Slow the flow and let the soil drink its fill

Bushfires have been rampaging across the country for months and emergency services are stretched to exhaustion. Lives, and more than 700 homes have been lost. Millions of people have suffered hazardous air quality for weeks, and immeasurable damage has been done to native biodiversity.

The fire emergency coincides with severe heatwaves. Australia has been steadily getting hotter for 70 years, with national average temperatures about 1.5C above the 1910-1950 average. The long-term trend dominates, and largely follows global trends, which relate to human-caused climate change.

Even for those unconvinced about ­climate change, however, the sharp rise in the frequency and intensity of heatwaves speaks for itself. So what can we do? The ­answer is in two parts: we must address the acute symptoms (fire outbreaks), and deal with the underlying environmental issues.

Longer-term improved funding support for the heroic fire services is essential. We need more of them and must retain those we have. And we need more equipment, including high-volume water bombers. We should also look at new approaches and materials for firefighting and fire prevention. Fires and smog affect us all — it is hard to think of a better use for the budget surplus.

The underlying issues are a harder nut to crack. For this, it is essential to move the focus away from climate change rhetoric and on to tangible action.

Global warming is related to the total amount of greenhouse gases accumulated in the atmosphere. Even without any new emissions, the legacy of accumulated global emissions would maintain temperature where it is now. Worse, some additional year-on-year warming would continue ­because of slow responses in the climate system. To reverse the warming trend, a large net drawdown of carbon dioxide from the ­atmosphere is needed. Bringing this to the scale required is humanity’s grand challenge for the rest of this century.

But for the shorter term, we need ­additional efforts that reduce fire risks, and that help repair fire damage in ways that ­reduce the chance of a recurrence.

Among the key issues that determine fire risk are temperature and fuel humidity, or moisture availability in the landscape. ­Others are fuel availability, wind speed, and landscape slope.

Local conditions are important too, ­especially via evaporative cooling. When it’s hot, evaporation increases. Evaporative cooling then occurs in soils and in the overlying air; plants and trees increase this effect. In arid regions, soil moisture availability limits the evaporation rate. As a result, evaporative cooling reduces when soils dry out, causing temperatures to increase. It’s a rapid cascade towards totally dry, hot conditions. Good soil moisture is key to avoiding this.

On parched land, with degraded soils, little water penetrates the soils during rainstorms. The water washes away over the surface into gullies, carrying topsoil with it. High-energy flows develop in the gullies, and cause these to erode rapidly. Over time, more and more topsoil is washed away, and winds remove much of what remains, ­especially when vegetation has died off. Vegetation die-off can be caused by overgrazing, or by drought and temperature stress.

But this runaway sequence of destruction may be stopped by slowing the rush of water through valleys and gullies with barriers (from logs, boulders, and so on), and by creating ponds and slow-flowing creeks in flatter areas by means of successions of weirs. Encouraging vegetation such as reeds helps to further manage water flow and improve water quality, while water lilies help to ­reduce evaporation.

Water retained in the created creeks and ponds gradually recharges surrounding soils over a wider area. This can be accelerated by shaping the landscape with contours that lead overflow water away from the gully-barriers and/or weirs. Once established, groundwater accumulated in the wider region ensures that water remains present in the creeks and ponds during droughts, even when conventional dams become dry.

The wider region’s soil moisture improves vegetation growth and root systems to help rebuild the soil’s organic content. Interactions between organic compounds and minerals further promote soil water retention. On farms, this helps to increase productivity. Meanwhile, grazing can be changed from low-density grazing over an entire area, to successive intensive grazing of smaller sub­divisions. Once grazed, that allows an area to recover with the benefit of manure trampled into the ground.

This regeneration of the soil helps restore balance in ecosystems and results in more ­efficient use of precious rainfall. And it will increase productivity on farms, especially in marginal settings, and help revitalise land razed by fires or logging. The overall increase in water reduces the quantity of tinder-dry fuel, and is a resource for fire services. Regenerating the landscape also promotes biodiversity, which increases future resilience to drought and bushfire. And, if carefully monitored, the resulting build-up of soil carbon represents a drawdown of carbon from the atmosphere, which may help Australia meet its emission-­reduction targets.

None of this can stop weather extremes, or fires, but it will reduce their scale and ­impact.

And improved soil moisture and vegetation increases humidity and the potential for cloud and rain formation.

Professor Eelco Rohling is at the ANU’s Research School of Earth Sciences.