The Gascoyne River Catchment
The Gascoyne River Catchment (Figure 1) is located in the mid north-west of Western Australia. The catchment covers an area of about 80 400 square kilometres. Most of the area is rangeland and the native vegetation is characterised by scattered perennial shrubs of various genera with a very scattered acacia overstorey. The rangelands support an extensive pastoral industry. On the river levee and floodplain delta near the town of Carnarvon there is about 1000 hectares of irrigated horticulture.
Why we assessed catchment condition
In December 2010 an extreme tropical storm caused widespread flooding in Carnarvon and across the catchment. There were 2 more floods that summer that caused significant soil erosion and damage to the horticultural enterprises and infrastructure at Carnarvon (Figure 2) and Gascoyne Junction. The damage bill was estimated at $90 million. At the time of the floods the catchment was considered to have low vegetative groundcover following an extended period of dry seasons.
To answer the question 'could damage from flooding have been reduced with increased vegetation condition?', the department assessed groundcover and catchment condition, particularly the influence perennial vegetation groundcover had on the risk of flooding and soil loss in the catchment.
How we assessed catchment condition
Our staff collated information about conditions in the catchment at the time of and preceding the floods, and then examined the relationship between groundcover and soil loss.
Information used in the assessment included:
- rainfall records sourced from the Bureau of Meteorology (BoM)
- comparisons of the December 2010 flood and previous flood events, using hydrographs from the Department of Water and Environmental Regulation
- assessment of soil loss based on the area of sediment plumes at the mouth of the Gascoyne River — from MODIS satellite data and estimates of the plume sediment load
- existing information and datasets relevant to the catchment
- an assessment of the condition of the middle to upper catchment between June and August 2011, primarily east of Gascoyne Junction. Survey teams recorded perennial plant numbers and landscape function at 96 long-term Western Australian Rangeland Monitoring System (WARMS) sites
- soil infiltration rates at 50 sites, to assess the relative importance of perennial vegetation cover and soil texture on infiltration rates
- traverse notes and photographs compiled while travelling to all sites of interest, to provide an overall perspective of catchment condition.
Experienced rangeland advisers assessed catchment condition using defined, visual assessments.
What we found
A record storm and flood resulted in high sediment loss
The tropical storm that crossed over the Gascoyne River Catchment between 16 and 19 December 2010 resulted in falls of more than 250–300mm over a 24-hour period, the highest falls on record at the time. The record rainfall brought record flooding with a peak depth of 7.77m at Nine Mile Bridge, near Carnarvon. The previous high was 7.63m in 1960, followed by 7.60m in 2000.
While major erosion has been occurring in the catchment at least since the 1960s, the sediment plume from the December 2010 flood was 2 to 7 times larger than previous flood events. Soil erosion from the December flood was estimated to be at least 5.6 million tonnes.
The catchment was in fair to poor condition
From historical records and more recent investigations, it is clear that the catchment has lost a significant amount of vegetative cover, including desirable perennial plants, and has had extensive soil loss (poor condition). These losses were recorded since at least the 1960s, and many areas are continuing to decline.
Assessments from 2002 to 2009 showed that over 3.6 million hectares were in poor condition, with a 15% decline in perennial shrubs from 2005 to 2009. There was a 39% decline in perennial plant numbers in the above-average seasons of 1995 to 2000, reduced resource capture (13% decline overall and 22% decline in mulga groves/run-on sites) and an increase in erosion features.
The overall condition of vegetation cover from 1989 to 2010 was stable, with areas assessed as poor condition in 1989 still in poor condition in 2010. However, large contiguous areas had declining vegetation cover between the central Gascoyne and Lyons rivers, with plant numbers in 2011 declining to 1995 levels.
A series of poor seasons were coupled with low levels of groundcover
Satellite images and BoM rainfall records indicate that the seasonal conditions were poor for 4 or more years before the December 2010 flood. A low greenness index at the time of the 2010 flood indicates that groundcover was also low.
Annual Return of Livestock and Improvement forms (from the Pastoral Lands Board of Western Australia) show stocking levels through consecutive dry years that was in excess of the estimated carrying capacity of the land. Those stocking levels and the poor seasonal quality would have contributed to high grazing pressures and a decline in vegetation cover in the catchment.
Catchment condition (and perennial groundcover) may influence flooding
Records from major floods associated with substantial rainfall events since 1960 indicate that the overall catchment condition has not changed substantially since the 1960s. That is, it has been in a poor condition since the 1960s.
The catchment is naturally a high water-shedding catchment. The soils are generally shallow, frequently less than 30cm deep and often consist of a sandy loam over clay, hardpan or weathered rock. Vegetated 'patches' which capture water and nutrients and limit run-off, occupy about 12% of the area, within larger areas of sparsely vegetated 'interpatches' comprising about 88% of the total land area. Water infiltration rates in the patches were about 4 times higher than in the interpatches.
Based on infiltration measurements, more perennial groundcover would reduce the severity of flooding from minor and moderate storms. However, the magnitude and intensity of the December 2010 rainfall would have exceeded the soil's capacity to store water and there would have been run-off from vegetated patches and interpatches regardless of the area occupied by vegetation.
Erosion is associated with loss of perennial groundcover in different parts of the landscape
Many of the slopes and upland drainage flats, which rely on perennial vegetation groundcover for protection, have been overgrazed. This results in these features also contributing to erosion problems in the catchment. Stony hills and ranges in the catchment shed a lot of the rain falling on them, and this has increased the erosion risk on less-protected soils downslope.
Vegetation loss has been considerable along the valley floors and in the drainage lines, where existing rill and gully erosion has increased water shedding.
Fertile, potentially productive patches were mostly overgrazed, with reduced forage value and capacity to retain water. These conditions would have contributed to erosion downslope. Many riparian pastures in the catchment have been overgrazed and degraded. Where buffel grass has become established, it has helped stabilise soil and prevent further erosion.
What the results mean
Given the magnitude of the December 2010 rainfall and the large, naturally occurring water-shedding areas with relatively low infiltration rates in the Gascoyne River Catchment, a major flood would have occurred regardless of catchment condition. Based on the historical and recent review of the catchment, heavy rainfalls will result in localised flooding, soil loss and damage to infrastructure. This is because Carnarvon and the horticultural area are situated on a river floodplain with a levee built up by successive flood events, and the tropical cyclones that cause flooding occur in summer when groundcover is naturally minimal.
Improving catchment condition by increasing the area of perennial vegetation cover, combined with infrastructure improvements could reduce soil loss from minor and moderate flood events.
For a copy of this report
Download 'A report on the Gascoyne River Catchment following the 2010–11 flood events', Resource management technical report 382 (PDF 6.5MB).