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In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Australian Agriculture Assessment 2001
Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
APPENDIX 2. Australian Soil Resources Information System
Available water capacity (topsoil and subsoil)
Available water capacity is a measure of the store of water available for plants to use. It is presented as the estimated total for the horizon (topsoil or subsoil) and measured in millimetres.
This assessment provides an approximation of the water storage capacity of Australia’s agricultural soils. It can be used in association with other soil (hydraulic conductivity, nutrient status, erodibility), climate and topographic characteristics to determine the suitability of land for either dryland or irrigated agriculture. An understanding of the soil-water regime is also important for moisture management in intensive agriculture (e.g. horticulture and viticulture) where optimising the supply of water to the plant at critical periods is important for plant growth and controlling ripening or perhaps optimising protein or sugar content.
How does it vary and what is it related to?
The amount of water held by the soil varies with soil texture, organic matter content, bulk density and soil structure development. Available water is defined as the amount of water held in the soil between two critical thresholds:
- field capacity or the amount of water held in the soil after being saturated and allowed to drain to an equilibrium;
- wilting point or the point at which most plants cannot draw the water from the soil because it is bound too tightly to the soil particles.
Across Australia, available water capacity varies closely with thickness of the soil layers with favourable water holding properties.
Available water capacity is the amount of water in the soil horizon that can be extracted by plants. (Table A14) presents the estimated total for the solum (topsoil plus subsoil). Figures A18 and A19 depict the distribution of the topsoil and subsoil available water capacity respectively.
Level of uncertainty
The scale of the soil maps used in deriving this map is shown in Figure A2.
Level of uncertainty associated with estimates of available water capacity are very high. McKenzie et al. (2000) note the many physical and practical reasons why such an estimate of available water capacity is only an approximate, and sometimes erroneous, estimate of the actual plant available water capacity (see Hillel 1980). Despite these limitations, it provides a reasonable first approximation of the water storage capacity of a soil.
Table A14 Total profile available water capacity (topsoil plus subsoil) (mm of water) by percent of land use type across Australia.
| 0 - 100 | 100 - 150 | > 150 | Total land use class area (ha) | |
|---|---|---|---|---|
| Conservation and natural environments | 25 | 50 | 25 | 227 605 100 |
| Production from native environments | 40 | 39 | 22 | 426 286 100 |
| Cropping | 52 | 32 | 16 | 22 466 200 |
| Grazing modified pasture | 58 | 29 | 12 | 19 181 300 |
| Horticulture | 32 | 43 | 25 | 347 900 |
| Irrigated cropping | 21 | 46 | 33 | 948 000 |
| Irrigated modified pasture | 23 | 67 | 10 | 1 076 200 |
| Total area | 697 910 800 |
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