Ash and soil water repellency effects on soil hydrology in fire-affected Mediterranean ecosystems

Merche B. Bodí
SEDER-Soil Erosion and Degradation Research Group, Departament de Geografia. Universitat de València. Valencia, Spain
GEA-Grupo de Edafología Ambiental, Departamento de Agroquímica y Medio Ambiente, Universidad Miguel Hernández, Elche, Spain


Cover. Click to enlarge.

After a wildfire, changes in the vegetation, micro and macro fauna, biochemical cycles, soil properties, and hydrological and geomorphological processes may occur. The most studied cause of these changes is the heat input, but the environmental conditions after the fire and the new ash cover also play an important role.

Ash is therefore, one of the key elements affecting burned areas. In the context of my PhD study, ash is considered the residue of the organic material after the fire and blankets the soil temporally with a layer of variable thickness and physical and chemical properties, which depend on fuel characteristics and fire severity. Ash is of major importance for the biogeochemical cycles, including the carbon cycle. It also modifies soil infiltration and runoff generation, affects erosion rates, and changes physical and chemical soil properties. Even after being removed from the soil surface by slope wash, wind erosion or dissolution, ash may still affect the processes of the environment where it is deposited. Despite its implications, ash remains one of the least understood elements of the post-fire ecosystem. To date, ash has been mainly considered only as an important element in the nutrient cycle and as a soil fertiliser. Very few studies have explored ash effects on soil hydrology and erosion, and from the research until now, we known that ash properties are very variable, which results in different and sometimes contrasting effects.

The general objective of this PhD thesis was is to study the effects of ash on pedological, hydrological and geomorphological processes. Particular attention is given to the wettability of ash and its effects on soil water repellency because of their potential relevance in controlling water, sediment and nutrient fluxes in the fire affected ecosystem. The thesis was presented in five scientific papers based on work carried out between July 2008 and June 2012. Three of them have been published in

international journals (Geoderma and Catena), and all have been presented at national and international conferences for their public discussion before publication.

The starting point of the Thesis was a bibliographic review on the effect of fire in soil properties published in the “Boletín de la AGE” (Journal of the Spanish Geographers Association) with the title “Efectos de los incendios forestales en la vegetación y el suelo en la cuenca mediterránea: revisión bibliográfica”. After putting the research in context, different studies and experiments were carried out. In the first one, which results are in press in Catena with the title: “Spatial and temporal variations of water repellency and probability of its occurrence in calcareous Mediterranean rangeland soils affected by fires”, it was studied the changes in soil water repellency immediately after a forest fire and in catchments burned 10 and 20 years ago. This work also examined the seasonal variations in water repellency related with soil moisture and spatial variations under different vegetation types and within microplots under the same vegetation type, and proved that ash can affect soil water repellency after a fire. The probability of water repellency occurrence was also calculated. This work raised new questions about water repellency of ash, and since no studies had been published on this issue, this became the main objective of the following part of this work. Its outcomes are now published in Geoderma under title “The wettability of ash from burned vegetation and its relationship to Mediterranean plant species type, burn severity and total organic carbon content”. Involving laboratory experiments, this work also examined the relationships between ash water repellency, organic carbon content and its colour, and how ash incorporation into the soil affects soil water repellency. At this point, the need became evident to also investigate the effects of ash on water repellency when ash covers the soil. Therefore, different experiments were carried out to explore, at the same time, the role of ash in runoff generation and chemical composition of runoff water and its role in the erosion produced after the fire when different types of ash cover different soils with various thicknesses. The results obtained from the related laboratory experiments are in press in the journal Geoderma, “Hydrological effects of a layer of vegetation ash on underlying wettable and water repellent soil”, and the results obtained in the field experiments are published in the Proceedings of the III International FESP Conference “Fire effects on soil properties” in Guimaraes (Portugal) with the title “Runoff rates, water erosion and water quality from a soil covered with different types of ash”.

The most important contribution of this PhD Thesis to the scientific knowledge are:

  1. Regarding the spatial and temporal variations of water repellency in Mediterranean calcareous soils and the prediction of its probability of occurrence:
    1. Water repellency can be a common soil property in the calcareous soils of Mediterranean forests. In general, more than 60 % of the samples were wettable, but water repellency changed seasonally and some samples showed extreme water repellency in dry-summer conditions. Water repellency was generally reduced at 1 cm depth, although an increase at 1 cm depth was found after a forest fire only during the first year. Water repellency decreased later to very low levels at the surface and at 1 cm depth. It is suggested that this decrease was due to the gradual washing out of hydrophobic substances and the lack of fresh organic and hydrophobic material inputs after fire, together with the topsoil’s cover of wettable material composed of ash and soil. The results obtained at the Sierra de Enguera study sites indicate that the recovery to the pre-fire water-repellent conditions may take more than 10 years.
    2. Variability of soil water repellency under Pinus halepensis, Quercus coccifera and Rosmarinus officinalis within the same site of 1 ha was as high as within the same vegetation type and as within a plot of 10 cm × 10 cm. The lower variability was found in bare soil plots, which were mostly wettable. Water repellency was also variable depending on soil moisture, being highest under dry summer conditions.
    3. The statistical model performed, allows estimation of the probability of water repellency occurrence in mature Mediterranean calcareous soils and was derived using covariates and fixed factors that are straightforward and economic to measure (vegetation type, soil depth and moisture content). This simple model is a first step towards a model that can be a powerful tool to estimate water repellency under different seasonal and meteorological conditions.
  2. Concerning ash water repellency: Ash from vegetation fires can be water repellent. A greater frequency and persistence of water repellency in ash from Pinus halepensis and Quercus coccifera was found for a low severity fires and, in laboratory generated ash the generated at temperatures between 200–300 °C, whereas above 400 °C repellency was absent. Rosmarinus officinalis exhibited lower levels of water repellency. Water repellency was related to the ratio of organic/inorganic carbon content in ash and type of organic carbon compounds. Ash water repellency might be reduced after wetting. The colour of ash was found not to be a good predictor of either water repellency or total organic carbon content when ash from different wildfires was compared.
  3. Regarding the effects of ash on soil water repellency:
    1. If ash covers the soil, a layer of wettable ash of more than 5 mm thickness can reduce soil water repellency by increasing the hydraulic pressure and the contact between water and soil, and hence promoting fingered subsurface flow especially during the first rain event after the fire. After the first rainfall event, and if the soil is still wet, ash did not produce a significant reduction on soil water repellency compared with bare soil.
    2. When ash is incorporated into the soil, it can increase or reduce soil water repellency depending on its wettable or water repellent nature. In the case of water repellent ash, the increase can be especially important if ash is incorporated by dry mechanisms as such as wind erosion and deposition, or bioturbation.
  4. With respect to the effects of an ash layer covering the soil on the hydrological response:
    1. Ash may enhance overland flow if the hydraulic conductivity of ash (Kash) is lower than rainfall intensity, which can happen when ash is compacted and crusted due to its high content of calcium carbonate, or when ash is water repellent.
    2. In the other cases, when Kash> rainfall intensity and especially for the first rainfall event, ash delays the onset of overland flow proportionally to its thickness due to its high water storage capacity.
    3. Once ash is saturated, overland flow and subsurface flow between the ash and soil starts until both layers reach an equilibrium, after which the soil infiltration rate dominates the process.
    4. Soil infiltration rates can be modified by ash, (a) resulting in a higher soil infiltration rate by decreasing the degree of soil water repellency (see point 3) or by reducing the soil’s susceptibility to crusting (ash acts as a mulch), and (b) reducing the soil infiltration rate by enhancing pore clogging, which depends on the particle sizes of soil and ash.
  5. Ash does protect the soil from splash erosion and prevents sheet erosion as long as the ash layer is not saturated and overland flow does not occur. This reduction is especially important for bare water repellent soils, for which erosion is usually higher than for wettable soil conditions after a fire. The sediment yield produced from soils covered with ash is associated with overland flow (see point 4). During intense rainfall events, and if ash is already saturated, ash can increase the transport capacity of the sediment, contributing to high erosion events.
  6. The nutrients released from ash can modify water quality increasing its pH, electrical conductivity and especially cation content. The chemical composition of runoff depends on the volume of water produced (see point 4), on the solubility of the ash components and on the chemical interactions with water from rainfall and soil. After the first intense rain event, most of the nutrients are solubilised and lixiviated or washed out, however, some of them may increase some weeks later due to chemical interactions with the soil nutrients.

Finally, if you would like to discuss some of the questions or have a copy of my work, please contact me, I will be very pleased.

This text has been published in FLAMMA 4(1): 61-64 under a Creative Commons license.


About Antonio

I am a Biologist (BS in 1996) and PhD in Soil Science (2000), and work teaching and researching at the University of Sevilla (Spain). My on-going work includes the study of soil degradation processes in Mediterranean areas, soil erosion and the impact of wildfires on soils.
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