Dealing with salinity in Wheatbelt Valleys - Department of Water

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Hatton and Ruprecht water from one to another when and if they overflow. Grades along the Lockhart and Pingrup rivers (the southern tributaries of Salt River) are very low (0.04 m/km and 0.24 m/km respectively), and from this Beard (1999) concluded that significant discharges are unlikely (however, see Flooding section below). Unlike the valleys of most rivers, which usually broaden downstream, the valley of the Avon is wide near its source (77 km) and narrows to 5 km or less after Toodyay. These broad shallow valleys of the upper Avon are characteristic of the wheatbelt rivers. The flatness of the bulk of the wheatbelt river systems leads to historic, and amusing, arguments regarding catchment boundaries, the putative connections between systems, and even which way water flows. It is essential to appreciate that these river systems do not all flow as one linked system except in the most extreme events. In the Blackwood catchment, the Coblinine River and Dongolocking Creek are two headwater streams draining to Lake Dumbleyung. This section of the river has very low grades, approximately 0.17 m/km. The combined drainage enters Lake Dumbleyung, which is a permanent salt lake that is said to have been dry before land clearing (Beard 1999). Since clearing, Lake Dumbleyung is thought to have overflowed into the Lower Blackwood only three times since the 1870s. The chains of (mostly dry) lakes form a series of local storages that in most years are not overtopped by the surface flows from upstream. Commander et al. (2001, this proceedings) gives the geological history and background to the evolution of the wheatbelt systems, and George & Coleman (2001, this proceedings) provides a description of the hydrogeology with special reference to regolith Station No River salinity. The key feature of the latter issue is that the flat, inland portions of these catchments hold massive amounts of salt, accumulated as a result of atmospheric deposition of marine salts, low rainfall, the natural patterns of native vegetation water use, highly weathered regolith, and the low gradients described above. Vegetation The wheatbelt catchments were essentially completely vegetated by a diverse range of woody plant communities whose distribution was controlled by climate and soil type (Beard 1981). About 65% of the Avon catchment has been cleared for agriculture with most clearing taking place between 1940 and 1970. However, many catchments in the upper Avon and Blackwood rivers have cleared proportions ranging from 85 to 95%. HYDROLOGY In describing the hydrology of the wheatbelt catchments, it is important to realise that their hydrology is characterised by high variability and nonstationarity. The rivers of this region have higher variability of streamflow than others worldwide (McMahon et al. 1992), indicating a usually unpredictable climate from year to year and season to season (Table 1). The smaller rivers may not flow for many years, then either a major summer event or a wet winter will lead to flow events. The larger rivers cease flowing in the wheatbelt regions during summer, except when extreme tropical cyclonic events or severe thunderstorm activities lead to heavy, intense summer rainfall. In “normal” years, winter rainfall is not enough to move water continuously through the catchment from the extreme eastern and southern boundaries to the coast. Table 1: Comparison of hydrology statistics for Avon River tributaries. Area (km 2 ) Mean annual rainfall (mm) Clearing (%) Mean annual flow (ML) Median annual flow (ML) Mean annual runoff (mm) 615012 Lockhart River 32,000 350 85 7,900 1,960 0.24 1.7 615015 Yilgarn River 56,000 300 50 6,500 890 0.12 2.1 615022 Yenyenning 92,000 340 70 12,980 87 0.1 1.4 – 4 – CV
Effect of clearing on runoff The other key feature of the hydrology, nonstationarity, refers to the fact that the underlying hydrological processes in these systems are still undergoing profound but subtle changes as a result of historic clearing. These changes include increasing runoff source areas due to rising regional groundwater, soil acidification, and decadal climate variability. In the longer-term, climate change may lead to decreased winter rainfall, increased summer Mean annual runoff (mm) 25 20 15 10 5 Hatton and Ruprecht rainfall and possibly more extreme events. The hydrological systems today are neither the same as they were on settlement nor as they will be into the future. Median annual streamflow for many of the smaller catchments in the Wheatbelt would have been close to zero prior to settlement. For the Avon catchment the annual average streamflow prior to clearing was estimated to be 18% of the current water yield, approximately consistent with experimental results in Ruprecht & Schofield (1991) (Figure 5). 0 0 20 40 60 80 100 Clearing (%) Figure 5: Mean annual runoff for wheatbelt rivers (mean annual rainfall less than 460 mm) as a function of clearing. As a percentage, the runoff equates to less than 5% and in most cases
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Foreword Dealing with Salinity in W
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1Day one: Monday 30th July. Field t
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Papers Dealing with Salinity in Whe
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Ali and Coles biggest challenges fa
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Ali and Coles erosion, maintenance
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Ali and Coles expected under these
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Ali and Coles Coles, N.A. & Ali, M.
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Commander, Schoknecht, Verboom and
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Page 51 and 52: REFERENCES Agstats (1997). Agricult
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Page 85 and 86: THE FUTURE In looking at the future
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Page 105 and 106: INTRODUCTION The Lake Chinocup Catc
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Pannell been conducted for the Cent
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Pannell investments in long-term la
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Pannell Pannell, D.J. (2001). Dryla
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Porter, Bartle and Cooper Table 1:
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Porter, Bartle and Cooper operation
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Porter, Bartle and Cooper Increase
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Porter, Bartle and Cooper support i
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Porter, Bartle and Cooper that is a
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CASE STUDY - TOOLIBIN LAKE AND CATC
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• The distribution, storage and m
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Secondly, there is a set of practic
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within three categories: relationsh
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CONCLUSIONS Actions to protect and
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FIELD TOUR LOCATIONS FIELD TOUR NOT
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COLES FARM “Avon River Basin Over
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Conference Outcomes Dealing with Sa
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with Salinity in Wheatbelt Valleys:
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with Salinity in Wheatbelt Valleys
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Photo Gallery 2 Dealing with Salini
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APPENDIX 1 BACKGROUND HYDROLOGY AND
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WHEATBELT HYDROLOGY Flow in the Yil
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Flow (ML) STREAM SALINITY Salt Rive
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APPENDIX 2 WATER MANAGEMENT OPTIONS
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APPENDIX 3 MANAGING WATER AND SALIN
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SUMMARY- TIMELINE OF RECENT SIGNIFI
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RECENT HISTORY OF WATER AND SALINIT
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Figure 1. Hydrological cross-sectio
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Depth (m) 316 315 314 313 312 311 M
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FURTHER INVESTIGATIONS AND MANAGEME
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Dealing with salinity in Wheatbelt Valleys - Department of Water

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