Ord: Climate and water overview
Climate conditions
Rainfall
The total area-averaged rainfall over the Ord region during the 2014–15 year was 691 mm, which is marginally above the long-term area-averaged rainfall of 680 mm (based on the 1900–2015 period). Rainfall ranged from more than 900 mm in the coastal north of the region to less than 600 mm in the southern and eastern parts of the region (Figure C12).
Figure C12 Total annual rainfall for the Ord region during the 2014–15 year
Rainfall across the majority of the region was generally average for the 2014–15 year (Figure C13). These conditions represent a decrease in rainfall compared to the 2013–14 year total of 1,042 mm (see the 2014 Account). Consequently, flows in the major rivers within the region were much lower than the previous year (see Streamflow).
Figure C13 Annual and monthly rainfall deciles for the Ord 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 only 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 C14). Because of the very low rainfall observed during these two wet season months, streamflow in the region was below average (see Streamflow), despite the total annual rainfall being equivalent to an average year. A similar rainfall pattern was observed during the 2012–13 year, which also resulted in well below average streamflow conditions (see the 2013 Account).
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 C14 Total monthly rainfall for the Ord 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 Ord region during the 2014–15 year was 2,470 mm, which is similar to the long-term area-averaged potential evapotranspiration of 2,457 mm (based on the 1911–2015 period). Potential evapotranspiration was highest along parts of the coast in the north and relatively uniform across the remainder of the region (Figure C15).
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 C15 Total annual potential evapotranspiration for the Ord region during the 2014–15 year
Potential evapotranspiration across the majority of the Ord region was generally average for the 2014–15 year (Figure C16).
Figure C16 Annual evapotranspiration deciles for the Ord region during the 2014–15 year
Soil moisture
Figure C17 shows that soil moisture, in the root zone (0 to 1m depth) for the 2014–15 year was average across most of the Ord region and above average in the southern part of the region (compared with the 1911–2015 period).
Figure C17 Annual and monthly soil moisture deciles in the root zone (0 to 1m depth) for the Ord region during the 2014–15 year
Figure C17 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 closely for the majority of months in the 2014–15 year.
Despite generally average rainfall and soil moisture conditions across the Ord region during the 2014–15 year, streamflow in the primary rivers was below average (see Streamflow), which can be attributed to the below average rainfall experienced during the typically wet months of February–March (Figure C14).
More information on soil moisture distribution across the Ord region is available in the Australian Landscape Water Balance.
Streamflow responses
Significant water events
On 5 January 2015, daily flow along the lower Ord River at Tarrara Bar (Station 809339: see Figure C7 in 'Physical information' for location) peaked above 875 m3/s, greater than a 1 in 2-year flood event (Department of Water 2013). This event was attributed to a large amount of rainfall occurring on 3 January 2015, where 24-hour totals of around 100 mm were observed in some parts of the region.
This rainfall-runoff event contributed to the above average monthly flows along the Ord River during January 2015 (see Streamflow section below) as well as meeting requirements outlined in the environmental water provisions for the lower Ord River (see Environmental benefit note).
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 C7 in 'Physical information'.
Total annual flows in these rivers during the 2014–15 year were less than the mean annual flow, particularly in the Negri River where flow only occurred in the river for four months of the year. The 3-month period between January and March is usually when the majority of the total annual flow occurs in these rivers. Total flow during this period was below the long-term average, particularly during February–March (figures C18–C19), which contributed to the overall below average annual flow during the 2014–15 year.
Interestingly, total flow during January in the Negri River (Figure C19) was less than expected given the total rainfall during this month was well above average (see Rainfall). It’s likely that the storm event that occurred on 3 January 2015 (see Significant water events), which contributed to a large portion of the high monthly flow for January in the upper Ord River, was primarily concentrated over the western part of the Ord region and largely missed the upper reaches of the Negri River.
Figure C18 Total monthly flow along upper Ord River during the 2014–15 year compared with the long-term average and percentiles for the river
Figure C19 Total monthly flow along Negri River during the 2014–15 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 C7 in 'Physical information' 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 2014–15 year in the lower Ord River was less than 50% of the mean annual flow. Except for December–January, monthly flow for the majority of the year was around the 10th percentile (Figure C20), which reflects the rainfall observed throughout the region (see Rainfall). Flow during January was the only month that was above average and can be attributed to high runoff from the unregulated catchment area during the rainfall event on 3 January 2015 (see Significant water events).
Figure C20 Total monthly flow along lower Ord River during the 2014–15 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 Plain area (Figure C21). Construction of essential infrastructure for these areas, including irrigation channels and roads, commenced in 2013 and continued throughout the 2014–15 year. The expansion project's first sorghum crop was planted in the Goomig Farmlands in mid-2015.
Figure C21 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 120,000 ML/year from the Ord River to the Goomig Farmlands. 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 C24). 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: