Published by Murray-Darling Basin Authority Postal Address gpo box 1801, Canberra act 2601



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4.Hydrology


Significant changes have occurred to the Wimmera River Terminal Wetlands’ hydrology due to development and water extractions upstream in the Wimmera catchment. The description of hydrological changes compared to without development conditions below is comprised of two components – the first is the change to hydrology prior to construction of the Wimmera Mallee pipeline; and the second is the change to hydrology post Wimmera–Mallee pipeline construction.

The Wimmera–Mallee Pipeline Project (WMPP) was a large-scale project that replaced about 17,700 km of the highly inefficient Wimmera–Mallee stock and domestic open channel system with a piped water distribution system. The project began in 2001 and was completed in early 2010. The WMPP resulted in water savings of about 103 GL a year, with 83 GL of this allocated to the environment.


4.1.Lake Hindmarsh


Ecological Associates (2004) modelled historical without development and pre WMPP hydrology of Lake Hindmarsh for the period 1903-2000 (Table 2). Under modelled without development conditions, the lake was always above the shallow water level, filling 35 times in 97.5 years, or once every 2.8 years on average. In addition, the lake made the full +2 m level 25 times in the 97.5-year period, for an average duration of nearly three months. Both the full and full +2 m levels would have watered the fringing woodland communities once every three years on average.

Table 2 Spell analysis of Lake Hindmarsh volume thresholds under without development and pre Wimmera Mallee Pipeline construction conditions for the period 1903-2000 (Source: Ecological Associates 2004).

Scenario

Thresholda

Lake level below threshold

Lake level above threshold




Number of events (in 97.5 years)

Average spell duration (months)

Maximum spell duration (months)

Number of events (in 97.5 years)

Average spell duration (months)

Maximum spell duration (months)

Without development

Empty

(always above threshold)

(always above threshold)

Shallow

(always above threshold)

(always above threshold)

Full

36

9.2

55

35

24.0

92

Full +2 m

26

42.4

226

25

2.7

6

Pre Wimmera Mallee Pipeline

Empty

14

9.0

19

14

74.6

329

Shallow

10

34.8

94

9

91.3

306

Full

22

43.6

366

21

10.0

32

Full +2 m

4

290.3

642

3

3.0

4

a For Lake Hindmarsh: empty = 10 GL; shallow = <80 GL; full = 378 GL (spill level into Outlet Creek); full +2 m = 630 GL + level required to flow in Outlet Creek at 57 GL/m.

Modelling indicates that under pre WMPP conditions, the lake filled far less often and for a shorter duration. Instead of being above the shallow threshold all the time, the lake would have dried to empty. Lake-full frequencies dropped to once every 4.6 years on average, with average durations shortening by over half. Larger flows (full +2 m) showed an even greater (92%) reduction in occurrence. Dry spells also increased, with the lake failing to reach full for 22 years after 1928, affecting the health of the fringing river red gum communities.

MDBA has undertaken analysis similar to Ecological Associates (2004) of Lake Hindmarsh key thresholds under without development and post Wimmera Mallee Pipeline conditions for the period 1895-2009 (Table 3). MDBA modelling of without development conditions indicates that some significant changes in the hydrological regime of Lake Hindmarsh occur by extending the modelling period to include the Federation Drought and the more recent Millennium drought post 1996. Significantly, Lake Hindmarsh would have dried during the Millennium Drought under modelled without development conditions. Lake full and lake full + 2m events results are similar in terms of the number and duration of events to those reported by Ecological Associates (2004); however, the maximum spell between lake full events more than doubles to 138 months or 11.5 years.

Extending the modelling period to include the Federation and Millennium Droughts provides a highly relevant insight to the hydrology of Lake Hindmarsh during extreme drought conditions. However, direct comparison between pre and post WMPP using analysis presented within Table 2 and Table 3 is difficult. Table 3 indicates that compared to without development conditions, Lake Hindmarsh post WMPP still experiences an increase in the number, average duration and maximum duration of empty and shallow events with a corresponding decrease in lake full events. However, some benefits associated with the WMPP are evident for example the maximum duration between full and full + 2m events decreases compared to pre WMPP conditions. Further analysis of changes to key ecologically relevant thresholds pre and post WMPP for Lake Hindmarsh is presented in Section 5.2.



Table 3 Spell analysis of Lake Hindmarsh volume thresholds under without development and post Wimmera Mallee Pipeline construction conditions for the period 1895-2009 (Source: MDBA analysis).

Scenario

Thresholda

Lake level below threshold

Lake level above threshold




Number of events (in 114 years)

Average spell duration (months)

Maximum spell duration (months)

Number of events (in 114 years)

Average spell duration (months)

Maximum spell duration (months)

Without development

Empty

3

10

17

3

446

1327

Shallow

2

38

67

2

646

1289

Full

40

12

138

40

22

68

Full +2 m

26

50

226

26

3

6

Post Wimmera Mallee Pipeline

Empty

4

14

43

4

328

1292

Shallow

16

11

91

16

74

415

Full

35

24

320

35

15

44

Full +2 m

12

112

393

11

2

5




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