57%) of total water input into the catchment is either
evaporated, transpired or moves into to the local and regional groundwater system.
The majority of the catchment rainfall and runoff occurs in the northern and eastern headwaters
and as a result maximum runoff is converted to river flows in these regions. Low rainfall and
higher evaporation in the southeast of the catchment has contributed to low conversion of the
runoff into the cumulative yields in the Lower South Esk regions. Considerable climate
variability within the catchment results in high spatial variability of runoff yield. The Upper Esk
subregion and Nile River catchment are identified as the most productive areas in relation to
converting rainfall into runoff and hence yield as river flow.
Average floods (1 in 2 year floods) in the lower reaches of the South Esk River catchment peak
at around 400 m3s
-1, while floods range from 100 to 150 m
3s
-1 in the major tributaries. At Perth
the observed streamflow recession after an average flood event is around 72 m3s
-1 per day and it
takes roughly five days for the peak flow to recede to an average river flow of 24 m3s
-1. Low
flow probability analysis indicated that within a given year the likelihood of occurrence of
average flows ≤ 1.0 m3s
-1 over a five day consecutive period is around 60%.
The current annual total allocated water in the catchment is around 44,415 ML. The subregion
breakdown of water usage shows that the bulk of the allocation is in the Lower Esk subregion.
The consumptive water usage for the entire catchment upstream of Longford is about 5% of the
total annual yield and this is reflected in the low hydrological disturbance index for the
catchment. Low water usage in the catchment is also a product of the seasonal flood extraction
rules that protect power generation at the Trevallyn power station. Strategies to balance the
consumptive and environmental uses of the water resources from the catchment must therefore
take into account the differences in flows, the variability of catchment yields and flood
extraction rules for the catchment.
Currently, surface water resources are allocated solely on surface water information, with only
cursory regard for groundwater influences. While it is recognised that river flow in this
catchment suggest a predominantly groundwater-driven system, there is little understanding of
groundwater in the catchment, and water allocation is presently based on the premise that water
is being allocated from surface water only, when in fact it may be drawing indirectly on the
groundwater resource. Conversely, the level of groundwater extraction is largely unknown and
may be having some impact on surface water. With increasing demand for better management
of the State’s water resources, greater emphasis must therefore be put into implementing an
holistic approach to understanding surface and groundwater connectivity within this and other
catchments in Tasmania.
4
Table of Contents
Executive Summary................................................................................................................................... 3
1.0 Introduction.......................................................................................................................................... 5
2.0 Catchment Description........................................................................................................................ 5
3.0 Catchment Hydrology ......................................................................................................................... 6
3.1 RAINFAL, RUNOFF AND EVAPORATION................................................................................................. 6
3.2 GAUGED FLOW MONITORING AND CHARACTERISTICS .......................................................................... 9
3.3 FLOOD FREQUENCIES ....................................................................................................................... 10
3.4 LOW FLOWS...................................................................................................................................... 12
3.5 FLOW RECESSION ............................................................................................................................. 13
3.6 WET AND DRY SEASON COMPARISON ................................................................................................. 15
3.7 HYDROLOGICAL CHARACTER OF THE SOUTH ESK RIVER CATCHMENT ................................................. 15
4.0 Catchment Water Balance Model .................................................................................................... 17
4.1 NATURAL FLOW ESTIMATION............................................................................................................. 18
4.2 HYDROLOGICAL DISTURBANCE INDICES ............................................................................................ 20
5.0 Flow Characteristics at Environmental Flow Assessment Locations............................................ 21
5.1 ENVIRONMENTAL FLOW ASSESSMENT LOCATIONS. ............................................................................. 21
5.2 NATURAL AND CURRENT FLOW CHARACTERISTICS ............................................................................. 21
5.3 FLOW DURATION ANALYSIS ............................................................................................................... 23
6.0 Catchment and Subregion Water Budget........................................................................................ 25
6.1 CATCHMENT WATER ALLOCATIONS.................................................................................................... 25
6.2 FLOOD TAKE RULES – SOUTH ESK BASIN .......................................................................................... 26
6.3 CATCHMENT WATER YIELD ............................................................................................................... 26
References................................................................................................................................................. 30
5
1.0 Introduction
This report provides relevant hydrological information that will support the development of a
water management plan for the South Esk River catchment. The key features of this report
include
• analyses of streamflow data collected at various locations around the catchment to
characterise the current hydrology of the catchment
• outputs from a catchment hydrological model to provide hydrological information for
environmental flow assessment, calculations of a catchment water budget and
assessment of yields for water usage and allocation decisions.
Six environmental flow assessment sites have been established in the catchment, and time series
flow data was extracted from the model and used to describe the flow at each of these sites.
Five water management subregions have been designated for the catchment, and modelled flow
data and catchment water allocations were analysed to provide water balances for these water
management subregions and the South Esk River catchment as a whole. From this water
balance approach, a view of the overall water budget is provided for each of the water
management subregions
2.0 Catchment Description
The South Esk River catchment (upstream Longford) is located in the northeast and midlands of
Tasmania and covers an area of approximately 3,350 km2 (Figure 2.1). The catchment rises in
the Fingal Tier in the East and is bounded by Ben Lomond and Mt. Saddleback to the North. Its
principal sub-catchments are drained by the Nile, St Pauls and Break O’Day Rivers.
Downstream of Longford, the South Esk River receives inflow from the Macquarie and
Meander Rivers before flowing into Tamar Estuary, thus forming part of the greater Tamar
River Basin.
6
Figure 2.1 South Esk River catchment.
The topography and the drainage pattern of the catchment are largely controlled by the local
and regional geology of the area. The upper South Esk catchment is characterised by
quartzwacke and mudstone whereas the middle parts of the catchment are dominated by
Jurassic dolerite. The lower part of the catchment is characterised by alluvial sediments derived
mainly from volcanic rocks. The drainage system generally follows a mixture of parallel to
dendritic pattern and has a density of around 2.0 km of river length per square kilometre of
catchment area. The mean elevation ranges from AHD 140 m at the Longford outlet to around
1,600 m at the top of the Ben Lomond Massif.
The South Esk River is essentially unregulated. There are no major storages within its
catchment and natural flows are altered primarily due to the combined influences of water
abstraction during the summer irrigation season and other land use practices. The primary land
uses in the catchment are agriculture and forestry. The lower part of the catchment is primarily
developed for grazing and irrigated agriculture, with the majority of the catchment cleared for
pasture.
3.0 Catchment Hydrology
3.1 Rainfall, Runoff and Evaporation
The South Esk River catchment is situated in one of the drier parts of eastern Tasmania, and
receives variable rainfall ranging from about 500 mm in the southwest to 1,800 mm in the
northeast (Figure 3.1). Higher rainfall generally occurs in the upper reaches of South Esk River
whereas the low lying valley and flatlands experience a drier climate and are prone to drought.
Spatial average of rainfall data indicates an average annual rainfall of 835 mm within the South
Esk River Basin for the period of 1960-2003. Some 15% of its 3,350 km2 catchment lies within
the high rainfall (≥800mm annual average) area of the Ben Lomond and Eastern Highlands
areas.
7
Figure 3.1 Distribution of mean annual rainfall in the South Esk River catchment.
Distribution of monthly rainfall varies widely across the catchment (Figure 3.2). Monthly
average rainfall in the south and west of the catchment is generally less than 50 mm. The
highest monthly rainfall generally occurs in the Ben Lomond area and as a result the Nile River
catchment receives comparatively greater runoff than the rest of the catchment. The Fingal and
Avoca region of the South Esk basin receive less than 100 mm of monthly rainfall.
0
50
100
150
200
250
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rainfall (mm)
Longford
Avoca
Mathinna
Ben Lomond
8
Figure 3.2 Mean monthly rainfall at selected regions of the South Esk River catchment.
Analysis of the mean monthly rainfall for the entire catchment indicates that rainfall exceeds
potential evaporation during the months of April to September when rainfall is highest (Figure
3.3). The average winter (May-Oct) rainfall is around 80 mm compared to 60 mm in summer
(Nov-April). The total average rainfall for the winter period is 478 mm while it is
approximately 357 mm in summer. The long-term average winter evaporation is 47 mm
compared to 117 mm during the summer. Evaporation is highest in the south and west of the
catchment. Low evaporation and maximum runoff occur at the head waters of the South Esk
River and its major tributaries.
0
20
40
60
80
100
120
140
160
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rainfall/Evaporation (mm)
Rainfall
Evaporation
Figure 3.3 Mean monthly rainfall / evaporation in South Esk River catchment.
A 5-year moving average analysis of annual rainfall data (1960-2003) indicates a roughly
decadal cycle of wet and dry years over the 43 years of the record period (Figure 3.4). The years
1972, 1982 and 1994 were identified as the driest years with annual average rainfall of
<600 mm across the catchment. The 1970's were the wettest years with peak annual rainfall in
the range 1,000-1,300 mm. Since 1975, the annual average rainfall in the South Esk catchment
has not exceeded 1000 mm.
9
500
600
700
800
900
1000
1100
1200
1300
1400
1960 1965 1970 1975 1980 1985 1990 1995 2000
Mean Annual Depth (mm)
Rainfall
5-year moving average
Evaporation
Figure 3.4 Variation in annual rainfall and evaporation data (1960-2003) superimposed with 5-
year moving average.
3.2 Gauged Flow Monitoring and Characteristics
There are currently five streamflow monitoring stations in the South Esk River catchment
(Table 3.1). Historical flow records also exist for an additional five streamflow monitoring sites
with various records available from 1937 to 1994.
Table 3.1 Streamflow monitoring stations in the South Esk River catchment.
Site_Name Easting Northing Area (km2) Period of Record
181_South Esk at Perth 516900 5394750 3280 19/12/1956 – present
150_South Esk at Llewellyn# 546850 5370340 2242 18/11/1952 – present
25_Nile at Deddington 538200 5397199 226 03/06/1982 – present
18311_St Pauls at Avoca 560400 5373500 495 04/05/1988 – present
191_Break O’Day at Killymoon 588000 5394500 240 04/11/1983 – present
# maintained by Hydro Tasmania.
The summary statistics for the streamflow monitoring sites are presented in Table 3.2. All the
streamflow monitoring sites exhibit a high variability in average flow from year to year. Flows
at Perth and Llewellyn are highly correlated with approximately 32% more gauged flow at
Perth. The area between Llewellyn and Perth contributes approximately 15% less runoff per
unit area than the catchment upstream of Llewellyn. This result is not surprising since the
average rainfall in the upper part of the catchment tends to be greater than that in the lower part.
10
Table 3.2 Summary statistics of stream flow (ML/day) at the selected streamflow monitoring
sites.
South Esk @ Perth (181) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean 2064.0 955.1 735.1 650.2 1284.3 1879.5 2753.7 3305.4 4145.7 3549.9 2382.5 1539.6 1492.3
Std.Dev. 1062.2 1180.2 1217.8 1442.0 1686.1 2140.2 3136.9 2644.8 3025.7 2479.3 2088.7 1521.7 2081.6
Minimum 4.9 14.5 4.9 10.9 11.2 48.4 68.7 218.9 363.3 192.2 143.1 48.9 15.1
Maximum 199715.0 81857.0 38596.0 114480.0 71818.0 199715.0 133193.0 48895.0 82230.0 58512.0 81989.0 38116.0 65169.0
South Esk @ Llewellyn (150) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean 1678.3 801.5 627.9 561.8 1257.1 1963.5 2344.0 2689.0 2951.1 2647.9 1699.7 1418.9 1385.7
Std.Dev. 974.0 922.5 1273.6 890.0 1895.0 3352.5 2954.4 2281.2 2123.1 2010.8 1153.8 1357.0 2011.0
Minimum 29.6 39.6 33.7 29.6 29.8 56.3 125.4 301.5 247.9 211.7 133.5 74.4 52.7
Maximum 203816.0 49938.0 42106.0 20678.0 61008.0 203816.0 104842.0 82131.0 41455.0 58271.0 22100.0 35496.0 54701.0
Nile at Deddington (25) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean 305.4 175.7 79.9 61.8 212.2 276.5 379.7 558.8 581.5 544.1 322.7 163.5 242.2
Std.Dev. 82.8 158.1 88.7 62.3 200.4 218.1 210.3 217.9 333.8 347.8 222.1 86.7 296.5
Minimum 3.3 7.1 5.5 4.1 3.3 11.7 18.0 36.9 36.8 34.8 20.9 13.8 7.5
Maximum 9743.8 6794.1 1926.0 2573.4 6874.6 7893.2 7798.5 6543.9 9743.8 5713.2 7863.2 2678.0 6250.1
St Paauls at Avoca (18311) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean 270.6 237.1 127.6 67.1 331.5 239.1 223.4 398.3 349.2 288.2 269.0 237.7 253.3
Std.Dev. 175.5 324.9 185.7 83.2 529.4 499.3 269.8 401.2 468.6 282.1 363.6 289.4 527.1
Minimum 0.1 0.1 0.1 0.3 0.8 0.8 1.5 9.5 13.2 7.5 1.0 0.9 0.0
Maximum 36095.0 36095.0 4197.8 2812.2 14081.0 12710.0 9063.4 9694.2 22333.0 5876.8 16132.0 6707.1 28274.0
Break O'Day at Killymoon (191) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean 138.4 155.5 58.6 30.2 128.4 179.8 79.7 207.3 186.6 183.2 131.7 129.6 162.7
Std.Dev. 82.3 255.0 87.2 37.9 239.6 393.7 89.0 233.5 289.7 290.7 232.6 176.7 304.4
Minimum 0.3 0.4 0.3 0.4 1.0 3.4 3.2 6.7 4.3 5.7 2.6 1.6 0.6
Maximum 31008.0 21921.0 2132.2 2125.1 7772.4 31008.0 2623.2 12523.0 13299.0 6880.3 14773.0 6903.9 10158.0
One way of assessing the efficiency of a catchment to yield flow is by calculating the specific
yield based on the size of the catchment area. Specific yield is simply a flow volume (ML) per
unit pickup area (km2). This specific yield can also be used to determine the proportional
amount of runoff (mm) that is converted to flow. A summary of specific yields from the gauged
flows is presented in Table 3.3, and shows that the Nile River catchment has the highest yield
per unit area of catchment. Both the St Pauls and Break O’Day Rivers have low specific yields
indicating poor performance of these catchments to yield surface flow.
Table 3.3 Summary of catchment yields.
Area Flow Specific Yield
Site (km2) (ML/day) (ML/day/km
2)
South esk at Perth 3280 2064.0 0.63
South esk at Llewellyn 2242 1678.3 0.75
Nile at Deddington 226 305.4 1.35
St Pauls at Avoca 495 270.6 0.55
Break O'Day at Killymoon 240 138.4 0.58
The winter average rainfall and evaporation for the South Esk River catchment upstream of
Perth are approximately 500 mm and 300 mm respectively resulting in approximately 200 mm
of potential direct runoff. During winter the efficiency of the catchment pickup area to translate
rainfall into effective river flow at Perth is approximately 68%. This means that about 32% of
the potential winter runoff is taken by consumptive water storages, wetlands and groundwater
recharge.
3.3 Flood Frequencies
Flood frequency analysis of flows from selected streamgauge sites in the South Esk River
catchment was carried out for the duration of gauged flow. The results of this analysis are given
in Table 3.4 and a selected number are graphically represented in Figure 3.5. The average flood
(1:2 years) in the lower reaches of the South Esk River (Lower Esk) peaks at around 400 m3s
-1
11
while it is 98 m3.s
-1 in the Nile River at Deddington. Both the St Pauls and Break O’Day Rivers
show a similar average flood magnitude of around 150 m3.s
-1.
1
Table 3.4 Annual exceedance probabilities (AEP) and average recurrence interval (ARI, years)
of peak floods (m3.s
-1) at selected stream gauging sites in the South Esk River catchment.
AEP ARI South Esk at Perth South Esk at Llewellyn Nile at Deddington St Pauls at Avoca Break O'Day at Killymoon
1.00 1 35 27 30 5 5
0.50 2 431 404 98 154 150
0.20 5 909 851 139 298 292
0.10 10 1301 1202 164 400 394
0.05 20 1721 1564 186 500 493
0.02 50 2319 2057 214 630 623
0.01 100 2802 2437 233 729 722
The South Esk River is the main source of major flood flows affecting the low lying areas of the
Fingal Valley, Longford, Hadspen and Launceston. The areas around Gray near St Marys (the
upper reaches of the Break O’Day River) and near Mathinna are well known as locations of
high rainfall events and flash flooding.
1
10
100
1000
10000
1 10 100
Annual Exceedance Probailityl (1:X years)
Flood Peak (m
3s-1)
South Esk at Llewellyn
St Pauls at Avoca
Nile at Deddington
Figure 3.5 Frequency curves of annual floods at selected streamflow monitoring sites in the
South Esk River catchment.
1 The units used in the flood frequency analysis are cubic metres per second (Cumecs) as the flood
frequency represents a peak rate of discharge. If mega litres per day were used as the units this would
provide a volume and the results may be misleading or misinterpreted.
12
3.4 Low Flows
Low flow frequency curves have been derived for a range of durations from 5 days through to
90 days (Figure 3.6). The curves give the probability that any given minimum flow will occur
over various time periods. For example, for a 5-day period the probability that a minimum
average daily flow of about 100 ML will occur in any given year is approximately 60%, while
for a longer period such as 90-days this probability decreases to around 15% for the South Esk
River at Perth. South Esk at Llewellyn shows a similar low flow probability range while it
varies widely for the major tributaries, Break O’Day, Nile and St Pauls Rivers.
This information has implications for environmental water provisions, water quality monitoring
and for the assessment of risk in supply of water from the river for purposes such as irrigation
and domestic use. Such risks will need to be taken into account during the planning process for
the six management subregions within the catchment.
Figure 3.6 Frequency curves of low flows at selected streamflow monitoring sites in the South
Esk River catchment.
South Esk at Perth
0
100
200
300
400
500
600
700
800
900
1000
0 10 20 30 40 50 60 70 80 90 100
Probability (%) of flow less than that shown in any given year
Flow (ML/day)
Expon. (90-Day)
Expon. (60-Day)
Expon. (30-Day)
Expon. (5-Day)
South Esk at Llewellyn
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50 60 70 80 90 100
Probability (%) of flow less than that shown in any given year
Flow (ML/day)
Expon. (90-Day)
Expon. (60-Day)
Expon. (30-Day)
Expon. (5-Day)
St Pauls at Avoca
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100
Probability (%) of flow less than that shown in any given year
Flow (ML/day)
Expon. (90-Day)
Expon. (60-Day)
Expon. (30-Day)
Expon. (5-Day)
Break O'Day at Killymonn
0
10
20
30
40
50
0 10 20 30 40 50 60 70 80 90 100
Probability (%) of flow less than that shown in any given year
Flow (ML/day)
Expon. (90-Day)
Expon. (60-Day)
Expon. (30-Day)
Expon. (5-Day)
Nile at Deddington
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80 90 100
Probability (%) of flow less than that shown in any given year
Flow (ML/day)
Expon. (90-Day)
Expon. (60-Day)
Expon. (30-Day)
Expon. (5-Day)
13
3.5 Flow Recession
A recession curve is a specific part of the flood hydrograph after the crest where streamflow
diminishes. The recession segment shows how the water storage in the river decreases over time
following a significant rain event. The recession curve basically reflects the baseflow
component of river flow and how groundwater storage influences and sustains flows in rivers.
Each recession segment of the flood hydrograph is described by a classic exponential decay
function of the form:
Qt = Qo e-αt
where Qt is the streamflow at time t, Qo is the initial streamflow at start of the
recession, e is natural logarithm and α is cut-off frequency constant.
Recession curves for peak flows in the South Esk River and its major tributaries are presented
in Figures 3.7 and 3.8. The recession equations for the South Esk River were:
Flow = 576 * e-0.0003*Time (Minutes)
South Esk River at Perth
Flow = 336 * e-0.0003*Time (Minutes)
South Esk River at Llewellyn
The upper part of the recession curve is dominantly surface water and as flow recedes the
surface flow contribution gradually diminish until the flow is comprised almost entirely of
baseflow (or groundwater discharge), depicted by the lower section of the curve. The curves in
Figure 3.7 show that it takes approximately 7000 minutes (approx 5 days) for the peak flow in
the South Esk River at Perth to recede from an average flood of 400 m3.s
-1 (during winter) to 50
m3.s
-1, which is a rate of decrease of about 72 m
3.s
-1 per day.
Flow Recession Curves
0
100
200
300
400
500
600
0 1000 2000 3000 4000 5000 6000 7000 8000
Recession Time (Minutes)
Flow (m
3s-1)
South Esk at Perth
South Esk at Llewellyn
Figure 3.7 Recession curves for peak observed flow in the South Esk River at Perth and South
Esk River at Llewellyn.
14
Recession curves for observed flows in the Nile, Break O’Day and St Pauls Rivers are shown in
Figure 3.8. The peak flow recession for sites in these rivers is described by the flowing
equations.
Flow = 96 * e-0.0006*Time (Minutes)
Nile at Deddington
Flow = 138 * e-0.0008*Time (Minutes)
Break O’Day at Killymoon
Flow = 194 * e-0.0006*Time (Minutes)
St Pauls at Avoca
The Nile and Break O’Day Rivers show a very similar recession rate of approximately 25 m3s
-1
per day, while the recession rate for St Pauls River is markedly steeper, at around 40 m3s
-1 per
day.
0
50
100
150
200
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Recession Time (Minutes)
Flow (m3s-1)
Nile at Deddington
Break Oday at Killymoon
St Pauls at Avoca
Figure 3.8 Recession curves for peak observed flow in the Nile, Break O’Day and St Pauls
Rivers.
15
3.6 Wet and Dry Season Comparison
Observed flow data (1960-2007) from the South Esk at Perth was examined to identify
relatively wet and dry years. This indicated that the years 1974 and 2006 represent years of
wetter-than-average conditions and drier-than-average. Figure 3.9 provides a comparison of the
hydrographs for these years plotted along with the monthly median flow for the record period
1960-2006. This has been included to demonstrate the degree of interannual variation that
occurs in runoff from this catchment.
Figure 3.9 Median monthly flow compared to wet and dry year hydrographs of flows from
South Esk at Perth.
3.7 Hydrological character of the South Esk River Catchment
The catchment experiences widely varying climatic conditions with rainfall ranging from 500
mm in the low lying areas to up to 1,500 mm in the highlands. With an annual average rainfall
of 835 mm, the total water input into the South Esk River catchment is approximately 3,000
GL/year. The total catchment annual yield at Longford is around 900 GL/year and therefore
comprises about 43% of the total annual water input. This simple water budget indicates that the
vast majority (57%) of total water input into the catchment is either evaporated, transpired or
moves into to the local and regional groundwater system.
The majority of the catchment rainfall and runoff occurs in the northern and eastern headwaters
and as a result maximum runoff is converted to river flows in these regions. Low rainfall and
higher evaporation in the southeast of the catchment has contributed to low conversion of the
runoff into the cumulative yields in the Lower South Esk regions. Considerable climate
variability within the catchment results in high spatial variability of runoff yield. The Upper Esk
subregion and Nile River catchment are identified as the most productive areas in relation to
converting rainfall into runoff and hence yield as river flow.
16
All flow gauge sites exhibit a high variability in average flow from year to year. Flows in the
South Esk River at Perth and Llewellyn are highly correlated with approximately 25% more
flow at Perth. The area between Llewellyn and Perth contributes approximately 40% less
runoff per unit area than the catchment upstream of Llewellyn over a year. This result is not
surprising since the average rainfall in the upper part of the catchment tends to be greater than
that in the lower part.
Perth and Llewellyn exhibit strong seasonal patterns generally peaking in the period July
through September. The Break O’Day and St Pauls River catchments display high variability
of flows. The Break O’Day River is dominated by very low and extremely high monthly flows.
The South Esk River is the main source of major flood flows affecting the low lying areas of the
Fingal Valley, Longford, Hadspen and Launceston. The areas around Gray near St Marys (ie.
in the upper reaches of the Break O’Day River) and near Mathinna are well known as locations
of high rainfall events and flash flooding.
Average floods (1 in 2 year floods) in the lower reaches of the South Esk River catchment peak
at around 400 m3s
-1, while floods range from 100 to 150 m
3s
-1 in the major tributaries. At Perth
the observed streamflow recession after an average flood event is around 72 m3s
-1 per day and it
takes roughly five days for the peak flow to recede to an average river flow of 24 m3s
-1. Low
flow probability analysis indicated that within a given year the likelihood of occurrence of
average flows ≤ 1.0 m3s
-1 over a five day consecutive period is around 60%.
17
4.0 Catchment Water Balance Model
Under the National Action Plan (NAP) a rainfall and runoff water balance model was developed
for the South Esk River catchment (HEC, 2005).
Whilst gauged data provides a good picture of the hydrology of a catchment, they are generally
limited or intermittent in the period of record at any one site. They are not able to provide a
clear picture of the catchment hydrology under natural2 conditions, nor can they provide a
picture of long term trends related to climate variability. A water balance model is used to
generate natural flow and current3 flow time series data over a much greater time period, using
rainfall and evaporation data. These models allow an assessment of changes in hydrology due
to current water abstraction, and allow catchment yields to be determined at several locations
within catchments so that various water allocation scenarios can be tested.
Calibration of the water balance model was achieved by adjusting catchment parameters (eg.
infiltration, baseflow, storage capacities) so that the modelled natural2 flow output best matches
the observed flow record at the calibration site. A detailed description of the development and
calibration of the South Esk River catchment model can be found in HEC (2005). The model
can generate a daily time-series of natural flow based on daily rainfall and evaporation records
over 100 years (1901-2003). The model can also generate a daily time-series of current flow
for the same period; that is, what flow would have been from 1901 to 2003 has current water
abstractions occurred in these years.
The catchment model has been used to generate natural flow time series data for the
environmental flow assessment locations and yield analysis for the water management
subregions. The model is also used to derive a daily time-series of current3 flow, which takes
into account water abstractions from the system and gives an indication of impact on the natural
streamflow and hence on the gross catchment yield. The impact of water abstractions from the
system has been used to derive the hydrological disturbance indices described in section 4.3.
Information on the current water abstractions in the catchment was obtained from entitlement
allocations in the DPIW Water Information Management System database (WIMS). The
allocations in the catchment are of a given Surety (from 1 to 8) and they have a specific period
of applicability. This period was used in the modelling to allocate an average daily abstraction
for each licensed abstraction. For example, a 184 ML allocation for the period May 1 to
October 31 (184 days) would have an average extraction of 1 ML/day in the modelling process
for May 1 to October 31. While it is realised that this even spread of abstraction may be
unrealistic (as abstractions will occur at different rates within the allocation period), there are
minimal historical records to assist in defining how each individual licence is utilised. This is
further discussed in the next section.
Furthermore the model does not explicitly account for changes in landuse and vegetation over
time within each of the subcatchments.
2 Natural flow: Flow that is expected to occur in a river where there is no water extracted for
consumptive use. This is generally produced from a hydrologic model for a catchment using rainfall and
evaporation as the primary input data. It does not take into account any changes in land-use that may
have occurred over the period of interest.
3 Current flow: Flow in a river where water has been extracted for consumptive use. For the purpose of
this report this involved subtracting the licensed water use for 2003 from the entire ‘natural’ flow record
produced by hydrologic catchment models.
18
4.1 Natural Flow Estimation
Natural flow was estimated for the environmental flow assessment locations and for the water
management subregions. This was done by calibrating the natural flow output with observed
winter flow at the South Esk River at Perth. Observed winter flow was assumed to be
representative of natural flow with minimal impact by water abstractions, although this may not
be strictly true. The result of the calibrated data is shown in Figure 4.1a. Time series plots
(Figure 4.1a) of the modelled and observed flow for an average rainfall year (1977) show that
the South Esk catchment model gives a fair representation of the natural flows at South Esk
River at Perth. The daily flow regression coefficient value (modelled flow vs observed flow) at
the calibration site was around 66%. The regression coefficient value is further improved to
85% if comparison is made between the monthly flow data. Overall, there is a general tendency
for the model to underestimate or overestimate flows depending on the state of catchment
saturation in any given year. The model predicts flows more accurately during wetter years,
when the losses mentioned in Section 3.3 are low.
Although modelled data from 1901 is available, data from 1960 onward was used for the
analyses. This period of data was used for the following reasons
• the rainfall and evaporation data prior to 1960 is less reliable
• the period 1960 to 2003 is more representative of the prevailing climatic conditions.
Prior to 1960 data reflects a wetter climate.
Duration analysis of the daily flow for the period 1960-2003 in Figure 4.1b indicates that at
exceedance greater than 60%, natural flow and observed flow correspond reasonably well.
Current flow is markedly less in this region of the duration curve, and this is likely to be a result
of the manner in which allocation data is used in the modelling process, as mentioned in Section
4.0. In addition, current flow is derived by subtracting water usage data for 2003 from the
entire period of the natural flow record (1960-2003). However, historical extraction from the
river would have increased from a low level in the 1960’s to the current level. Application of
the 2003 abstraction data to the entire record accentuates the differences between both natural
and current flow, and between current and observed flow.
In addition, the current quantity of water extracted from the catchment is unverified. It relies on
water licence information (WIMS, 2003) for estimates of extractions, and these may not
represent the true quantity of water being extracted. It also needs to be pointed out that the
modelled current flow does not take into account periods of water restriction, which will result
in observed flows during dry periods (exceedance greater than 90%) being closer to modelled
natural flow.
All of the issues highlighted above must be borne in mind when viewing the disturbance indices
in the next section and the plots in Figure 5.1.
19
Figure 4.1 Time series and flow duration analysis of the, current, observed and natural flow data
for South Esk River at Perth.
20
4.2 Hydrological Disturbance Indices
Indices of hydrological disturbance provide and assessment of the change in hydrology due to
water abstraction in the catchment. These indices were derived from a comparison of the natural
and current flows estimated from the NAP catchment model (Table 4.1). The indices were
calculated using the formulas stated in the Natural Resource Management (NRM) Monitoring
and Evaluation Framework developed by SKM for the Murray-Darling Basin (SKM, 2003).
Table 4.1 Indices of hydrological disturbance at selected water management subregions of the
South Esk catchment.
Hydrologic Disturbance Indices Longford Neck of Bottle Glen esk Ormley Malahide Glen Mavis Benham
Mean Annual Flow Index 0.96 0.96 0.99 0.99 0.99 0.95 0.99
Flow Duration Curve Difference Index 0.81 0.82 0.91 0.92 0.94 0.69 0.86
Seasonal Amplitude Index 0.94 0.94 0.98 0.98 0.99 0.86 0.98
Season Period Index 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Hydrological Disturbance Index 0.90 0.90 0.95 0.96 0.97 0.83 0.93
Index of Mean Annual Flow: This provides a measure of the difference in total flow volume
between current and natural conditions. It is calculated as the ratio of the current and natural
mean annual flow volumes and assumes that increases and reductions in mean annual flow have
equivalent impacts on habitat condition.
Index of Flow Duration Curve Difference: The difference from 1 of the proportional flow
deviation, averaged over p monthly flow percentile point. A measure of the overall difference
between current and natural monthly flow duration curves. All flow diverted would give a score
of 0.
Index of Seasonal Amplitude: The change in amplitude of the seasonal pattern of monthly
flows. It is defined as the average of two current: natural ratios, firstly, that of the highest
monthly flows, and secondly, that of the lowest monthly flows based on calendar month means.
Index of Seasonal Periodicity: The change in seasonal timing of flows. It is defined as the
difference from 1 of one twelfth of the sum of the absolute values of the differences between
current and natural of first, the numerical values of the months with the highest mean monthly
flows, and second, the numerical values of the months with the lowest mean monthly flows.
Hydrological Disturbance Index: This provides an indication of the hydrological disturbance
to the river’s natural flow regime. A value of 1 represents no hydrological disturbance, while a
value approaching 0 represents extreme hydrological disturbance.
21
5.0 Flow Characteristics at Environmental Flow Assessment Locations
5.1 Environmental Flow Assessment Locations.
Six sites were chosen in the South Esk River catchment for the purposes of environmental flow
assessment (shown in Figure 5.1, grid locations listed in Table 5.1). These sites were selected
as representative of the physical river characteristics of the main subcatchment areas. Natural
and current flow was generated from the hydrological model for the six environmental flow
assessment locations.
Figure 5.1. Environmental flow assessment locations in the South Esk River catchment.
Table 5.1 Approximate location of environmental flow assessment sites in the South Esk River
catchment.
Site_ID Easting Northing Upstream Area (km2)
South Esk @ Malahide (u/s Pig Creek) 583250 5399770 726
South Esk @ Ormley 568030 5380770 1370
South Esk @ Glen Esk 540550 5375230 2360
South Esk @ Neck of Bottle 521700 5394070 3165
Nile @ Glen Mavis 529350 5389930 255
St Pauls @ Benham 561900 5372400 293
5.2 Natural and Current Flow Characteristics
A summary of natural and current flow characteristics at the environmental flow assessmentlocations are presented in Table 5.2.
22
Table 5.2
Longte
rm a
ver
age
flow
(M
L/d
) ch
arac
teri
stic
s under
nat
ura
l an
d c
urr
ent
wat
er u
se c
ondit
ions
at t
he
six e
nvir
onm
enta
l fl
ow
asse
ssm
ent lo
cati
ons
in the
South
Esk
Riv
er c
atch
men
t. F
low
dat
a an
alyse
d f
or
the
reco
rd p
erio
d 1
960-2
003.
Neck of Bottle
Glen Esk
Ormley
Malahide
Glen Mavis
Benham
NaturalCurrent
NaturalCurrentNaturalCurrent
NaturalCurrent
NaturalCurrent
NaturalCurrent
Mean
2392
2312
1735
1711
1347
1329
962
952
317
302
180
177
Median
962
878
660
637
510
491
358
346
120
104
42
40
CV
2.1
2.2
2.4
2.4
2.4
2.4
2.4
2.4
2.1
2.2
3.9
4.0
Daily Minimum
7.7
0.0
5.4
0.0
4.1
0.0
2.3
0.0
0.6
0.0
0.4
0.0
Daily Maximum
154605
154499
149684
149659
116335
116310
88178
88163
14906
14889
24377
24375
10th%ile
146
83
100
80
71
58
45
40
14
17
4
30th%ile
426
356
298
277
221
206
149
142
45
31
19
17
90th%ile
5517
5418
3793
3766
3014
2992
2200
2185
769
752
331
328
Jan
763
702
581
563
427
416
264
260
72
59
86
84
Feb
536
476
416
397
302
291
170
167
49
37
60
58
Mar
649
589
539
520
396
384
228
224
56
44
91
89
Apr
1342
1282
1081
1061
875
863
576
572
159
147
133
131
May
2703
2617
2016
1990
1604
1582
1165
1150
357
342
223
220
Jun
3769
3677
2786
2760
2107
2085
1477
1462
489
472
334
331
Jul
4678
4583
3327
3301
2653
2630
1986
1971
671
654
318
315
Aug
5613
5517
3898
3872
3031
3008
2267
2252
829
812
348
345
Sep
3978
3882
2740
2713
2066
2043
1527
1512
537
520
248
245
Oct
2086
1988
1455
1429
1149
1126
854
839
289
272
111
109
Nov
1271
1179
946
920
758
735
517
502
151
134
75
72
Dec
1177
1114
935
914
722
710
450
446
125
111
120
118
Winter
3804
3711
2704
2678
2101
2079
1546
1531
529
512
264
261
Summer
956
890
750
729
580
567
368
362
102
89
94
92
23
5.3 Flow Duration Analysis
A flow duration curve (FDC) is a plot of discharge vs. percent of time that a particular
discharge was equalled or exceeded. An FDC represents the relationship between the magnitude
and frequency of daily, weekly, monthly or yearly streamflow for a particular river basin,
providing an estimate of the percentage of time a given streamflow was equalled or exceeded
over a historical period. The area under the flow duration curve gives the average daily flows
for the natural and current conditions, and the median daily flow is the 50% value. An FDC
provides a simple, yet comprehensive, graphical view of the overall historical variability
associated with streamflow in a river basin.
A series of FDCs have been generated for the six environment flow assessment locations in the
South Esk River catchment (Figure 5.1). These curves compare long-term duration
characteristics of natural and current flow conditions.
A FDC also characterizes the ability of the basin to provide flows of various magnitudes. Table
5.2 provides a summary of the extracts from the FDC showing thresholds of natural flow at
varying probability of exceedance. Information concerning the relative amount of time that
flows past a site are likely to equal or exceed a specified value of interest is extremely useful for
the design of structures on a stream. For example, a structure can be designed to perform well
within some range of flows, such as flows that occur between 20 and 80% of the time (or some
other selected interval).
Table 5.2 Probability of exceedance (POE) of natural flows (ML/day) at the six environment
flow assessment locations in the South Esk River catchment
POE (%) Neck of Bottle Glen Esk Ormley Malahide Glen Mavis (Nile) Benham (St Pauls)
5 9135 6516 5232 3981 1355 567
10 5517 3793 3014 2200 769 331
20 3027 2030 1511 1039 375 170
30 2022 1360 1010 697 239 100
40 1385 945 727 514 172 63
50 962 660 510 358 120 42
60 649 449 346 240 75 28
70 426 298 221 149 45 19
80 267 188 139 90 26 12
80 267 188 139 89 26 12
90 146 100 71 45 14 7
95 90 61 44 28 9 4
99 46 31 22 14 5 2
The shape of a FDC in its upper and lower regions is particularly significant in evaluating the
stream and basin characteristics. The shape of the curve in the high-flow region indicates the
type of flood regime the basin is likely to have, whereas, the shape of the low-flow region
characterizes the ability of the basin to sustain low flows during dry seasons. A very steep curve
(high flows for short periods) would be expected for rain-caused floods on small watersheds. In
the low-flow region, an intermittent stream would exhibit periods of no flow, whereas, a very
flat curve indicates that moderate flows are sustained throughout the year due to natural or
artificial streamflow regulation, or due to a large groundwater capacity which sustains the base
flow to the stream.
The general effect of water abstractions and various landuse practices in the catchment is to
depress the natural flow scenario at low flow end of the FDC. Figure 5.1 shows the impact on
natural flow by water abstraction alone. The FDCs for all the environment flow assessment
24
locations show depression of natural flows at probability of exceedance greater than 60%,
however the least impact is displayed at the South Esk at Malahide site.
This is typical of many unregulated rivers in Tasmania, where the impact of water abstraction is
mostly on the low flow part of the flow regime. This is not surprising as summer irrigation
coincides with the low flow period for most Tasmanian rivers. Generally, the flow regime in
most of Tasmania’s unregulated rivers is close to natural apart from the low flow component,
and hence the focus of management is on sharing of water between the environment and
consumptive use during drier times.
Figure 5.1 Flow duration curves for the six environment flow assessment locations in the South
Esk River catchment.
Flow Duration Curve for South Esk at Neck of Bottle (1960-2003)
1
10
100
1000
10000
100000
1000000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
Flow Duration Curve for South Esk at Glen Esk (1960-2003)
1
10
100
1000
10000
100000
1000000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
Flow Duration Curve for South Esk at Ormley (1960-2003)
1
10
100
1000
10000
100000
1000000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
Flow Duration Curve for South Esk at Malahide (1960-2003)
1
10
100
1000
10000
100000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
Flow Duration Curve for Nile at Glen Mavis (1960-2003)
1
10
100
1000
10000
100000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
Flow Duration Curve for St Pauls at Benham (1960-2003)
1
10
100
1000
10000
100000
0 20 40 60 80 100
Percentage of time discharge was equalled or exceeded
Discharge (ML/d)
Natural Flow
Current Flow
25
6.0 Catchment and Subregion Water Budget
6.1 Catchment Water Allocations
The distribution of current consumptive water allocations in the South Esk River catchment is
shown in Figure 6.1. The bulk of the allocation lies within the Lower Esk subregion. The
current total annual consumptive water allocation in the South Esk River catchment is
44,415 ML. A summary of the water usage and subregion distribution of allocation is given in
Table 6.1, and shows that 55% of the volume of allocated water is used in the Lower Esk
subregion.
Figure 6.1. Distribution of water allocations in the South Esk River catchment.
Table 6.1. Summary of the water usage and subregion distribution of allocation
Subregions Area (km2) Allocation (ML/Year) Purpose Allocation (ML/Year)
Upper Esk 1016 6845 Irrigation 43002
Avoca-Break O'Day 514 3497 S & D 614
St Pauls River 524 1470 Water Supply 485
Nile River 318 7946 Industrial 314
Lower Esk 972 24658 Direct Take 21906
Total 3345 44415 Storage 22509Allocation excludes storage for aesthetic and recreation.
26
6.2 Flood Take Rules – South Esk Basin
Under an Act of Parliament, all of the water in the Tamar River Basin is managed to support
hydro-electric power generation by Hydro Tasmania. Under this management framework, some
water is made available for agricultural production. Through a memorandum of understanding
(MOU) developed in 2004 between Hydro Tasmania, the Tasmanian Farmers and Graziers
Association and the Department of Primary Industries, Water and Environment (now DPIW),
Hydro Tasmania recognised that other water users can take additional water from the South Esk
catchment during times of flood. In this MOU, Hydro Tasmania set out a number of rules by
which other water users in the catchment (mainly agricultural) can take water without affecting
the ability of Hydro Tasmania to maximise power production at Trevallyn Power Station.
These rules take the form of summer and winter ‘trigger flows’ at agreed locations, and in the
South Esk River, the trigger point is located at the ‘Llewellyn’ streamflow monitoring station
(Table 6.2). At this location, when flow at this station exceeds 20.3 m3.s
-1 (1,750 ML.day
-1) in
winter and 23.4 m3.s
-1 (2,450 ML.day
-1) in summer, flood extraction throughout the catchment
can occur for 5 days. This period can be extended based on the discretion of Hydro Tasmania,
and is dependant on continuing spill of Lake Trevallyn.
This management rule should be kept in mind when viewing the data regarding water use and
water availability.
Table 6.2. The “trigger flow” for the South Esk River at Llewellyn, which determines
when flood water can be extracted from the river system
Summer trigger Winter triggerSite Name
m3.s
-1ML.day
-1m
3.s
-1ML.day
-1
South Esk River at Llewellyn 23.4 2,450 20.3 1,750
6.3 Catchment Water Yield
The average annual natural flow yield of the South Esk River at Longford is 887,658 ML with
an average daily outflow of 2,430 ML/day. Table 6.3 shows a summary of the natural flow
yields for the South Esk River (at Longford) and its major tributaries. About 80% of the total
natural flow yield at Longford is contributed by the winter runoff mainly from the upper
reaches of the catchment. The major tributaries (Nile, St Pauls and Break O’Day Rivers)
together contribute about 28% of the total catchment yield.
Monthly flows are generally highest during August and lowest during February for all the
subregions. The Upper Esk shows the highest monthly and seasonal yields and this is expected
as maximum runoff occurs in this part of the region. Although the Nile River catchment has a
smaller catchment area (318 km2) than the St Pauls River (524 km
2), its winter yields are greater
by twice the amount.
27
Table 6.3 Distribution of average monthly, seasonal and annual yields (ML) in the South Esk
River catchment. Also included in the table is the annual allocation (ML).
Upper Esk Avoca-Break O'Day St Pauls River Nile River Lower Esk Catchment
Jan 11346 2896 2663 2265 8008 24515
Feb 7120 2125 1697 1409 4825 15479
Mar 10495 2591 2819 1704 5320 20109
Apr 23585 3910 3990 4635 7418 39549
May 44327 8499 6898 10873 20898 84598
Jun 53459 14992 10011 14695 31290 114436
Jul 72250 15678 9852 20733 38368 147029
Aug 80510 21727 10785 25654 48936 176827
Sep 52347 15846 7447 16337 38687 123217
Oct 30829 7868 3452 9097 18744 66538
Nov 20005 4514 2243 4556 9774 38850
Dec 19408 4501 3726 3864 8737 36510
Winter Yield (May-Oct) 333723 84610 48447 97389 196923 712646
Summer Yield (Nov-Apr) 91959 20538 17139 18434 44082 175013
Annual Yield 425682 105148 65585 115823 241005 887658
Annual Allocation 6845 3497 1470 7946 24658 44415
Figure 6.2 shows the catchment yields and relative distribution of water allocations within the
water management subregions. The Upper South Esk, Avoca-Break O’Day and St Pauls River
subregions show relatively lower water allocations compared to summer yields.
1000
10000
100000
1000000
Upper Esk Avoca-Break
O'Day
St Pauls
River
Nile River Lower Esk Catchment
Total Yield (ML)
Annual Yield Winter Yield (May-Oct) Summer Yield (Nov-Apr) Annual Allocation
Figure 6.2 Distribution of yields and current allocations in the water management subregions.
28
The seasonal and annual specific yields of the water management subregions are shown in
Figure 6.3 and compared to the whole of the catchment upstream of Longford. Both the Upper
Esk and Nile River subregions show significantly higher specific yields than the remainder of
the subregions. The Lower Esk subregion shows similar yield capability to that of the whole
catchment.
0
100
200
300
400
500
Upper Esk Avoca-Break
O'Day
St Pauls
River
Nile River Lower Esk Catchment
ML//day/km2
Summer Winter Annual
Figure 6.3 Distribution of summer, winter and annual specific yields in the water management
subregions.
The ability to convert maximum runoff to river flow by a catchment is best shown by analysing
the monthly specific yields (Figure 6.4). The figure shows that the Upper Esk and the Nile
River are the most productive subregions. Both these subregions show peak specific yields of
around 80 ML/day/km2 during mid-winter period. Winter specific yields are generally less than
20 ML/day/km2 for the St Pauls River subregion.
29
0
10
20
30
40
50
60
70
80
90
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
ML/day/km2
Upper Esk
Avoca-Break O'Day
St Pauls River
Nile River
Lower Esk
Figure 6.4 Monthly specific yields in the water management subregions.
The overall catchment water budget for the South Esk River catchment is given in Table 6.4.
The total average annual yield for the entire catchment up to Longford is approximately
887,658 ML with current annual water allocation of 44,415 ML.
Table 6.4 Distribution of annual yield (ML) in the water management subregions.
Subregions Annual Yield (ML) Annual Allocation (ML) Available Yield# (ML)
Upper Esk 425682 6845 418838
Avoca-Break O'Day 105148 3497 101651
St Pauls River 65585 1470 64116
Nile River 115823 7946 107877
Lower Esk 241005 24658 216347
Catchment 887658 44415 843244
#Yield subject to Hydro Tasmania's MOU on flood take rules.
Currently, about 5% of the total catchment yield is allocated water. Approximately
840,000 ML of catchment yield is available for resource allocation subject to licensing
agreements on flood takes rule for Trevallyn Dam. Future utilisation of water resources in the
catchment will need to focus on the development of management scenarios that include
groundwater resources in the catchment water budget.
Currently 44,415 ML allocated for agricultural and other consumptive uses out of a total of
887,658 ML. Hydro Tasmanias flood take rules stipulate that water cannot be taken until the
triggers have been crossed. This means that of the total annual yield from the catchment,
approximately 555,000 ML is protected for power generation, 44,415 ML is currently allocated
for agricultural and other consumptive uses and 287,000 ML contained in flows above the
trigger levels.
30
References
HEC 2005a. NAP Region Hydrological Model South Esk Catchment. Report 118783-2. Hydroelectric
Corporation: Hobart, Tasmania.
HEC 1999. South Esk – Great Lake Hydro Catchment
SRA 2003. Hydrology Theme Pilot Audit Technical Report – Sustainable Rivers Audit. MDBC
Publication 08/04. Murray Darling Basin Commission: Canberra, ACT.
SKM (2003) Sustainable Rivers Audit Hydrology Theme – Ovens River Basin Hydrology Report,
Murray-Darling Basin Commission. Sinclair Knight Mertz.
Recommended