Finally some rain; It has been a dry couple of months, especially in Sydney. My garden looks like dust bowl and not because of the recent dust storms in Sydney. The amount of soil that was lost in those dust storms is scary and will take years to rebuild. It stresses even more the need to sequester carbon in our soils and many people have already pointed this out. The agricultural community would like to be paid for the amount of carbon they sequester and that is understandable, but, while there is great potential there are difficulties in monitoring the amount of carbon changed. But what about the water? The last month drier weather got me watching the “El Nino” monitor again to see if we are again in for a long drought, but it all seems pretty uncertain. It also got me thinking again about how the rainfall would change under changes in the global climate and how carbon and water would work together.
A group of people, led by R.B. Jackson, a while know ecopysiologist wrote an article in Science in 2005: “Trading Water for Carbon with Biological Carbon Sequestration". This is one of my least favourite, or maybe my most favourite article because it drives me mad! It drives me mad because I cannot come up with a strong scientific counter argument, but I come up with lots of questions. Their argument was that planting more trees would lead to reduced flows in the rivers, an argument echoed in Australia by the Murray Darling Basin Commission and the well-known Professor Mike Young from Adelaide. Basically, trees use water to grow and this use of water is greatest in the first 10 – 20 or so years of their growth curve (the famous Kuzcera curve) and afterwards stabilises. This around the time that the plantations would be harvested and new ones planted and thus keep having high levels of water use. There is little to argue about the science. So maybe we should not be planting trees to sequester carbon as this will decrease the amount of water available.
Using that same argument means we should also not sequester any carbon in soil. More carbon in soil leads to higher water holding capacity and this water will be evaporated and used by plants and thus leaving less water to run off and replenish the rivers. You can see the silliness in these arguments, again we would argue that we should not sequester more carbon in our soils because it will lead to less water in the rivers.
The other question I have is about how much water there was before humans removed trees and reduced soil carbon in the Murray Darling Basin. Maybe we have always been spoiled with too much water because the land was cleared and depleted of carbon.
Would increasing carbon levels increase the recharge to groundwater from soils? If increasing carbon levels lead to better soil structure it might lead to greater porosity and thus better drainage (or worse drainage depending on your point of view). This was demonstrated already on conservation tillage and in paddocks with erosion control banks. Paddocks under conservation tillage and with control banks had much higher water fluxes below the root zone than conventionally tilled soils. Thus more water to groundwater.
If you take the point of view that most of the shallow groundwater is unusable and that there is little connection from groundwater to surface water (i.e. rivers in NSW are losing rather than gaining) than this water is essentially lost to humans, or is it? Do we really understand enough about the connections between groundwater and surface water.
Which landscape would be more resilient? We tried to argue this in a landscape with rainwater harvesting in India (essentially forcing the water to the groundwater) and found that increasing groundwater storage made the irrigated agriculture more resilient, but we could not yet answer this for the whole landscape.
Maybe it is all a question of equilibrium. Yes, initially we will see a reduction in streamflow due to greater storage and use of water in the landscape, but in the end it will reach a new equilibrium.
You can see my problem. I cannot believe that the answer to more water in the river systems (“to save the Murray”) has to be increased runoff and this banning tree planting and maybe carbon soil sequestration. If that is the argument than the catchments should all be concreted over.