Great rain but ...

February 16, 2009
By: Mark Kesby, Projects Officer, LPLM

Driving to town last week, it was great to see the good rain (it got even wetter that afternoon). I noticed that some paddocks were starting to run water while all the rain had soaked into others. Same rain, so why the difference? Obviously some paddocks had a better ability to allow the moisture to infiltrate.

The paddocks that were bare and compacted by stock were the worst, bare cultivated paddocks not much better while paddocks with good stubble cover or standing pasture still weren’t running water at all! Really there is nothing surprising about this. We have known for 30 years that ground cover is critical for protecting the soil and increasing water infiltration. The table below shows work done by the Soil Conservation Service in this area in the 70’s. They clearly showed that once groundcover gets below 70%, runoff and erosion increase markedly while water infiltration declines.

There are two main reasons for the differences. The groundcover protects the soil surface from raindrop impact. Rain drops fall with a lot of speed. They hit the bare soil hard and splash up again taking dirt with it. This process leads to surface sealing resulting in greatly reduced water infiltration. Groundcover also holds water back, giving it more time to infiltrate.

This can be tested simply on your farm. Take a shovel or moisture probe out onto your paddocks and see how far the moisture has infiltrated (make sure that its not somewhere that there has been ponding). You might find the results very revealing. There will be differences between paddocks and in some cases these could be large. The differences could be due to different soil types, slope, compaction (surface or at plough depth) or groundcover. The important thing is to discover if there is a difference and then why.

The practical implications of this are major. Soil moisture is the major limiting factor in our farming system, yet we are letting much of our rainfall run off rather than staying in the paddock to be used by our pastures and crops. A bare fallow could lose half the rainfall as runoff. That’s a whole lot of production lost!

All that runoff will take a lot of soil with it. One millimetre of soil lost over one hectare equals about 10 tonnes of soil. Some of the erosion losses that can be seen at the moment would be in the thousands of tonnes. Sorry to say they aren’t making any more soil and worse, the majority of nutrients are near the surface so you are losing a lot of nutrients with that soil.

How we can stop some of those losses is a topic for another day.