National Water Account 2016

Sydney: Climate and water

Annual rainfall in the region was above average, which is largely attributed to heavy rainfall associated with several east coast lows that occurred during the 2015–16 year. These events resulted in very high streamflows and flooding across parts of the region. Rainfall and streamflow during the majority of the year were below average.

 

 

Introduction

The Sydney region has a temperate climate. Rainfall occurs throughout the year with most rainfall falling during the summer and autumn months. Higher streamflows typically occur during these months.

See Water resources in 'Region description' for more information.

 

Climate conditions

Rainfall

Throughout most of the 12 month period, rainfall was very much below average across much of southeastern Australia. For most of this period, up until late March 2016, Australia's climate was largely influenced by one of the stronger El Niño events since 1950. El Niño is usually associated with below average rainfall for southeastern Australia. Following the breakdown of the El Niño, a strong negative phase of the Indian Ocean Dipole developed, contributing to two very wet months during May–June 2016.

The total area-averaged rainfall over the Sydney region during the 2015–16 year was 934 mm, which is above the long-term area-averaged rainfall of 891 mm (based on the 1900–2016 period). Annual rainfall ranged from more than 1,200 mm along the coast to less than 900 mm in the western part of the region (Figure C1).

 

Figure C1 Total annual rainfall for the Sydney region during the 2015–16 year

Figure C1 Total annual rainfall for the Sydney region during the 2015–16 year

 

Rainfall across most of the region was average for the 2015–16 year; rainfall around Sydney and the southern part of the region was above average (Figure C2).

 

Figure C2 Annual and monthly rainfall deciles for the Sydney region during the 2015–16 year

 

The above-average rainfall across the Sydney region during the 2015–16 year is largely attributed to several east coast low events that occurred during the year. The east coast low event that occurred on 25–26 August 2015 mainly affected the southeastern part of the region (Figure C2). At some locations in the area, more than 430 mm of rainfall was observed in the 48-hour period. The east coast low events on 5–7 January 2016 and 5–6 June 2016 were more widespread, and heavy rainfall occurred across the entire region (Figure C2). At some locations during the early-June event, more than 560 mm of rainfall was observed in the 48-hour period, equivalent to a greater than 1-in-50-year rainfall event.

Aside from these east coast low events, rainfall for the majority of the 2015–16 year was below average. Only four months recorded above average monthly rainfall (Figure C3). Three of these months were influenced by east coast low events.

 

Figure C3 Total monthly rainfall for the Sydney region during the 2015–16 year compared with the long-term average and percentiles for the region

Figure C3 Total monthly rainfall for the Sydney region during the 2015–16 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 201516 year was 1,542 mm, which is above the long-term area-averaged potential evapotranspiration of 1,514 mm (based on the 1911–2016 period). Potential evapotranspiration was relatively uniform across the region but marginally higher in the northern parts (Figure C4).

 

Figure C4 Total annual potential evapotranspiration for the Sydney region during the 2015–16 year
Figure C4 Total annual potential evapotranspiration for the Sydney region during the 2015–16 year

 

Potential evapotranspiration was above average across most of the Sydney region during the 2015–16 year (Figure C5), which may be attributed to the decreased rainfall and hence less cloud cover that occurred during the majority of the year.

 

Figure C5 Annual potential evapotranspiration deciles for the Sydney region during the 2015–16 year
Figure C5 Annual potential evapotranspiration deciles for the Sydney region during the 2015–16 year

 

Soil moisture

Soil moisture, in the root zone (0–1m depth) for the 2015–16 year was average across most of the Sydney region compared with the 1911–2016 period (Figure C6). 

 


Figure C6 Annual and monthly soil moisture deciles in the root zone (0–1 m depth) for the Sydney region during the 2015–16 year

 

Similar to rainfall, soil moisture was below average for the majority of the year (Figure C6). Soil moisture was only above average across most the region during the periods immediately following the heavy rainfall associated with the east coast low events—August 2015, January 2016 and June 2016.

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

 

Streamflow responses

Streamflow

The Hawkesbury–Nepean and Shoalhaven river systems are two primary river systems within the Sydney region. The location of key gauging stations on the Grose River at Burralow (Station 212291) and on the Shoalhaven River at Fossickers Flat (Station 215207) are provided in Figure R6 in 'Region description'. These two gauging stations were selected as key stations for the region because flows upstream of these stations are unregulated and diversions are negligible.

In the Grose River, monthly flows were well below average for almost the entire 2015–16 year (Figure C7). The exception was January 2016, where monthly flow was above average. This can be attributed to the east coast low event that occurred 5–7 January 2016 (see Rainfall).

Interestingly, total flows during August 2015 and June 2016 (Figure C7) were less than expected given the total rainfall during these months were well above average (Figure C3). Most of the rainfall during these months were associated with an east coast low event (see Rainfall). The late-August east coast low primarily affected the southeastern part of the region; much less rainfall was observed over the upper reaches of the Grose River. The early-June east coast low was more widespread, but the soil moisture conditions in the upper catchment area of the Grose River were relatively low following a very dry autumn period (Figure C6).

 

Figure C7 Total monthly flow along the Grose River during the 2015–16 year compared with the long-term average and percentiles for the river
Figure C7 Total monthly flow along the Grose River during the 2015–16 year compared with the long-term average and percentiles for the river
 

 

In the Shoalhaven River, monthly flows were also below average for most of the 2015–16 year and generally reflected the rainfall conditions experienced in the region (Figure C8). The well above-average monthly flows in August and June were primarily attributed to the east coast low events that occurred over 25–26 August 2015 and 5–6 June 2016 respectively (see Rainfall).  

Total flow during January (Figure C8) was less than expected given the total rainfall during this month was above the 90th percentile (Figure C3). Most of the rainfall during this month was associated with an east coast low event (see Rainfall); however, soil moisture conditions in the upper catchment area of the Shoalhaven River were relatively low following a very dry spring and early-summer period (Figure C6).

 

Figure C7 Total monthly flow along the Shoalhaven River during the 2015–16 year compared with the long-term average and percentiles for the river

Figure C8 Total monthly flow along the Shoalhaven River during the 2015–16 year compared with the long-term average and percentiles for the river 

 

Major water reforms

Organisational change

On 1 January 2015, State Water and Sydney Catchment Authority were amalgamated under the Water NSW Act 2014 to form WaterNSW. The administrative and operational changes in relation to this amalgamation have taken place. In addition, from 1 July 2016 a number of functions related to the delivery of water services in New South Wales was transferred to WaterNSW from Department of Primary Industries–Water.