Sunday, 18 March 2007

Rain Water Absorption Rates on Bare Soils

In Darryl Cluff’s booklet titled “Farming Without Farming” ISBN 0-9751118-09 Published in 2003 and available through Stipa Native Grasses Association, Inc. he comments at one point that as a genera rule on land which has an effective water cycle in operation one should expect to see 1 cm of soil profile moisture created from every 1 mm of rainfall. Thus where the soil profile is dry and 5 mm of rain falls there should be 5 cm of soil moisture in the soil, after allowing time for infiltration.

Recently on Ochre Arch we received 9 mm of very gentle rain over a period of 5 hours. Pretty much all of the soil on the property had zero grass ground cover, and prior to the rainfall event the soil profile was extremely dry. Approximately 1 hour following this event I headed off across a couple of paddocks with shovel-in-hand to see how much moisture had been absorbed by the soil.

On the flatter country in one place where the soil surface was not capped I dug down to find that there was about 9 cm of moisture penetration – consistent with Darryl’s comments, and evident in the accompanying photograph.
At another place on the property I dug to see what the soil moisture profile was underneath one of the well-worn sheep tracks. The results here were markedly different, and the moisture profile was only around 1 cm – almost 90 % less than on the flatter country. See second photograph. This was despite the fact that there had been considerably more water in the sheep track, what with the runoff of water along the sheep-track that had occurred. On reflection, the primary feature of the soil surface in the sheep-track was compressed / capped soil, making the bottom of the track very much like an impervious trough. The underlying cause of the compaction was over-grazing.

The other critical factor in rainfall effectiveness is retention of water in the soil post the rainfall event. Given there is was no ground cover on Ochre Arch the loss of moisture in the days following was extreme (especially given hot weather) and pretty much all of the moisture had evaporated within 48 hours.

For those wishing to learn more about the basics of the way the water cycle works, there is an excellent presentation on the website. Click HERE to go directly to the presentation.

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