National Water Account 2018

Daly: Climate and water

  • A monsoon trough in January 2018 contributed to above-average wet season rainfall.
  • The lower Daly region experienced 1 in 100 annual exceedance probability rainfall over a 4-day period during the monsoon.
  • The heavy rainfall caused a major flood in the Daly River; streamflow was average to below average for the remainder of the year.

 

 

For further information on the region's climate and water conditions during the 2017–18 year scroll down this page or click on the links below:

 

About the region

  • The Daly region lies within the wet–dry tropics of northern Australia. Almost all rainfall in the region occurs during the wet season (November–April). As a consequence, flow in the rivers is strongly seasonal.
  • Although most of the annual flow in the region's rivers occurs during the wet season, the Daly River system is one of the few in northern Australia to have perennial flow. The dry-season flow (May–October) is mostly dominated by input of groundwater from the two underlying limestone aquifers, Tindall and Oolloo.

 

Climate conditions

Rainfall

Figure C1 Annual and monthly rainfall deciles for the Daly region during the 2017–18 year

 

  • Total area-averaged rainfall for the year was 1,157 mm, above the mean value of 1,000 mm.
  • A monsoon trough over northern Australia during late-January 2018 contributed to above-average rainfall for that month across the entire region; parts of the lower Daly region recorded their highest January rainfall on record.
  • Except for November 2017 and January 2018, rainfall was below average for most of the year.

 

 


Figure C2 Total monthly rainfall for the Daly region during the 2017–18 year compared with the average and percentiles for the region

 

  • Almost 50% of the total annual rainfall occurred in January 2018, a large proportion of which was associated with the monsoon trough that impacted the region late in the month.
  • Total rainfall in January was above the 90th percentile and more than double the January average.

 

Actual evapotranspiration

Figure C3 Annual and monthly actual evapotranspiration deciles for the Daly region during the 2017–18 year

 

  • Total area-averaged actual evapotranspiration was above average across most of the region.
  • The above-average evapotranspiration may be attributed to a combination of above-average temperatures across the region and increased water availability following high rainfall in November 2017 and January 2018.
  • Actual evapotranspiration was above average for almost the entire year.

 

Soil moisture

Figure C4 Annual and monthly soil moisture deciles for the Daly region during the 2017–18 year

 

  • Soil moisture in the root zone (0–1 m depth) was average to above average across the region.
  • Rainfall and soil moisture are typically closely aligned. For example, above-average soil moisture during the 4-month period from November–February is due to above-average rainfall that occurred during November 2017 and January 2018. The below-average soil moisture during March–June can be attributed to the relatively poor rainfall that occurred during the latter part of the wet season (February–April).
  • More information on soil moisture distribution across the Daly region is available in the Australian Landscape Water Balance.

 

Flow responses

Significant water events

  • A high magnitude flood event occurred in the region during late-January to early-February 2018.
  • The event was attributed to a monsoon trough that formed over northern Australia. Higher rainfall occurred over the lower Daly region (see Rainfall). At a rain gauge near the township of Nauiyu (Daly River), more than 550 mm was recorded during the 4-day period between 28–31 January—equivalent to 1 in 100 annual exceedance probability rainfall (according to the Bureau of Meteorology's Design rainfall website).

 

Figure C5 Flood event in the Daly River during the 2017–18 year
Figure C5 Flood event in the Daly River during the 2017–18 year

 

Flood waters in the town of Nauiyu in late-January 2018 (photo supplied by Northern Territory Police)

  • On 1 February 2018, flow in the Daly River peaked at 6,125 m 3/s, one of the larger flood peaks recorded along the Daly River and equivalent to a 1-in-15-year flood event (according to the Bureau of Meteorology's Water Data Online website).
  • The recurrence interval of the flood peak was lower than that of the rainfall as the higher rainfall only occurred over a relatively small portion of the contributing catchment area.
  • Flood classifications have been assigned to flows in the Daly River based on river levels that would inundate the town of Nauiyu. During the flood event, major flood levels in the Daly River were exceeded for about 1 week; the entire township of Nauiyu (population of approximately 500 people) was evacuated for the event's duration.
  • 28% of the total annual flow in the Daly River occurred during the 1-week flood.

 

Streamflow

Figure C6 Map of key flow gauging stations along the main rivers within the Daly region
Figure C6 Key flow gauging stations along the main rivers within the Daly region

 

  • The Katherine and Daly rivers are two primary rivers within the Daly region.

 

 


Figure C7 Total monthly flow for major rivers in the Daly region during the 2017–18 year compared with average and percentiles

  

  • Total annual flows in the Katherine and Daly rivers were approximately 10% and 40% above the average annual flow respectively.
  • The very high monthly flows during January–February 2018 were primarily attributed to rainfall associated with the monsoon trough that occurred in late-January (see Significant water events).
  • During this event, higher rainfall occurred over the lower Daly region, which meant flows in the Daly River were relatively higher than flows in the Katherine River during this 2-month period.

 

Groundwater level

  • The Tindall Limestone and Oolloo Dolostone aquifers are important groundwater stores providing dry season flow for parts of the river system. The aquifers are also a primary source of consumptive water supply.
  • Key groundwater level monitoring bores for the Tindall aquifer are located near Katherine; a key bore for the Oolloo aquifer is located near the junction of the Douglas and Daly rivers.

 

 


Figure C8 Groundwater level in key bores within the Daly region from 2014–2018

  

  • Groundwater levels in the Tindall aquifer peaked in mid-February 2018 following very high rainfall in January 2018. Below-average rainfall for the remainder of the year meant groundwater levels on 30 June 2018 were well below those observed at the same time last year.
  • Dry season levels in 2017 in the Tindall aquifer were the highest in several years following the above-average wet season rainfall during 2016–17 (see 2017 Account). This followed very low dry season levels in 2015 and 2016, which were due to consecutive poor wet season rainfall in 2014–15 and 2015–16.
  • Groundwater levels in the Oolloo aquifer on 30 June 2018 were higher than those observed at the same time last year. This is likely due to the monsoon event in late-January 2018 when record rainfall occurred over the lower part of the region. Interestingly, despite above-average wet season rainfall in 2016–17, dry season levels in 2017 were relatively low in the Oolloo aquifer compared with the Tindall aquifer.