Wild land fire and bushfire fires are increasing in frequency and magnitude, due to climate change and human activities respectively (Thomaz, 2021; Di Piazzo, 2007). Recent challenges and policy developments are opportunities for soil physicists and other soil erosion modellers to respond with more accurate assessments and solutions as to how to reduce soil erosion and how to achieve Zero Net Land Degradation (ZNLD) targets of 2030 (Panagos and Katsoyiannis, 2019). Therefore, determining the relationship between bushfire and erosion development is imperative in achieving ZNLD. Post-wildfire responses are basically transitory, incident, variable in space and time, dependent on intensity and severity, and involve multiple processes measured by different methods (Moody et al., 2013). Wildfires or forest fire are known to be one of the major causes of soil erosion, slope instabilities, land degradation, and a times debris flow (Martini et al., 2020; Staley et al., 2018; Gabet, 2014; Benito, 2014). These effects are felt both in temperate region and in the tropics (Lal, 1985; Malkinson and Wittenberg, 2011; Keesstra et. al., 2012).
On-site effects are particularly evident on agricultural land where redistribution of soil particle within the area, loss of soil from the area, the breakdown of soil crumb and the reduction in organic matter and nutrient leading to reduction of cultivable soil depth and soil fertility. Erosion also lead to decrease in soil moisture, leading to more drought more drought-prone environment. The resultant effect is a loss of fertility vis-à-vis productivity and increased expenditure on fertilizer. Many countries spend so much on fertilizer leading to increase in cost of agricultural products. According to Morgan (2005), only 22% of the earth’s field of 14,900 million hectares is conceivably productive. Since this has to give 97% of the food supply, it is under rising constraint as global population continue to surge.
Off-site problems emanate from sedimentation downstream, reducing the capacity of watercourses, and drainage channels enhancing the risk of flooding, blocking irrigations canals, shortening the design life of reservoirs .Numerous hydroelectricity dams and irrigation projects have been rendered ineffective by erosion. Sediments have the potentials of increasing the levels nitrogen and phosphorus in water bodies leading to eutrophication. Erosion contribute to the weathering of soil aggregate into basic particles of sand, silt and clay. The process of breaking down of soil aggregate into primary particles results in the release of CO2 to the atmosphere, aiding green house effect.
Forest roads are often subject to intense runoff and erosion, and can be intensified by wildfires (Spanos et. al., 2010; Wittenberg, 2012; Lucas-Borja, and Zema, 2021). Their impacts on ecosystems are expected to increase in time due to changes in climate and land use (Girona-Garcia et al., 2021). It is therefore vital to mitigate the increased hydrological and erosive response after wildfires to maintain the sustainability of ecosystems. Forest in the tropics are regularly subjected to severe fire outbreak. Majorly, fire impact causes meaningful effect on soil bio physiochemical properties. It also leads to vegetation damage exposing the soil to erosion and land degradation (Shakesby, 2011). These effects depends on the density of vegetation; the severity, size and intensity of the fire; and predominant soil type (Keeley, 2009; Moody et al., 2013). Bushfire affects soil hydrology by breaking down the aggregated soil structure, decreasing moisture retention, development of water repellency (WR) and production of oxidized ash layer (DeBano, 2000; Mataix-Solera et al., 2011; Bodi et al., 2013). This in turn reduces erodibility of soil, making the soil susceptible to erosion. Many researchers have studied the causes, effect and extent of fire-triggered soil erosion ranging from laboratory studies to catchment studies (Moody et al., 2013), pointing to a common fact of increased soil erosion after bushfire or wildfire.
DeBano (2000) established that fire results to total vaporization of organic matter present in the surface soil and litter layers. The charred organic matter is partly vaporized into the atmosphere and the remaining part transferred as vapour, towards deeper soil layers where it condenses. Bushfire not only causes an increase in runoff due to the formation of hydrophobic layer, but also decreases of soil erodibility, as a result of destruction of organic matter (Prosser & Williams, 1998).
Low intensity fires result to increase in runoff only for the first erosive rainfall season, while higher intensity fires lead to increased runoff for 2–4 years (Soto & Diaz-Fierros, 1998; Inbar et al., 1998;Candela et al., 2005). Fire destroys organic layer in a soil rich in organic matter, whereas for soils less rich in organic matter the increase of erosion processes is mainly due to the destruction of the vegetation cover and its protective function (Inbar et al., 1998). The oxidation of soil organic content and reduced permeability, caused by soil hydrophobicity, affects erodibility.
The extent of organic matter vaporized at the surface and, the resulting quantity of hydrophobic substances depends on both fire intensity, fire duration and the organic substances present in the soil. DeBano (2000) stated that hydrophobic substances probably originate from aliphatic hydrocarbons, made up by different carbonyl groups with a high percentage of oxygen.
The vegetation cover serves as biofuel, so it directly influences the fire temperature attained in soil layers.
Bushfire affects the landscape by burning vegetation, depositing ash, influencing WR, and mechanically weathering boulders and bedrocks (Santi, 2013; Thomaz, 2018). Water repellency increases runoff and erosion (Robichaud and Hungerford, 2000; Wittenberg et al., 2011) whereas ash layer increases soil loss (Larsen et al., 2009; Bodi et al., 2013). Ash alters soil hydrological properties, increases water retention and reduces sediment movement (Cerda and Doerr, 2008). However, ash has the potentials of clogging soil voids, sealing the soil surface, and increasing surface erosion. The first post bushfire rainfall partly wash away the ash, while some penetrate into the soil viod (Larsen et al., 2009; Keesstra et al., 2014). The properties and extent of ash layer determine the duration and degree of surface soil protection (Woods and Balfour, 2010; Bodi et al., 2012). Considering the complicated dynamics, a very in-depth grasp of the ash-soil-water system interaction is vital in the immediate post-fire period for efficient management practices that will mitigate soil erosion development (Thomas et al., 2000; Smith et al., 2011). A variety of restoration strategies can be adopted, including natural regeneration (such as fire breaks, weed control, erosion control, topsoil replacement, peatland rewetting), enrichment cultivation (such as planting nursery-raised seedlings, direct seeding), commercial restoration (such as plantation forests, agroforestry), use of check dam and applying Engineering With Nature (EWN) principles (Scheper et al., 2021; Haring et al., 2021; González-Romero et al., 2019). Soil erosion estimation models are important for predicting fire impacts and planning post-fire emergency responses (Fernández and Vega, 2018; Chuvieco et al., 2014).
The technique used in this study is to model the controlled experiment of bush burning and using subsequent rainfall simulations to determine how the erodibility of the soil is affected.