National Water Account 2016

Ord: Climate and water

Annual rainfall in the region was slightly above average; however, a large portion of the annual rainfall occurred in November–December, in the early wet season. During January–February, the typically wettest part of the year, rainfall was well below average, which contributed to the low streamflows across the region.



 

Introduction

The Ord region lies within the wet–dry tropics of northern Australia, which has a distinct and predictable climatic seasonality. Almost all rainfall in the region occurs during the wet season (November–April). As a consequence, flow in the rivers is strongly seasonal. Many of the region's rivers, including those that flow into Lake Argyle, cease to flow during the dry season (April–October).

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

 

Climate conditions

Rainfall

In general, below-average rainfall conditions were experienced across much of northern Australia during the 2015–16 year. Climate across Australia was largely influenced by strong El Niño conditions in the Pacific Ocean, one of the strongest El Niño events since 1950. During September–November, in the months leading up to and into the early part of the wet season, Australia's climate was also influenced by a positive phase of the Indian Ocean Dipole, which also contributed to the relatively dry conditions throughout northern Australia.

The Ord region was one of the few areas in northern Australia where annual rainfall was above average. The total area-averaged rainfall over the Ord region during the 2015–16 year was 701 mm, slightly above the long-term area-averaged rainfall of 682 mm (based on the 1900–2016 period). Rainfall ranged from more than 900 mm in the north to less than 600 mm in the southern parts of the region (Figure C1).

 

Figure C1  Map of total annual rainfall for the Ord region during the 2015–16 year
Figure C1 Total annual rainfall for the Ord region during the 2015–16 year

 

Rainfall across the majority of the region was generally average for the 2015–16 year, with small areas of above-average rainfall in the southern and central parts of the region (Figure C2).

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

 

During the 2015–16 year, well above-average rainfall occurred during November–December, the early part of the wet season. Total rainfall during December was well above the 90th percentile (Figure C3), and most of this rainfall occurred as a result of a tropical low that formed over the Timor Sea in mid to late December.

During the typically wettest months of the year (January–February), the region experienced below-average rainfall (figures C2–C3).

 

Figure C3 Graph of total monthly rainfall for the Ord region during the 2015–16 year compared with the long-term average and percentiles for the region
Figure C3 Total monthly rainfall for the Ord 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 Ord region during the 2015–16 year was 2,530 mm, which is above the long-term area-averaged potential evapotranspiration of 2,459 mm (based on the 1911–2016 period). Potential evapotranspiration was highest along parts of the coast in the north and relatively uniform across the remainder of the region (Figure C4).

Typical of the climate conditions across northern Australia, annual potential evapotranspiration over the Ord 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 2009). Only during high-rainfall periods in the wet season does rainfall exceed potential evapotranspiration, which drives the seasonal streamflow.

 

Figure C4  Map of total annual potential evapotranspiration for the Ord region during the 2015–16 year
Figure C4 Total annual potential evapotranspiration for the Ord region during the 2015–16 year

 

Potential evapotranspiration across the majority of the Ord region was above average to very much above average for the 2015–16 year (Figure C5), which may be attributed to the decreased rainfall (and hence cloud cover) that occurred during the typically wettest months of the year (January–February). 

 

Figure C5  Map of annual potential evapotranspiration deciles for the Ord region during the 2015–16 year
Figure C5 Annual potential evapotranspiration deciles for the Ord region during the 2015–16 year

 

Soil moisture

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

 

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

 

Comparison of monthly rainfall and soil moisture deciles show the response of soil moisture to changes in rainfall aligns closely for the majority of months in the 2015–16 year. For example, above-average soil moisture during November–December 2015 (Figure C6) reflects the above-average rainfall that occurred across the region during this 2-month period (Figure C2). An exception occurred in March 2016 when soil moisture was below average despite above-average rainfall observed during that month (Figure C3). This indicates soil water stores were still low following the relatively poor rainfall that occurred during the preceding two months (January–February).

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

 

Streamflow responses

Streamflow

The upper Ord and Negri rivers contribute most of the inflows to Lake Argyle. The location of key gauging stations at the Old Ord Homestead (Station 809316) and the Mistake Creek Homestead (Station 809315) along these rivers are provided in Figure R7 in 'Region description'.

Despite the total annual rainfall during the 2015–16 year being slightly above average, total annual flows in these rivers were less than a third of the mean annual flow. The 3-month period from January–March is usually when the majority of the total annual flow occurs in these rivers. The combination of relatively poor rainfall and soil moisture conditions experienced during these months resulted in well below-average streamflows during this 3-month period (figures C7–C8), which contributed to the overall below-average annual flow during the 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.

Interestingly, total flow during December in these rivers (figure C7–C8) was less than expected given the total rainfall during this month was well above the 90th percentile (Figure C3). Most of the December rainfall occurred as a result of a tropical low that formed over the Timor Sea in mid to late December. The highest rainfall associated with this event occurred over the lower eastern part of the Ord region and much less rainfall was observed over the upper reaches of the Ord and Negri rivers (Figure C2). 

 

Figure C7 Graph of total monthly flow along upper Ord River during the 2015–16 year compared with the long-term average and percentiles for the river
Figure C7 Total monthly flow along upper Ord River during the 2015–16 year compared with the long-term average and percentiles for the river


Figure C8 Graph of total monthly flow along Negri River during the 2015–16 year compared with the long-term average and percentiles for the river

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

 

Streamflow in the lower Ord River below Lake Argyle is measured at Tarrara Bar (Station 809339: see Figure R7 in 'Region description' for location). Flows in this section of the river are influenced by dam operations and diversions for irrigation supply upstream.

Total annual flow during the 2015–16 year in the lower Ord River was less than 40% of the mean annual flow. Except for December, monthly flow for the majority of the year was around the 10th percentile (Figure C9), which reflects the relatively poor wet-season rainfall. Flow during December was the only month that was above average and can be attributed to runoff from the unregulated catchment area (downstream of Lake Argyle) during the rainfall event associated with the tropical low that formed in mid to late December (see Rainfall).

 

Figure C9 Graph of total monthly flow along lower Ord River during the 2015–16 year compared with the long-term average and percentiles for the river
Figure C9 Total monthly flow along lower Ord River during the 2015–16 year compared with the long-term average and percentiles for the river

 

Major water reforms

Ord–East Kimberley Expansion Project

The Ord–East Kimberley Expansion Project is a major initiative of the Western Australian Government. The State committed $322.5 million from the Royalties for Regions fund under the Ord Irrigation Expansion Project and a further $195 million was contributed by the Australian Government via the East Kimberley Development Package.

The first stage of the expansion will develop 74 km² of irrigated farmland in the Goomig Farmlands, with an option to develop a further 60 km² in the Knox Creek Plain area (Figure C10). Construction of essential infrastructure for these areas, including irrigation channels and roads, commenced in 2013 and continued throughout the 2015–16 year. The expansion project's first sorghum crop was planted in the Goomig Farmlands in mid 2015. During the 2015–16 year, approximately 5.5 km² of the Goomig Farmlands was cropped and irrigated.  

 

Figure C10 Map of the proposed expansion in the Ord River Irrigation Area
Figure C10 Proposed expansion in the Ord River Irrigation Area

 

The Ord Irrigation Cooperative currently holds a licence for diversion of 335,000 ML/year from the Ord River for the Ivanhoe and Packsaddle irrigation districts (see Water entitlements and other statutory water rights). Kimberley Agricultural Investment Pty Ltd were issued a licence in mid-2015 for diversion of 47,000 ML/year from the Ord River to the Goomig Farmlands. This is expected to increase to the total commitment of 120,000 ML/year in the next few years. Additional water demand from the Ord River to support irrigation development in the Knox Plain area is expected to be 60,000–110,000 ML/year.

There are proposals to develop other irrigated areas in the Ord region, such as the Ord West Bank and the Mantinea areas, the Carlton and Keep River plains and areas alongside the Packsaddle Plain (Figure C10). Development of these areas could take place in the next 5–10 years. Further details on these potential irrigation areas are given in Chapter 3 of the Ord Surface Water Allocation Plan.

Further information on the Ord–East Kimberley Expansion Project can be found at the following Government of Western Australia websites: