Drought and flooding rains

Walter Jehne, former CSIRO Climate Scientist and Microbiologist, founder of Healthy Soils Australia, has written a 3-part document on drought in Australia and in it he demonstrates many scientific facts about how we should be managing our soils. There are many fascinating revelations in his scientific analysis such as how the removal of vegetation affects rainfall or how the lowering of soil carbon lowers the capacity of the soil to hold onto water. There is also a cry for critical leadership to develop ways of enhancing the soil carbon sponge. It is beyond the scope of this newsletter to cover the scope of Walter’s papers, however the AIEA is prepared to send email copies to anyone who registers an interest. If interested please contact kcochrane@ecoag.org.au.

A. The evidence for the systemic aridification of Australia, beyond periodic droughts
B. The processes driving it
C. The case study evidence of how to rehydrate and regenerate landscapes to minimize these threats.

The following exerts are from Paper (A)

1.     The infiltration of raindrops by the Earth’s soil carbon sponge.

Our current industrial agricultural practices such as clearing, burning, cultivation, fertilization, excess irrigation, use of biocides, overgrazing and bare fallows often impair these microbial processes and oxidize carbon from soils carbon leading to their structural collapse. Hygrographs demonstrate how such soil degradation can impede rainfall infiltration and increase runoff, floods and soil erosion. 

Effectively this structural soil degradation and collapse can aridify soils and make them less resilient and bio-productive and more drought prone, even with normal or above normal rainfall. “Droughts’ in such soils and agricultural systems are therefore often man-made. Calls for irrigation to overcome such ‘droughts’ are often counterproductive as the soil can’t infiltrate and use this water efficiently.

Conversely, we can readily improve the rainfall infiltration in most soils, and thus their ability to avoid and be resilient to ‘drought’ by regenerating the natural microbial processes that aggregate and build structure in soils via their bio-sequestration of carbon. Extended regionally ecological grazing, cropping or horticulture that aids the regeneration of the carbon content and structure of soils is our only practical means to help limit hydrological extremes and ‘droughts’, whatever the rainfall.

4. Why we must extend the longevity of green growth and its cooling transpiration effects.

Industrial agriculture for more than 100 years has sought to maximize yields and the efficiency of yield increases per unit of limiting factor or input; mostly of added water and nutrients.

To do this it has selected ‘pioneer’ plants that can grow big quickly via ecological ‘weed’ strategies. These fast growing pioneer plants can often escape modest droughts via their rapid growth and maturation while conditions remain good and rapid seed set and senescence once stresses and limits intensify.

By contrast most natural perennial plants invest biomass to establish resilient soil interfaces to help them access limiting factors and buffer stresses so as to sustain their longevity of green growth despite these stress conditions.

While both strategies can be effective, humanities growth and demand for food, the degradation of soil resources and the locked in increase in climate stresses, dictate that our current rapid opportunistic growth model may no longer be viable even with inputs.

For example, if 100 mm of rain falls on degraded soil from which 80% runs off, the 20 mm of retained water  may only be able to sustain growth for 2-3 weeks, far too short for even opportunistic plants to grow and set seed. Conversely that same 100 of rain falling on a healthy soil with 95% infiltration may sustain slower growing plants for up to 20 weeks via the efficient use of that stored soil water.

As rainfalls decline and become less reliable we need to focus on conserving and efficiently using our limiting soil water resource so as to extend the longevity of green growth so we can still secure good yields with minimal inputs. To do that we need to regenerate the structure, health, water holding capacity and the longevity of green growth of our agro-ecosystem, not the growth rate of a crop.

Breeding ‘superior’ plants, often via just higher harvest indicies and input dependencies has at best increase yields by some 50%, but with significant externalized natural capital and strategic costs. By regenerating the structure, rainfall retention and the longevity of green growth of our soils we can readily grow plants for 10 times longer, yielding a potential 1000% growth response.

More importantly, as our industrially grown plants may not be able to finish and set seed as climates aridify, this relative difference in growth is infinite, and existentially of life or death significance.

5.The regeneration of shelterwoods to protect water in soils from evaporative losses.

Australia used to receive on average 452 mm/an of rain. Its potential evaporation rate is often four times this. Exposed surface water can thus be lost rapidly by evaporation and wind scour effects.

While most of the rain that fell on and infiltrated into the Earth’s soil carbon sponge was protected naturally from evaporation by surface litter and plant covers, our industrial agriculture with its clearing, fires, overgrazing and cultivation and its associated oxidation of soil carbon and collapse of soil structures has resulted in vast areas now becoming bare and exposed to increased evaporation. As the climate aridifies and is more variable and the risk of major water losses have intensified. Whereas over 90% of the rain that fell may have been naturally infiltrated and used efficiently by the native plant covers to extend the periods of green growth and cooling transpiration, up to 90% of the rain may now run off from the residual subsoils due to our 200 years of landscape degradation. While levels vary with site, seasons and the degree of past soil degradation, over half of that runoff may be lost to evaporation, a third may be transpired by the remaining vegetation with the remainder contributing to the 12% of rain that reaching streams and recharges deeper aquifers.

Naturally most of Australia’s inland was also covered by an open grassy woodland or shelterwood of scattered trees that resembled an open park and greatly reduced surface and wind speeds and evaporation losses from the protected shaded soils and grasslands. Our extensive clearing of these trees by fire, overstocking and for cultivation and their limited regeneration as they age has exposed vast areas of these residual landscapes to increased evaporation losses, aridification and droughts.

If even half of the 50% of rainfall that now evaporates could be again be infiltrated into soil for use by plants this could significantly reduce the frequency and severity of our current ‘droughts’. This could be achieved practically at minimal cost by regenerating the Earth’s soil carbon sponge over much of this landscape via the simple ecological grazing ecologies discussed subsequently and by regenerating the protective shelterwoods that naturally limited wind speeds and evaporation .

This contrasts with desperate calls and political talk to build more dams to ‘drought proof’ Australia. Currently 2% of Australia’s rainfall is able to be stored in dams. This water needs to meet Australia’s irrigated agriculture (70%), industrial (20%) and domestic (10%) needs. Even if suitable sites existed that would not evaporate during drought, this water would need to be available across Australia’s vast land area when and where needed at an affordable cost if it was to offset future ‘droughts’.

In fact, Australia already has major dam storages such as the 10,000 GL in Lake Argyll whose water is grossly under-utilized because of its distance from demand and the impossible cost of its transport.

Instead aiding land managers to regenerate their soil carbon sponges as proposed in Regenerate Australia could readily provide the equivalent to an extra 100 lake Argylls to supply water where and when needed and protected from evaporation at minimal capital or public cost. All that is required is leadership and recognizing our imperative to reverse the systemic aridification of Australia in time.

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