The topography in the lower catchment is relatively low lying ( Bogie River , Pelican Creek , Bowen River ), with increasing elevation in the upper catchment , particularly the Broken River sub catchment ( up to 1,070 metres ; Figure 8 ; Roth et al . 2002 ). The satellite image in Figure 7 emphasises these landscape features . Several sub catchments drain the western slopes of the coastal ranges , particularly the Bowen and Broken Rivers . Long-term annual mean rainfall ( 1890-2011 ) indicates that the Bowen-Bogie sub-catchments generally receive 600-1000 mm / yr , whereas the upper catchment areas including Broken River receive around 2000 mm / yr ( Figure 9 ).
2.3 Soils in the BBB catchment
As a result of the geology of the Broken , Bowen and Bogie being characterised by granitic parent material , soils in the three sub catchments are dominated by Chromosols and Sodosols ( Table 1 ). Chromosols are more associated with undulating terrain , while Sodosols tend to be more prevalent in plains and in alluvial deposits along the major rivers and creeks . Both are also called duplex soils because they are characterised by an abrupt change between a relatively loamy-sandy top layer , and a dense clayey subsoils , restricting water infiltration during high intensity rainfall . Hence both soil types are very vulnerable to erosion . Because of their topography , Chromosols can more easily lose their topsoils , and once the subsoil is exposed can form hillslope gullies and scalded areas . In the case of Sodosols the subsoil is highly dispersive , making them more susceptible to erosion than Chromosols . Once the topsoil has eroded Sodosols tend to form deep and extended gully networks , often along drainage lines or in the alluvial terraces adjacent to the major rivers . Exposure of only small areas of sodic subsoil can be enough to trigger the gullying process . Once initiated , the ‘ melting ’ of the subsoils is very difficult to stop . These soils are moderately productive in terms of pasture growth . Together they cover about 2 / 3 of total basin , which is why the three sub catchments are so erosion prone .
Table 1 . Areal extent and prevalent distribution of the three main soil types in the Bowen , Broken and Bogie sub catchments ( data extracted from the Atlas of Australian Soils , as modified by Isbell et al , 1997 )
Soil type 1
|
Key occurrences ( in order of dominant sub |
Approximate area ( ha ) |
|
catchment ) |
|
Chromosols |
Bogie R , Broken R , Pelican Ck , Little Bowen Ck , |
388,317 |
|
Bowen R , Glenmore Ck , |
|
Sodosols |
Broken R , Bogie R , Little Bowen Ck , Pelican Ck , |
340,968 |
|
Rosella Ck , Bowen R , Glenmore Ck |
|
Vertosols |
Rosella Ck , Bowen R , Pelican Ck , Little Bowen R , Bogie R |
151,351 |
|
1
Australian Soil Classification ( http :// www . clw . csiro . au / aclep / asc _ re _ on _ line _ V2 / soilhome .
|
htm ) |
Another very widespread soil type is black or grey cracking clay soils called Vertosols . These tend to be moderately to highly productive pasture soils , and are located on plains or in landscape depressions . They are often associated with Sodosols in the plains , and in some instances Vertisols can also express sodic properties in the subsoils . This makes these soils also prone to gully erosion , but at a lesser scale than Sodosols .
Other significant parent materials are constituted by sedimentary rocks and materials of volcanic origin . These rocks form a wide range of soils that occur in smaller areas or pockets . On the more steeply inclined areas ( south and south east , Broken River , and western boundary of the Bowen sub catchment ), shallow and less developed soils tend to dominate . They are less prone to erosion , but typically are low productivity soils because of their limited water holding capacity and low nutrient fertility .
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