S.C. Boucher.
School of Geography and Environmental Science.
Monash University, 3800 Victoria.
This article is a summary historical document about the pioneering research work on land degradation by R.G. Downes from the mid-to-late 1940's around the Dookie area in north-eastern Victoria. Unquestionably the material is old and much of the incidence of erosion will have changed, but the extent and density of erosion at the time is worth noting. Furthermore, Downes made attempts to explain the origin of tunnel and other forms of erosion in Victoria in the context of the environmental changes wrought by European settlement.
The area in question is located in the central-east part of the Goulburn-Broken CMA, near Violet Town. Whilst much of the mapped area was eroded by sheet, gully and tunnel erosion, degradation was most severe on land south of Broken River, especially in the Parishes of Caniambo and Gowangardie where the annual average rainfall was approximately 20-25” i.e. 508 – 635 mm (Downes 1949, Figure 2). A subset from a map of land degradation in the latter area has been scanned from the original 1949 report and reproduced below. Tunnel, sheet and gully erosion were evident with the latter two forms having been graded in severity.
Tunnel erosion in the County of Moira (Downes 1946). |
The rainfall pattern was winter-dominant with most rain falling between April and October. The summers were generally hot, dry and interspersed with rainfall “… irregularly distributed in large amounts” (Downes 1949, 12). These summer rains together with the autumn seasonal break were suggested to be responsible for water erosion. The soil tended to dry and crack to a depth of 2-3’ i.e. 60-90 cm, enhancing the potential for tunnels to form. Tunnels were located in the B1 horizon, samples of which dispersed more than those from the other horizons (Downes and Leeper 1940, Downes 1946). The land was selected and cleared from approximately 1870 onwards. Clearing of most of the Eucalyptus spp. trees (particularly E. hemiphloia i.e. grey box) left native grasses such as Danthonia and Stipa spp. that were overgrazed by kangaroos, rabbits and sheep. Further details were provided by Downes (1958, 1974). The minimum slope steepness for tunnel erosion to occur was approximately 4o. |
Profile Characteristics | Horizon | Horizon Depth (inches) | Horizon Depth (cm)a | Texture | pH (Water) 1:5 | Sum of Major Cations | ESPb |
Iron-impregnated chips; | A1 | 0-4 | 0-10 | Loam | 6.6 | 9.6 | 3 |
Iron-impregnated chips; | A2 | 4-6 | 10-15 | Loam | 6.9 | na | na |
Small, nutty when dry & plastic and sticky when wet | B1 B1 B1 | 6-10 10-17 17-36 | 15-25 25-43 43-91 | Clay Clay Clay | 7.4 7.5 7.8 | 12.2 na 26.4 | 9 na 20 |
Ordovician & Silurian slates, sandstones, shales & quartzites | BC | 36-72 | 91-183 | Clay | 5.1 | na | na |
Legend a inserted by the author. b calculated as ESP [m.e. 100g] = Exchangeable Na [100] Sum of Major Exchangeable Cations na not available. |
The following soil data were extracted from Downes (1946, 1949) as presented in the article on gully erosion by Boucher (2003b). M. Imhof (pers. comm.) classified the following soil profile as a Red/Brown Sodosol (Isbell, 1996). It is evident from the ESP data that the subsoils were strongly sodic, as was the case at Caniambo. |
Profile Characteristics | Horizon | Horizon Depth (inches) | Horizon Depth (cm)a | Texture | pH (Water) 1:5 | Sum of Major Cations | ESPb |
Iron-impregnated chips; | A1 | 0-2 | 0-5 | Loam | 5.6 | 6.9 | 6 |
Iron-impregnated chips; | A2 | 2-5 | 5-13 | Loam to Sandy Loam | 5.6 | na | na |
Small, nutty when dry & plastic and sticky when wet | B1 B1 | 6-15 15-27 | 15-38 38-69 | Clay Clay | 6.0 6.1 | 10.2 na | 14 na |
Ordovician & Silurian slates, sandstones, shales & quartzites | BC | na | na | Clay | na | na | na |
Legend a inserted by the author. b calculated as ESP [m.e. 100g] = Exchangeable Na [100] Sum of Major Exchangeable Cations na not available. |
Prediction of dispersive potential for soils affected by tunnel erosion near Gowangardie (after Fitzpatrick et al. 1994). | The relation between ESP and Exchangeable Mg/Ca for soils affected by tunnel erosion near Gowangardie (after Fitzpatrick et al. 1994). |
Horizon | Horizon Depth (") | Horizon Depth (cm)a | Texture | pH (Water) 1:5 | Sum of Major Cations | ESPb |
A1 | 0-8 | 0-20 | Friable clay loam | 6.6 | 9.7 | 2 |
B1 | 8-14 | 20-36 | Friable clay | 6.7 | 8.5 | 1 |
B21 B22 | 14-25 25-50 | 36-64 64-127 | Friable clay Friable clay | 6.4 7.4 | na 23.0 | na 2 |
BC | 50-72 | 127-183 | Friable clay | 8.7 | na | na |
Legend a inserted by the author. b calculated as ESP [m.e. 100g] = Exchangeable Na [100] Sum of Major Exchangeable Cations na not available. |
Horizon | Horizon Depth (") | Horizon Depth (cm)a | Texture | pH (Water) 1:5 | Sum of Major Cations | ESPb |
A1 | 0-8 | 0-20 | Friable clay loam | 6.6 | 9.7 | 2 |
B1 | 8-14 | 20-36 | Friable clay | 6.7 | 8.5 | 1 |
B21 B22 | 14-25 25-50 | 36-64 64-127 | Friable clay Friable clay | 6.4 7.4 | na 23.0 | na 2 |
BC | 50-72 | 127-183 | Friable clay | 8.7 | na | na |
Legend a inserted by the author. b calculated as ESP [m.e. 100g] = Exchangeable Na [100] Sum of Major Exchangeable Cations na not available. |