Australian Natural Resources Atlas

Natural Resource Topics

APPENDIX 2. Australian Soil Resources Information System

Saturated hydraulic conductivity (topsoil and subsoil)

Saturated hydraulic conductivity is a measure of the permeability of a soil (or how quickly water can move through the soil when it is saturated). Soil permeability, in conjunction with water storage capacity, is fundamental to controlling the soil-water regime, that determines land suitability for a range of purposes.

Soils with a slow hydraulic conductivity at or near the soil surface (e.g. less than 30 mm/hr) cannot transmit water from heavy showers of rain and this can lead to excessive run-off and potentially to erosion. Run-off also represents a loss of water that could have otherwise been available to plants. Subsoil layers are nearly always less permeable than surface layers because of the lower rates of biological activity. Soils with a strong texture contrast between topsoil and subsoil (e.g. Kurosols and Sodosols) may have a sharp reduction in hydraulic conductivity with depth. In this case, drainage of water is impeded and waterlogging can be a problem.

How does it vary and what is it related to?

Saturated hydraulic conductivity is controlled mainly by the texture, organic matter content and structure of the soil layer. Sandy soils are nearly always very permeable. Some clay soils can be more permeable than sands (e.g. Red Ferrosols) because of their strongly aggregated structure. Other clay soils (e.g. most Vertosols and the B horizons of Sodosols) are very impermeable.

The presence of worms, termites and other soil fauna increases soil permeability markedly. Earthworm burrows frequently make up a high proportion of the large pore space of soils, and soils with earthworms can drain two to ten times faster than soils without (Lee 1985).

Generally, heavy clay soils (Vertosols) have moderate permeability (Ksat < 50 mm/hr) in their topsoil, decreasing to very slow permeability (< 0.1 mm/hr) in their subsoil. Kandosols have rapid permeability in their topsoil (~500 mm/hr) which decreases to moderately slow in their subsoil. Sandy soils, Podosols, Chromosols and Tenosols in Western Australia and South Australia are very rapid permeable. Sodosols are rapidly permeable in their topsoil but only slowly permeable in their subsoil.

Table A15 Permeability by percent of land use type for topsoil when saturated (saturated hydraulic conductivity mm/hr) across Australia.
<0.3 0.3 - 3 3 - 30 30 - 300 >300 Total land use
class area
very slow slow moderate high extreme (ha)
Conservation and natural environments 2 0 7 90 0 263 893 800
Production from native environments 15 2 12 70 0 443 032 100
Cropping 5 5 13 72 5 22 519 000
Grazing modified pasture 2 2 15 76 5 19 237 500
Horticulture 4 1 15 79 1 350 900
Irrigated cropping 17 11 33 39 0 949 000
Irrigated modified pasture 15 2 50 33 1 1 079 100
Total area 751 061 400
Table A16 Permeability by percent of land use type for subsoil when saturated (saturated hydraulic conductivity mm/hr) across Australia.
<0.3 0.3 - 3 3 - 30 30 - 300 >300 Total land use
class area
Land use class very slow slow moderate high extreme (ha)
Conservation and natural environments 2 6 17 74 0 227 606 400
Production from native environments 14 12 35 39 0 426 250 500
Cropping 10 13 44 30 2 22 482 300
Grazing modified pasture 4 14 39 40 2 19 182 100
Horticulture 5 8 40 46 1 347 300
Irrigated cropping 28 13 47 12 0 947 400
Irrigated modified pasture 16 10 60 13 1 1 075 100
Total area 697 891 100

On average Australia applies about 18 000 000 ML of water onto agricultural soils every year.

Table A17 compares the permeability of saturated soil for topsoils and subsoils for irrigated lands. Of the 2 400 000 ha mapped as irrigated in the national land use map:

The most significant implication for irrigation operations are those lands (some 28%, see grey shading on table) that may be prone to waterlogging. These lands tend to have very slow or slow topsoil and subsoil permeability, or moderately permeable topsoils overlying slow or very slow permeability subsoils. Ripping and other farm tillage practices are commonly used to overcome these impediments for cropping.

Application of map

Permeability is one controlling factor in determining how susceptible a soil is to erosion. This map of saturated hydraulic conductivity has been used as one input to estimate erodibility, by combining with maps of %clay and organic carbon.

Level of uncertainty

Scale of soil maps used in deriving this map is shown in Figure A2.

Saturated hydraulic conductivity (Ks) typically exhibits substantial short-range variation and is relatively difficult to measure (there are few reliable sets of Ks data for Australia). Estimates of Ks on used by McKenzie et al. (2000) are based on experience gained in CSIRO Land and Water and published data sets. Estimates for Western Australia and South Australia soil groups are based on expert knowledge of Ks for different horizons (based primarily on texture and structure), and on interpolation of McKenzie et al (2000). Attribution Ks was estimated using the classes presented in Table A17, where the median values for each class are approximately equidistant on a logarithmic scale, since Ks data are generally log-normally distributed.

Table A17 Comparison of saturated hydraulic conductivity (mm/hr) of topsoil and subsoil for irrigated areas (%) (includes all horticulture, irrigated crops, irrigated pastures).

Topsoil <0.3 0.3 - 3 3 - 30 30 - 300 >300
Subsoil very slow slow moderate high extreme
<0.3 very slow 14 4 1 0 0
0.3 - 3 slow 0 1 8 1 0
3 - 30 moderate 0 0 29 24 0
30 - 300 high 0 0 0 17 0
300 extreme 0 0 0 0 0
Distribution of saturated hydraulic conductivity (mm/hr) of the topsoils across Australia. Distribution of saturated hydraulic conductivity (mm/hr) of the subsoils across Australia.

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