Can rain suppress smoldering peat fire? (original) (raw)

Introduction

Peatlands are important ecosystems in the boreal and tropical regions, which not only support the biological diversity for a wide range of wildlife habitats, but also store 25% of the planet's terrestrial organic carbon, i.e., approximately the same mass of carbon that is in the atmosphere (Freeman et al., 2001; Page et al., 2011). Peat fire is the driving phenomenon of wildfire in peatlands, such as those that cause widespread destruction of ecosystems and episodes of haze in South Asia, North America, and north-east Europe (Page et al., 2002; Pellegrini et al., 2018; Turetsky et al., 2002, Turetsky et al., 2015). Peat fire is one of the largest and longest-lasting fire phenomena on Earth, and it can sustain for months and even for years despite extensive rain, weather changes, or fire-fighting attempts (Rein, 2013). Recently in September of 2019, large deposits of peat in Kalimantan and Sumatra were ignited and burned for several months, covering Indonesia and nearby countries with haze and causing the cancellation of enormous flights due to poor visibility (Normile, 2019). Moreover, the annual release of ancient carbon from peat fires is approximately equivalent to 15% of human-made carbon emissions (Ballhorn et al., 2009; Hu et al., 2018; Page et al., 2002; Prat-Guitart et al., 2016; Turetsky et al., 2015; Wakhid et al., 2017).

Peat fire is dominated by smoldering, a slow, low-temperature, and flameless form of combustion (Rein et al., 2008). Smoldering peat fire is different from regular flaming wildfire in its chemistry, transport processes, and time scales (Rein et al., 2009). Peat can hold a high water content to prevent the ignition, but natural or anthropogenic-induced droughts can increase the risk of peat fire (Sinclair et al., 2020; Turetsky et al., 2015). The ignition source for peat fire can be natural, such as lightning, flaming wildfire (Lin et al., 2019b), self-heating (Restuccia et al., 2017), and volcanic eruption, or anthropogenic, such as deforestation, poor land management, accidental ignition, and arson (Rein, 2013). Most recent peat fires were initiated on the surface by the flaming wildfires. The probability of ignition depends on the moisture content, mineral content, and other physicochemical properties (Benscoter et al., 2011; Frandsen, 1997; Huang et al., 2016; Lin et al., 2019b; Restuccia et al., 2017). Once ignited, smoldering fire can easily burn out an organic soil layer of >50 cm deep over an extensive area (Ballhorn et al., 2009; Huang and Rein, 2017; Rein, 2013; Wilkinson et al., 2018).

Fundamentally, three approaches can be used to extinguish the fire, that is, burnout, smothering, and cooling (Quintiere, 2006). For peat fire, burnout of peat soils is unacceptable since it will severely destroy the essential peatland resources and ecosystem, as well as release a significant amount of toxic and greenhouse gases into the atmosphere (Hu et al., 2019b, Hu et al., 2019a; Turetsky et al., 2015). Smothering is to extinguish the fire by removing or reducing oxygen. However, peat fire can be sustained in an extremely low oxygen concentration (Belcher et al., 2010; Huang and Rein, 2016), and there is neither a natural mechanism nor a manmade technique to prevent the diffusion of oxygen into the soil layer in the field scale. Therefore, quenching the peat fire by different cooling methods is the only practical approach, and water is the most widely used cooling agent in firefighting efforts. In reality, peat fire can also be quenched under several conditions, (i) the presence of an inorganic soil layer; (ii) the presence of a thick wet soil layer; (iii) the suppression of heavy continuous rains, and (iv) active firefighting (Migalenko et al., 2018; Rein, 2013). However, compared with extensive studies on the ignition and development of peat fire, very few studies are available on how to extinguish these smoldering wildfires.

Because of the persistence of peat fire, a short-term man-made water spray is not able to stop the fire spread (Ramadhan et al., 2017). Compared to flaming wildfires, smoldering wildfires require at least 50% more water to extinguish the same amount of burning fuel (Rein, 2013). Some chemical foaming agents can easily penetrate into peat soil and shield the burning peat particles from the oxygen supply (Ratnasari et al., 2018), but the required quantity to suppress any real peat fires is enormous. In fact, limited trials in the literature have demonstrated the ineffectiveness of all man-made suppression methods in controlling or extinguishing any massive peat fire (Dianti et al., 2018; Mikalsen et al., 2018; Ramadhan et al., 2017). On the other hand, the authors have identified a research gap surrounding the natural suppression of peat fire by rain.

Rain is a crucial part of the Earth's water cycle (Jash et al., 2019; Seely and Louw, 1980), and it may decelerate the wildfire spread by wetting the fuels and even directly extinguish the flame. For the recent wildfire in Amazonas, Brazil, the burning area decreased significantly when the regional rainfall increased (Vasconcelos et al., 2013). Although the suppression effect of rain on smoldering wildfire is still mostly unknown, it is hypothesized that the rain droplets can penetrate into the burning peat layer, and if the rainwater can overcome the combustion heat, eventually peat fire can be quenched. Nevertheless, if the peat fire was not entirely extinguished by rain, re-ignition could happen after an extended time, especially when the drought season arrives (Huang and Rein, 2019; Ramadhan et al., 2017). Thus, it is necessary to thoroughly explore the effectiveness of rain in suppressing peat fire and identify the critical rainfall intensity and depth.

Herein, well-controlled experiments were conducted to explore the possibility of the suppression of smoldering peat fire by rain. Rainfall intensities (I) of ‘light (<2 mm/h),’ ‘moderate (2–10 mm/h),’ ‘heavy (10–50 mm/h),’ and ‘violent (>50 mm/h)’ were tested up to 400 mm/h. The required rainfall duration (∆t), rainfall depth (d), as well as, the mass loss per unit area of peat sample (∆m _p_′′) and carbon emissions of peat fire (∆m _C_′′) under different rainfall intensities were analyzed in detail. The minimum rainfall intensity (I min) and rainfall depth (d min) to extinguish the peat fire was also quantified.

Section snippets

Peat soil sample

The organic-rich moss peat soil (Fig. 1) tested in the experiment came from the Netherlands, and it had an organic matter of about 96%. The bulk density of oven-dried peat was measured to be 145 kg/m3 (±5%). The peat sample had an open-pore structure and an overall porosity of about 0.90. The element analysis for the peat organic matter showed 44.2/6.1/49.1/0.5/0.1% mass fraction for C/H/O/N/S, respectively (Lin et al., 2019b). Because peat soils could become hydrophobic under a

Effectiveness of fire suppression by rain

The effectiveness of rain of different intensities on suppressing peat fire is quantified against the rainfall duration (∆t), rainfall depth (d), mass loss of burning peat (∆m _p_′′), and carbon emissions per unit area of peat fire (∆m _C_′′) in Fig. 2. We found that the minimum rainfall intensity (I min) to be roughly 4 mm/h (Fig. 2a), below which the peat soil could completely burnout like those without any rain suppression. Therefore, the smoldering peat fire may not be suppressed by a light rain

Conclusions

In this research, we assess the underlying mechanism of rain in suppressing the peat fire in the shallow soil layer up to 15 cm deep through laboratory experiments. The minimum rainfall intensity to extinguish the peat fire is found to be roughly 4 mm/h, so that the persistent light rain cannot extinguish such smoldering wildfire. The required rainfall duration, ∆t (min), for extinguishing peat fire decreases with the rainfall intensities, I (mm/h), as log10_∆t_ = − 1.15log10_I_ + 3.3. For example,

CRediT authorship contribution statement

**Shaorun Lin:**Investigation, Writing - original draft, Formal analysis.**Yau Kuen Cheung:**Investigation, Resources.**Yang Xiao:**Investigation, Resources.**Xinyan Huang:**Conceptualization, Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research is funded by National Natural Science Foundation of China (NSFC) No. 51876183. The authors thank for the inspiration from Prof. Guillermo Rein (Imperial College London).