Catalyst: Water through cracked soil

GRAHAM PHILLIPS:

Dry, cracked earth is synonymous with drought in the Australian bush. But when the rains eventually come, how much do we know about how the water flows down through the cracks? Well, surprisingly little, it turns out. But a group of researchers at the University of NSW hopes to change all that. And their work may one day help struggling farmers cope better with it being dry.

DR PAUL WILLIS:

When it comes to irrigation, there a lot we don't know. We don't know how much water we should be putting on our crops or where that water goes once it hits the ground. The answer to these and many other questions is quite literally written in the soil.

PROF IAN ACWORTH:

We're really trying to understand the processes beneath the root zone and above the groundwater table.

DR PAUL WILLIS:

This team from the University of NSW are investigating soil crack dynamics and pioneering a new way to monitor water movement. As the land dries and the soil starts to crack farmers need to reconsider the effectiveness of their irrigation techniques. But how do you work out what's going on inside a soil, both as it cracks during the drought and then when it gets wet when the rains come? You need a set-up like this. That's a nasty-looking weapon you've got there. What is it?

ANNA GREVE:

It's a resistivity probe. We actually use four of them in the field. We stick them in the soil, then we can actually measure the resistivity in the area between the probes. That gives us a three-dimensional profile of the moisture content.

DR PAUL WILLIS:

So what does happen when you get it wet?

ANNA GREVE:

Well, let's find out.

DR PAUL WILLIS:

By drying, wetting, re-drying and then re-wetting the soil in this tank they've discovered that, unlike the conventional wisdom of some farmers, cracks don't appear in the same place twice. That discovery has changed our understanding of the hydrology of cracking soils. Nothing? We've put in what? 50 litres of water there? None has come through the cracks?

ANNA GREVE:

No.

DR PAUL WILLIS:

So what's going on in here? That water is just being absorbed?

ANNA GREVE:

Yes, the soil is just like a sponge. It can take in a lot of water. It's just sucking it in. And we're not getting any deep drainage until the soil is really, really moist.

DR PAUL WILLIS:

Knowing how water moves through the soil has benefits when it comes to gaining and understanding exactly how much water is required to irrigate the crop. And that's the key.

ANNA GREVE:

We can see where water is appearing and where it's moving, but we don't have the numbers yet to back that up, to really see how much water is moving. The next step is to hopefully make this a device that's cheap enough to produce so that the farmer can actually use it out in his field and make his irrigation more efficient.