Climate
The Mediterranean-type climate of the Avon River Basin is typified by mild wet winters and hot dry summers controlled primarily by 'southern oscillation of the of the anticyclonic belt' with relatively small influence of the 'El Nino' effect. There is some cyclonic influence during summer months. Annual rainfall decreases from about 700 mm in the west to 325 mm in the east of the basin. Approximately 70% of annual rainfall falls during the 5-month growing period (May-September) and is of relatively low variability. Frosts also occur during this period.
Wet winters or intense rainfall events cause occasional flooding. Major flooding has occurred in 1910, 1917, 1926, 1930, 1945, 1946, 1955, 1958, 1963, 1964, 1983 and 2000. The potential for change in climate may be significant to natural resource management in the Avon River Basin although the extent to which his could occur remains uncertain. Some changes predicted (CSIRO, 2001) by about 2070 (compared with 1990) include:
- higher temperatures (1-5 degrees Celsius warmer)
- changing rainfall trends (seasonality and annual averages)
- higher evaporation rates
- more frequent extreme weather events
The agricultural industry is a substantial contributor of the ‘greenhouse gas' emissions that are considered to be a cause of climate change.
Regional Landform and Soils
The landform and soils of the Avon Region have been mapped within Soil-Landscape Zones that are related to the ancient and rejuvenated drainage zones of the region. There are 9 Land Resource Sub-regions within these zones in the Avon Region. Approximately 150 Land Systems and 900 Land Sub-systems (soil-landform units) have been mapped within the sub-regions. The application of digital elevation models (DEMs) and high-resolution radiometric information adds value to understanding the distribution of soils in the region. The Land Systems can be identified in three broad categories for management:
1.Uplands
2.Mid-slopes
3.Valleys
The Land Resource assets are identified by grouping of Land Systems within the broad categories. However, available information does not allow the Land Systems to be consistently grouped according to these categories so the Land Resource Sub-regions are currently adopted as the assets. The topographical sequence of Land Systems from uplands to valleys provides an important basis for integrated action for NRM implemented at a landscape scale.
The soils of the Avon River Basin can be generally described as being of low fertility. Duplex textured profiles are most common in which sub-soils have low hydraulic conductivity. Deep sandy soils have a naturally acidic trend. The dominant soil types are shown in the Land Resource Supporting Document.
Groundwater and Salinity
The deeply weathered soil profiles of the ancient landscapes in the Avon River Basin have extensive regional groundwater aquifers and localised unconfined aquifers. Groundwater resources in the Avon Region are generally not suitable for irrigated agriculture due to poor water quality however there are some fresh superficial aquifers with commercial use potential and farm water supply. Some saline groundwater is used for mining.
Groundwater in the Avon River Basin is monitored locally where wells are installed for salinity risk investigations. An inventory of long-term monitoring information of regional aquifers is maintained in 'AQUABASE' which is coordinated by the Department of Environment. Community-based monitoring information is coordinated through 'COMBORES' which is managed by the WA Department of Agriculture. An analysis of monitoring information shows a trend of regional groundwater rise in most areas. Monitoring has also shown groundwater in the region to be acidic.
The regional groundwater aquifers are generally saline, with water quality generally exceeding 30 000 mg/L. Local groundwater flow systems range from fresh to saline. The groundwater processes of the region are well understood although groundwater flow systems of the Avon River Basin have not been mapped. Regional groundwater aquifers between catchments are thought to not be connected. The very low gradient and poorly transmissive soils in the valley floors delay the rate at which groundwater aquifers change but also delays the response to aquifers by groundwater management.
The full potential impact of salinity has been estimated from interpretation of Digital Elevation Models and satellite imagery (the 'Land Monitor Project'). This shows there to be 5.4% of land used for agriculture within the region to be currently effected by salinity but this could eventually increase to 27.5%. This information is consistent with estimates for the region by the National Land and Water Audit (NLWA, 2001). An analysis of the threat of salinity within the Avon Region is provided in the Land resource Supporting Document.
