Daly: Climate and water

  • Total annual rainfall was above average following a very dry 24-month period.
  • Streamflows were above average in the Daly River for the first time in three years.
  • Groundwater levels in the Tindall Limestone Aquifer increased by more than 5 metres during 2020–21.

Schematic representation of the Daly region. In 2020–21, rainfall was 1,307 mm, above average for the first time in three years. Streamflow in Daly River in 2020–21 was 9,645 GL compared to the average value of 7,200 GL. Groundwater levels increased by more than 5 metres.

For further information on the region's climate and water conditions during the 2020–21 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 aquifers—Tindall Limestone and Oolloo Dolostone.

 

Climate conditions

Rainfall

Figure C1 Annual and monthly rainfall deciles for the Daly region during the 2020–21 year

 

  • Total area-averaged rainfall for the year was 1,307 mm, above the mean value of 1,035 mm.
  • The above-average rainfall in 2020–21 follows the region's driest 24-month period in more than 50 years.
  • The climate across northern Australia was influenced by weak La Niña conditions in the Pacific Ocean, the first La Niña event since 2012.
  • Heavy rainfall associated with widespread thunderstorm activity in late-December 2020 and a monsoon trough that crossed the region in late-February 2021 contributed to the above-average rainfall across large parts of the region during those months.

 

Figure C2 Total monthly rainfall for the Daly region during the 2020–21 year compared with the average and percentiles for the region

 

  • Rainfall was generally average to above average throughout the 2020–21 wet season (November–April).
  • Total rainfall in December 2020 and February 2021 was well above average and above the 90th percentile for those months.

 

Actual evapotranspiration

Figure C3 Annual and monthly actual evapotranspiration deciles for the Daly region during the 2020–21 year

 

  • Total area-averaged actual evapotranspiration was average to above average across most of the region.
  • Evapotranspiration was well below average during July–September 2020 prior to the 2020–21 wet season. This was due to very dry conditions across the region following two consecutive years of poor wet-season rainfall.
  • Evapotranspiration was above average over large parts of the region during January and March 2021 due to increased water availability following high rainfall in late-December 2020 and late-February 2021 respectively.

 

Soil moisture

Figure C4 Annual and monthly soil moisture deciles for the Daly region during the 2020–21 year

 

  • Soil moisture in the root zone (0–1 m depth) was above average across the entire region.
  • Very low rainfall during the previous 24-month period between 2018–20 meant root-zone soil moisture was well below average across most of the region during July–August 2020.
  • By the start of the 2020–21 wet season, soil moisture was above average across the region following late dry-season rainfall during September–October 2020.
  • Soil moisture remained above average during the 2020–21 wet season due to generally average to above-average rainfall occurring throughout the entire wet season.
  • More information on soil moisture distribution across the Daly region is available in the Australian Landscape Water Balance.

 

Flow responses

Streamflow

Map of key gauging stations. Katherine River at the Railway Bridge, station number G8140001, in the central part of the region near Katherine. Daly River at Mount Nancar, station number G8140040, in the region's northwest near Nauiyu.
Figure C5 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 C6 Total monthly flow for major rivers in the Daly region during the 2020–21 year compared with the average and percentiles

 

  • Total annual flows in the region's major rivers were above average reflecting the wet conditions experienced across the region. For example, total annual flow in the Daly River at Mount Nancar (Station G8140040) was 9,645 GL compared to the average value of 7,200 GL.
  • The above-average flows in the Katherine and Daly rivers in 2020–21 followed two consecutive years of very low flows in these rivers.
  • Very high streamflow occurred in February–March 2021 due to heavy rainfall in the latter part of February 2021. This rainfall caused moderate flooding in the Daly River in late-February to early March 2021.

 

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.
  • A key groundwater level monitoring bore for the Tindall Limestone Aquifer is located near Katherine; a key bore for the Oolloo Dolostone Aquifer is located near the junction of the Douglas and Daly rivers.

 

 

Figure C7 Groundwater level in key bores within the Daly region from 2014–2021

 

  • At 30 June 2021, groundwater levels in the Tindall Limestone and Oolloo Dolostone aquifers were well above those observed at the same time last year due to above-average annual rainfall in 2020–21.
  • Groundwater levels in the Tindall Limestone and Oolloo Dolostone aquifers peaked in late-February 2021 following very high rainfall during that month.
  • During January–February 2021, groundwater levels in the Tindall Limestone and Oolloo Dolostone aquifers increased by more than 5 metres.
  • Prior to the 2020–21 wet season, groundwater levels in the Tindall Limestone Aquifer were the lowest in more than 20 years and lowest on record in the Oolloo Dolostone Aquifer. Low rainfall during the previous two years meant that almost no recharge occurred in these aquifers during the 2018–19 and 2019–20 wet seasons.