It’s hard to believe, when stepping off an airplane in Canberra, that anywhere in the South East of Australia can ever get too wet. With the first steps onto the tarmac, stinging dry air enters my nose, my eyes start to water, and my lungs hesitate to breathe because the air, which tastes of frost in the winter and dust in the summer, is so dry.
My first few months in Canberra coincided with a drought and the dryness caused my skin to peel as if I had sunburn.
If one looks up Canberra’s climate data, it’s not surprising that I have this reaction: Canberra gets on average 614 mm of rain a year and evaporation is 1 715 mm based on the annual mean pan evaporation. Humidity (in the afternoon) ranges between 37 and 60%, although I remember some summer days when it has been below 10%. Basically, water is more prone to evaporate than accumulate in this part of Australia much of the time, and this makes the air really dry.
So it’s hard to believe there could be such a thing as too much water here. Even when not in drought, there are days over 35 degrees celsius every summer when tired looking gum trees have embolisms, dropping their branches (or limbs) and leaves everywhere.
There are also hundreds of thousands of kilometres of deep clays which have much more capacity to take and hold the water received as rainfall than the stony soils (like this and this) on the alluvial plains of my homeland.
And here is where we come to the crux of the problem discussed in this blog.
These amazing clays which are like ecological banks for nutrients and water, have unique properties which mean all this rain is causing an excess of water and some nutrients in the soil. This is, in turn, decreasing crop yields and plant health.
Let me explain.
In the South East of Australia, and indeed much of Western Australia too, the most common type of soil is texture-contrast soil. These soils have a layer of coarser (i.e. sandier) soil above a finer (i.e. a more clay rich) layer. The clay rich layer—the subsoil—is much denser (has a higher bulk density) than the layer above it. This means that when water filters down into the soil, it hits a layer which is much more difficult to penetrate than the layer above. Flow slows and water fills up the pores in the upper layers of soil where plant roots live.
This phenomenon is called water logging. It means that air in the soil, which fills between 10% and 60% of pores in well drained soils is replaced by water, and plant roots cannot exchange oxygen and carbon dioxide. This causes damage when air-filled porosity drops below 8%. Microorganisms which depend on air also die.
There are also problems with how chemicals in the soil move when soil becomes saturated.
Chemicals which otherwise act as nutrients, such as sodium and boron for example, as well as others like aluminium, can become available to plants in toxic concentrations when waterlogging occurs. Salinity is also likely to increase, meaning that the salt in the soil can pull water from the soil better than plants.
The impact of all these waterlogging related factors is reduced plant growth and in extreme cases plant death.
I saw the impact of waterlogging first hand just two weeks ago when I was out conducting fieldwork in a canola paddock. The canola at the top of the hill was knee height but at the bottom of the hill, where waterlogging was occurring, the canola was in places only ankle height. The year before the paddock had been sown in wheat, and I saw no such contrast in crop growth between the top and bottom of hill.
The aspect/s of waterlogging that are causing the reduced canola growth in this instance—whether it be anaerobic conditions (absence of oxygen), salinity, chemical toxicity, or maybe a bit of each—have yet to be made clear. But the effect of the wet season on crop productivity is unmistakable.
Statistics can probably show us better than words can just how much of a problem waterlogging is in Australia.
Some researchers reckon that crop production in South Eastern Australia can be increased by 50% if constraints such as waterlogging, lack of adapted cultivars (crop types), limited root penetration due to poor soil structure and acidity, and poor nutrient supply, are overcome. Others have said that in Western Australia wheat, oat, and lupin yields decrease by 15, 11, and 6 kg per hectare respectively for every 1 mm of rain received in August (when plant growth and therefore evapotranspiration is lowest).
So while my skin feels great and my nose doesn’t react at all to the breeze blowing in from out west these days, the negative impacts of this season will be felt by many farmers in South Eastern Australia.
It doesn’t seem quite fair does it, this constant struggle for more water at some times and for less water at others?
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