2.2 Results and analysis
2.2.2 Catchment condition
2.2.3.2 Analysis of Infiltration measurements
2.2.3 Soil infiltration measurements
Figure 23 Examples of different resource capture zones where comparison infiltration site measurements were undertaken on interpatch and patch surfaces
C I = 29 t + 10.21 R2 = 0.993
B I = 119 t + 7.99 R2 = 0.995 A I = 264 t + 6.57
R2 = 0.989
0 10 20 30 40 50 60
0.000 0.500 1.000 1.500
Time t (h)
Cumulative Infiltration I (mm) A B C
Figure 24 Three infiltration measurements taken at the one WARMS site. A and B are infiltration replicates in the patch while C is the infiltration in the interpatch
The differences in infiltration rates (Q) observed in Figure 24 are seen across the soil infiltration measurements as a whole. The average steady-state infiltration for the patches was four times higher than the interpatches (Table 10). For all the sites, patch infiltration rate was 305 mm/hr compared to 75 mm/hr for the interpatch infiltration rate. With the 14 paired infiltration measurements, Q was 73 mm/hr and 326 mm/hr for the interpatch and patch respectively (P0.05; Two tail Paired t-Test), thus supporting the whole data set.
With all the measurements, there were more than twice the number of infiltration measurements made in the interpatch compared with the patches—a reflection of the proportion of interpatch and patch at WARMS sites (Appendix Table 4.3). There are
observable differences in soil texture with patches tending to have lighter soil textures than interpatches (Section 2.2.3.1). Light textured soils are often associated with high infiltration rates. Here however, Tables 10 and 11 indicate infiltration rates for all texture groups are higher in the patches than in interpatches, and importantly, average steady-state infiltration rates are of similar magnitude for a range of texture classes within a patch class at either depth. In addition there is only a weak relationship between infiltration rate and soil texture, with infiltration rates slower for heavier textures.
Differences in infiltration due to vegetation have been observed by others. Tongway and Ludwig (1996) demonstrated that infiltration under a simple mulga branch stack (in effect fallen timber) improved tenfold, from 11.6 mm/hr to 118 mm/hr. Dunkerley (2000, 2002) showed in the interpatches between mulga groves infiltration was a function of distance from the stems of shrubs, with infiltration rates near the stem up to 20 times greater than those measured 6 m away. These differences are similar to those observed here.
The results suggest that in the Gascoyne River catchment vegetation may determine infiltration rate across the catchment and not soil texture. Therefore, it is reasonable to expect soil infiltration rates are more likely to change with changes in vegetative cover from season to season and in the longer term as a result of droughts and land management practices.
Table 10 Comparison of average steady-state infiltration rates (mm/h) for each patch class and texture group at soil depths 0 to 5 cm
Soil depth 0–5 cm Interpatch Patch Average
Sand 74 333 218
Sandy loam 76 424 124
Loam 89 133 101
Clay loam 73 435 182
Light clay 39 134 96
Clay 22 N/A 22
Overall average 75 305 151
Table 11 Comparison of average steady-state infiltration rates (mm/h) for each patch class and texture group at soil depths and 5–10 cm
Soil depth 5–10 cm Interpatch Patch Average
Sand 130 366 280
Sandy loam 73 448 136
Loam 85 133 101
Clay loam 86 312 173
Light clay 29 134 68
Clay 26 N/A 26
Overall average 75 305 151
3 Effect of soil and vegetation condition on landscape stability
This section describes the mechanisms and processes within the Gascoyne River
catchment associated with landscape function using photographic evidence to demonstrate the role perennial vegetation groundcover has in maintaining landscape stability and in reducing the risk of soil loss for different rangeland landscapes.
The information presented in this section is based on field work in the Gascoyne River catchment between June and August 2011. All photographs used in this section are of the Gascoyne River catchment. Aerial photography has been provided by and with the
permission of the Western Australian Land Information Authority trading as Landgate.
Evaluations of catchment condition that form the basis of this section are based on the methodology established from the DAFWA rangeland survey project and from previous flood reviews (Section 2.1). Landforms and landscape patterns of interest, predetermined through aerial photograph interpretation, were visited whilst on route to WARMS sites. To formulate an overall perspective of catchment condition, traverse notes and photographs were collected throughout the field work period.
To explain the landscape processes responsible for influencing soil and vegetation condition in the catchment it is first necessary to present the landscapes of the catchment in a broad geological and geomorphic context. This is provided in ‘Physiographic regions of the
Gascoyne River catchment’ (Section 3.1). A soil-landscape mapping hierarchical framework is used to describe the different landscapes. Landscapes with similar topographic features, soils, vegetation and drainage patterns are grouped at a systems level. The spatial
organisation of the catchment’s landscapes are explained through descriptions of landforms, geomorphology and vegetation.
Prior to summarising the condition of the catchment’s landscapes it is also necessary to describe how rangeland ecosystems function, and how the present condition of the soil and vegetation is affecting landscape stability. Landscape processes operating within, but not unique to, the Gascoyne River catchment are briefly described in ‘Landscape organisation and function’ (Section 3.2). Whilst focusing on the Gascoyne River catchment much of the information presented on landscape organisation and function is relevant to many of the southern rangeland environments of Western Australia.
The findings from traverse notes and aerial photograph interpretation form the basis of the
‘Gascoyne River catchment condition summary’ (Section 3.3). Degradation and erosion processes common to the various land types are discussed to explain how catchment function is being impaired and therefore affecting landscape stability. This section concludes with a brief summation of the catchment.