The hydrological properties and the effects of hydration on vegetative ash from the Northern Rockies, USA

Vegetative ash is known to alter post-fire runoff rates, in some cases decreasing runoff by protecting the underlying soil and in other cases aiding in the formation of a surface seal increasing runoff. Variability in this hydrological response of ash may reflect differences in the hydrologic properties of ash caused by shifts in its physical and chemical characteristics. This study investigated the physical (particle size and shape, particle density, bulk density and porosity) chemical (mineralogy and elemental composition) and hydrologic (water retention, hydraulic conductivity, and sorptivity) characteristics of laboratory and wildfire ash from the Northern Rocky Mountain region of the United States. A variety of techniques were used including scanning electron microscope, X-ray diffraction, thermogravimetric analysis, and conventional soil science techniques. Laboratory ash samples were prepared by combusting fuel from three dominant trees species of the region (Pinus contorta, Pinus ponderosa and Pseudotsuga menziesii) at various combustion temperatures (300°C, 500°C, 700°C or 900°C). The main objectives of this study were to determine (i) how the physical (particle size and shape, particle density, bulk density and porosity) and chemical (mineralogy and elemental composition) properties of ash might explain variations in hydrologic properties (water retention, hydraulic conductivity, and sorptivity); (ii) whether wetting alters the physical, chemical and hydrologic properties of ash; (iii) whether ash characteristics could be linked to various stages of thermal decomposition and whether ash produced in the laboratory could be beneficial to understanding wildfire ash characteristics and (iv) whether ash hydrologic properties can be linked to simple metrics, such as calcium carbonate content or color, which can be used in rapid post-fire hazard assessment.Results of this study indicate that variation in the hydrological response of ash can be partially explained by differences in ash characteristics associated with combustion temperature. Depending on ash mineralogy, which was dictated by the level of thermal decomposition of organic material, ash particles can (a) contain micro-porosity contributing to total porosity values as high as 98%; (b) shift hydraulic conductivity values orders of magnitude from 0.12±0.03 to 0.002±0.0003cms−1; and (c) chemically transform after initial hydration suggesting that ash layers could alter post-fire runoff response by creating a chemical crust. Furthermore while ash color is often used as an indicator of burn severity it may not be an accurate indicator regarding the hydrologic response of ash in post-fire systems. Instead carbonate content is suggested as a more informative aid in predicting post-fire hydrologic response.