Daly: Climate and water overview
Climate conditions
Rainfall
The total area-averaged rainfall over the Daly region during the 2014–15 year was 1,000 mm, which is marginally below the long-term area-averaged rainfall of 1,032 mm (based on the 1900–2015 period). Rainfall ranged from more than 1,200 mm in the northwest of the region to less than 900 mm in the southern parts of the region (Figure C8).
Figure C8 Total annual rainfall for the Daly region during the 2014–15 year
Rainfall across the majority of the region was generally average for the 2014–15 year (Figure C9). These conditions represent a decrease in rainfall compared to the 2013–14 year total of 1,138 mm (see the 2014 Account). Consequently, flows in the major rivers within the region were much lower than the previous year (see Streamflow).
Figure C9 Annual and monthly rainfall deciles for the Daly region during the 2014–15 year
Generally average rainfall conditions were experienced across much of Australia's northwest 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. There were no strong influences on the region from other large-scale drivers during the year (such as the Indian Ocean Dipole).
During the 2014–15 year, above average rainfall occurred during the early part of the wet season (November–January). For the remainder of the year, the region experienced below-average rainfall, particularly during the typically wet months of February–March (Figure C10). Because of the very low rainfall observed during these two wet season months, total annual flow in the region was below average (see Streamflow), despite the total annual rainfall being equivalent to an average year.
This is typical of catchments in northern Australia: periods of high rainfall during the early part of the wet season, when the soil is relatively unsaturated after the dry season, will have much less impact on the runoff in the region compared to periods of high rainfall during the middle of the wet season.
Figure C10 Total monthly rainfall for the Daly 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 Daly region during the 2014–15 year was 2,368 mm, which is above the long-term area-averaged potential evapotranspiration of 2,326 mm (based on the 1911–2015 period). Potential evapotranspiration was highest in the southern parts of the region and relatively uniform across the remainder of the region (Figure C11).
Typical of the climate conditions across northern Australia, annual potential evapotranspiration over the Daly region is much greater than the total annual rainfall. This means that on an annual basis the landscape is generally dry and can be described as water-limited (CSIRO 2009b). Only during high-rainfall periods in the wet season does rainfall exceed potential evapotranspiration, which drives the seasonal streamflow.
Figure C11 Evapotranspiration for the Daly region during the 2014–15 year
Potential evapotranspiration across the majority of the Daly region was above average to very much above average for the 2014–15 year (Figure C12).
Figure C12 Annual evapotranspiration deciles for the Daly region during the 2014–15 year
Soil moisture
Figure C13 shows that soil moisture, in the root zone (0 to 1m depth) for the 2014–15 year was above average in the southern parts of the region and generally average across the remainder of the Daly region (compared with the 1911–2015 period).
Figure C13 Annual and monthly soil moisture deciles in the root zone (0 to 1m depth) for the Daly region during the 2014–15 year
Figure C13 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 to a greater degree from September 2014 to June 2015.
Despite generally average rainfall and average to above average soil moisture conditions across the Daly region during the 2014–15 year, streamflow in the Katherine and Daly rivers was below average (see Streamflow). This can be attributed to the below average rainfall experienced during the typically wet months of February–March (Figure C10).
More information on soil moisture distribution across the Daly region is available in the Australian Landscape Water Balance.
Streamflow responses
Significant water events
Several high flow events occurred in the Daly region between January–April of the 2014–15 year, which is typical of wet season flow conditions in the region.
Flood classifications have been assigned to flows in the Katherine and Daly rivers. These are based on river levels that would inundate the towns of Katherine and Daly River. During the 2014–15 year, none of these high flow events reached a flood classification. Although some roads were impassable at stages throughout the wet season, the main towns within the region were not impacted by river flows.
Streamflow
The Katherine and Daly rivers are two primary rivers within the Daly region. The location of key gauging stations at Mount Nancar (Station G8140040) and the Katherine Railway Bridge (G8140001) along these rivers are provided in Figure C5 in 'Physical information'.
Total annual flows in these rivers during the 2014–15 year were less than the mean annual flow. Above average flows occurred during the early part of the wet season (December–January), which reflects the high rainfall observed throughout the region during this period (see Figure C10). The flow, however, was well below the long-term average during the 3-month period between February and April when the majority of the total annual flow usually occurs in these rivers (figures C14–C15). The low flow during this 3-month period contributed to the overall below average annual flow during the 2014–15 year.
Figure C14 Total monthly flow along the Katherine River compared to long-term average and percentiles for the river
Figure C15 Total monthly flow along the Daly River compared to long-term average and percentiles for the river
Major water reforms
No major water reforms occurred in the Daly region during the 2014–15 year.