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National Water Account 2015

Sydney: Climate and water overview

Warragamba Dam, New South Wales (iStock © Kokkai Ng)

Climate conditions

Rainfall

The total area-averaged rainfall over the Sydney region during the 201415 year was 1,052 mm, which is above the long-term area-averaged rainfall of 890 mm (based on the 19002015 period). Annual rainfall ranged from more than 1,800 mm to less than 900 mm. Rainfall was relatively high close to the coast and decreased gradually towards the west (Figure C11).

 

Figure C11 Total annual rainfall for the Sydney region during the 2014–15 year
Figure C11 Total annual rainfall for the Sydney region during the 2014–15 year

 

During the 2014–15 year, rainfall was above average across most of the Sydney region and average elsewhere (Figure C12). These conditions represent an increase in rainfall compared to the 201314 year, when rainfall was below average to very much below average across most of the region (see the 2014 Account). As a result, there was increased runoff into the storages during the 201415 year (see the Surface water note).

 

Figure C12 Annual and monthly rainfall deciles for the Sydney region during the 2014–15 year

 

Climate across Australia was largely influenced by persistent near El Niño conditions in the Pacific Ocean from late 2014 and the Bureau of Meteorology declared an El Niño event in May 2015.

In New South Wales, rainfall was above average along the east coast. The area-averaged monthly rainfall for the Sydney region (Figure C13) reached or exceeded the 90th percentile for four months of the year and was around the 10th percentile for two months. Overall, the four very wet months contributed to the higher runoff in Shoalhaven River. Also, runoff volume in Colo River exceeded the 90th percentile in April 2015. 

 

Figure C13 Total monthly rainfall for the Sydney region during the 2014–15 year compared with the long-term average and percentiles for the region
Figure C13 Total monthly rainfall for the Sydney region during the 2014–15 year compared with the long-term average and percentiles for the region

 

Evapotranspiration

The total area-averaged potential evapotranspiration over the Sydney region during the 201415 year was 1,496 mm compared with the long-term area-averaged potential evapotranspiration of 1,504 mm (based on the 19112015 period). Potential evapotranspiration was between 1,200–1,400 mm and was uniform for most of the region, with patches of slightly higher or lower potential evapotranspiration elsewhere (Figure C14).

 

Figure C14 Total annual potential evapotranspiration for the Sydney region during the 2014–15 year
Figure C14 Total annual potential evapotranspiration for the Sydney region during the 2014–15 year

 

Potential evapotranspiration was average across most of the Sydney region, tending to below average, and very much below average towards the east and along the coastal boundary (Figure C15).

 

Figure C15 Annual potential evapotranspiration deciles for the Sydney region during the 2014–15 year
Figure C15 Annual potential evapotranspiration deciles for the Sydney region during the 2014–15 year

 

Soil moisture

Figure C16 shows that soil moisture, in the root zone (0 to 1m depth) for the 2014–15 year was average to above average in the Sydney region (compared with the 1970–2015 period). 

 


Figure C16 Annual and monthly soil moisture deciles in the root zone (0 to 1m depth) for the Sydney region during the 2014–15 year

 

Figure C16 also shows monthly soil moisture deciles in the root zone (0 to 1m depth) during the 2014–15 year. Comparison of monthly rainfall and soil moisture deciles show the response of soil moisture to changes in rainfall and align closely for the majority of months in the 2014–15 year.

The very much above average rainfall and  average soil moisture conditions in Shoalhaven catchment during August 2014 have contributed to above 90th percentile flow volume experienced in some reaches of the Shoalhaven River. Similarly, very much above average rainfall and above average soil moisture conditions in Hawkesbury–Neapean River catchment during April 2015 caused above 90th percentile streamflow in the Colo River (See Streamflow). 

More information on soil moisture distribution across the Sydney region is available in the Australian Landscape Water Balance.

 

Streamflow responses

Significant water events

During April 2015, most of the Hawkesbury–Neapean River catchment experienced a high rainfall event that was very much above average and also the highest on record in some parts (Figure C11).  This resulted in high streamflows in Colo River (Figure C18). The rainfall in April did not have a notable impact on the soil moisture and streamflow of the Shoalhaven River catchment located further south in the region.

 

Streamflow

Figures C17 and C18 show streamflow pattern, volumes and variation at two major rivers in the region:

  • Shoalhaven River at Fossickers Flat in the Shoalhaven catchment (Station 215207)
  • Colo River at Upper Colo in the Hawkesbury–Nepean catchment (Station 212290).

The two gauging stations are shown in Figure C6 and were selected for the following reasons:

  • long-term flow data are available
  • flows upstream of both river locations are unregulated and diversions are negligible
  • both are major rivers in terms of flow volumes.

The Shoalhaven River experienced flows that exceeded the 90th percentile volumes in three months of the 2014–15, with generally average to below average flows in the other months (Figure C17). Total annual flow for the Shoalhaven River for the 2014–15 year was above the long-term average for the period 1977–78 to 2014–15.

The Colo River experienced below average flows throughout the 2014–15 year except for April 2015 which easily surpassed the 90th percentile flow volume (Figure C18). Total annual flow for the Colo River for the 2014–15 year was approximately 75% of the long-term average for the period 1975–76 to 2013–14.

 

Figure C17 Total monthly flow along the Shoalhaven River during the 2014–15 year compared with the long-term average and percentiles for the river
Figure C17 Total monthly flow along the Shoalhaven River during the 2014–15 year compared with the long-term average and percentiles for the river
 

 

Figure C18 Total monthly flow along the Colo River during the 2014–15 year compared with the long-term average and percentiles for the river
Figure C18 Total monthly flow along the Colo River during the 2014–15 year compared with the long-term average and percentiles for the river

 

Major water reforms

Organisational change

State Water and Sydney Catchment Authority were amalgamated on 1 January 2015 to become WaterNSW. WaterNSW is responsible for developing infrastructure solutions for improved water supply and reliability, protecting water quality in its designated catchments, catchment protection in the Greater Sydney drinking water catchments, asset management and flood operations and mitigation.