Assessing the influence of watershed conditions or perennial vegetation cover on downstream flooding in the Gascoyne River watershed. Assessment of the influence of the watershed or land cover of perennial vegetation on soil erosion in the Gascoyne River watershed.
Purpose
Objectives
Background
As a result, the watershed's shape, topography, and soil types would have affected each of the three flood events differently. In Section 3, we present illustrative evidence of the relationship between catchment condition (as defined by DAFWA, Appendix 3.1) and erosion.
Methodology
Rainfall, flooding and erosion
MODIS images were used to obtain estimates of the Total Suspended Solids (TSS) based on calibrations obtained from other studies. Visual assessment of the type and severity of erosion in the catchment not directly attributable to the December 2010 flood was considered part of catchment condition assessment (see below) and provides an overview of erosion and catchment condition.
Catchment condition
- Condition (Traverse data)
- Trends and condition over time (WARMS)
- Vegetation cover (Remote sensed data)
Erosion, as a result of the December 2010 flood, was assessed in terms of the area of the sediment plume obtained by digitizing MODIS images (NASA/GSFC, Rapid Response). In addition, photographs of erosion and deposition along the Gascoyne River taken during field surveys, coupled with oral accounts of erosion by pastoralists, provide qualitative evidence of the extent and severity of erosion.
Soil infiltration measurements
The stable fraction of the plotted infiltration data was determined visually and a linear regression (Snedecor & Cochran 1989) was fitted (Equation 1). Infiltration rates were assessed in relation to vegetation associations (ie patchwork or patchwork) and surface and subsoil soil textures.
Results and analysis
Rainfall, flooding and erosion
- Summer 2010-2011 weather events
- Rainfall and flooding
- Erosion
The December flood was predominantly over the western end of the catchment from the Carnarvon area to Gascoyne Junction. During major floods, large parts of the main river channels are eroded and the material is
Catchment condition
- Condition (Traverse data)
- Trends and condition over time (WARMS data)
- Vegetation cover
- Soil and site data
- Analysis of Infiltration measurements
Based on WARMS locations assessed in 2011, an average of 11.6% of the landscape has been defined as catch zones. Large parts of the Gascoyne catchment area are characterized by a stony mantle and a high watershed area (Appendix Table 3.2). Residual coverage is variable between land systems of the same land type, depending on the state and.
The percentage of the catchment by year of the last reasonable season is shown in Table 7. Low levels of perennial vegetation ground cover are indicative of conditions within the Gascoyne River catchment (Section 2.2.2.1) and have not shown improvements in scale. widely for the analyzed period. For example, sandy textures dominated the surface of the patches (44%), while sand balls dominated the subsurface of the patches (44%).
The difference in texture between the resource capture zones reflects the accumulation of sand and resources in the patches and the stripping of the surface in the intervening patch. With all the measurements, there were more than twice the number of infiltration measurements made in the patch compared to the spots - a reflection of the ratio of patch to patch at WARMS sites (Appendix Table 4.3).
Physiographic regions of the Gascoyne River catchment
- Spatial organisation
- Upland source areas
- Transfer zones (Sheet wash plains)
- Bottomland deposition areas
While focusing on the Gascoyne River catchment, much of the information presented on landscape organization and function is relevant to many of the southern rangeland environments of Western Australia. A small section along the central southern margin of the catchment comprises the Murchison Province of Tille (2006), based on the Murchison Province of Bettenay (1983). This boundary is based on the northern half of the Yilgarn Craton tectonic unit of Tyler and Hocking (2001).
The provinces of the Gascoyne River Basin are divided into the following zones based on geomorphological or geological criteria, as described by Tille (2006). Coastal plains make up only a small portion of the watershed (0.1%) and are confined to coastal areas in the west. Hills and mountain ranges comprise some of the highest features in the landscape (relief > 30 m), with gently sloping to steep slopes.
Rocky plains usually surround mountainous areas of higher relief and occupy a larger part of the landscape. The vegetation of the riparian zone is typically dense and dominated by river gum (Eucalyptus camaldulensis), with mulga, curara (Acacia tetragonophylla) and miniritchie (Acacia cyperophylla).
Landscape organisation and function
Rangeland ecosystem function and soil moisture balance
With proximity to the coast, the amount of sand increases and its availability facilitates the development of the dunes that characterize the sandy plains. Limited to the western part of the watershed, various dune formations have been formed and modified mainly by wind; land systems are differentiated based on dune pattern (ie gridded, linear). Groves of trees, clumps of shrubs and banks of stragglers act as fertile patches and are important in water and nutrient capture processes.
Consequently, greater floristic diversity generally occurs within vegetated habitats or under tree-based clumps (Figure 32a). Inside bushes, branch and leaf litter accumulates around their bases and impedes soil surface winds and water flow. Wind- and water-dispersed material, (i.e. leaf litter, seeds, animal droppings, general debris) collects within and immediately against the bush or clump.
The continuous grazing of groves and clumps of bushes during dry periods results in the deterioration of the structure and composition of these habitats. At all stages of the catena sequence throughout the basin the landscape has lost or has a greatly reduced capacity to regulate resources through water and nutrient retention (Sections 2.2.2.2 and 2.2.3).
Landscape incision and the desiccation process
- Headward erosion
- Lateral erosion
- Erosion exacerbated by infrastructure
The process of erosion that cuts deeper into drainage channels tends to slow as the cut approaches harder substrates, such as rocky pediments or cemented soil horizons, typical of the region. The sheet flow is channeled into the incised drainage channel (left), leaving the interfluve water starved except during heavy rainfall events when the water can fill and flow out of the channel. Infrastructure (roads, tracks and fences) further disrupts the mechanisms that regulate resources through the landscape, primarily sheet flow.
Similarly, if a track is cut below the surface of the land, water will flow into and along it until it encounters an outlet. Similarly, if a track runs parallel to the leaf stream, the water is directed downhill quickly, similar to a runoff pipe. The bottom half of the image shows the ascending areas, which are covered in annual windgrass (Aristida contorta) in good seasons; this appears yellow in the image.
In contrast, the upper half of the picture cannot support any significant ground cover because it is deprived of water due to the road. Water from upstream areas is directed down the road to drain into drains instead of continuing as a flat flow across downstream areas; Gascoyne River Basin (2007 aerial photo provided by Landgate).
Gascoyne River catchment condition summary
Upland source areas
Transfer zones (Sheet wash plains)
In severely degraded areas, drift banks may become isolated hummocks subject to sand drift and surface redistribution (Figures 40a, b). As sandy banks contract through fragmentation, interbank sections merge and increasing sheet flow escalates burning and sheet erosion. Both processes can strip the soil surface on wide fronts, leading to terrace erosion (Figures 42a, b).
When erosion of terraces, rills, and gullies begins, reduction in the capacity to limit sheet flow facilitates erosion. Wilcox and McKinnon (1972) observed that erosion removed the sandbanks and obliterated the patterns that separated the component land systems.
Bottomland deposition areas
The heavy clay and duplex soils in once sluggish drainage areas are commonly sealed by scalding. The proximity to major rivers and alluvial plains has resulted in overgrazing of calcrete platform vegetation communities (Figure 53) and associated highly favored areas prone to preferential grazing, such as saline plains and drainage foci, are often severely degraded and eroded (Figure 54a, b, c). Sandy splays on the right side of the photograph are eroding from the downslope side of the bank, Peedawarra land system.
Fluvial plains form the lowest parts of the catenary succession, where sediments and nutrients are thought to be deposited upward. Widespread erosion in the upper basin increased sedimentation in the main channels of the Gascoyne and Lyons rivers. Erosion of watercourses is considerable; its role in drainage of the watershed is important and a major factor contributing to the loss of resources and the reduction of watershed resilience.
In the lower parts of the catchment, sand dunes and dunes become more common, between which interdunal areas occur where the flow is concentrated (Figure 58). The increased connectivity between interdunal areas as sand areas fragment and banks erode, along with reduced infiltration, associated with scalding and coating, has increased water flow through these areas (Figure 59).
Summation
In the catchment's lower reaches, saline alluvial plains are one of the more dominant land types, the others being sand plains. Previous reports determined that the condition of the Gascoyne River catchment was in poor condition (Wilcox & McKinnon 1972; However, from the present analysis it is not possible to determine the impact catchment condition on the extent of the December 2010 flood not.
With many upper slopes, interfluves and drainage surfaces severely degraded, the transport capacity of the Gascoyne River catchment is significantly diminished. The condition of the Gascoyne River catchment is poor and has been deteriorating since at least the 1930s. Lightfoot, LC 1961, Soil erosion on the Gascoyne River catchment, Report of the Gascoyne River Erosion Committee Memo, Department of Agriculture, WA.
Wilcox, DG & McKinnon, EA 1972, A report on the condition of the Gascoyne catchment, Western Australia Department of Agriculture, Technical Bulletin No.
Acronyms
Data sets and sources of data
Vegetation condition assessment and summary (2002–2009)
Wash flats and sandbanks on hardpan, with mulga scrub and wanderrie grasses or spinifex.
Soil infiltration and profile data
Peak river heights at Nine Mile Bridge 1960 to 2011
Average soil surface attributes for capture and shedding
Physiographic regions of the Gascoyne River catchment
Glenburgh Soil System Rugged granite hills, rocky outcrops and lower plains, supporting scattered tall mulga and other acacia bushes. Two Hills Land System Long, low hills and stony slopes of sedimentary rock, supporting tall mulga and other acacia bushes. Pelle soil system Low hills, mesas and ridges of supporting sedimentary rocks, tall bushes of mulga and other acacias.
Collier land system Rolling rocky uplands, low hills and ridges and rocky plains supporting mulga scrub. Phillips land system Low hills and rolling highlands of crystalline rock, supporting mulga and other acacia-dominated tall scrub. Yagina land system Rocky plains and alluvial plains with occasional low dunes and clay fields supporting sparse tall shrublands.
Stonehut soil system Non-saline alluvial plains with large cross sand banks supporting mulga and other tall acacia shrubs with low non-halophytic shrubs. Winmar Soil System Rocky plain with sandy banks supporting mulga and other acacia shrubs with low eremophila and cassia shrubs and riparian grasses.
