Fire effects on water quality: a synthesis of response regulating factors among contrasting ecosystems
The key components of watershed processes are inputs in precipitation, interactions of vegetation, soil and water including evapotranspiration (water yield) overland flow (erosion) and storage and filtering (nutrients) and outputs in streamflow. Fire effects occur at the vegetation soil interface and can result in altering overland flow and infiltration rate of water. Fire can affect infiltration rates by collapsing soil structure and reducing soil porosity, contributing ash and charcoal residues which can clog soil pores, and raindrop splash can compact soil and firther contribute to loss of soil porosity. An extreme example is the development of hydrophobic soils as observed in the western U.S. following severe wildfire. Watershed responses to fire depend on intensity and severity. Many factors influence fire severity including the quality and quantity of fuels, soil properties, topography, climate, and weather. The most important factors influencing the response to fire are vegetation mortality and the loss of the forest floor which are directly proportional to fire severity. Vegetation mortality reduces nutrient and water uptake, soil stability with root death, and the litter source for forest floor replenishment. The forest floor litter and humus (duQ layers provide soil cover, act as a sponge, and enhance infiltration. Large storm events immediately after a fie can accelerate surface runoff and compact soil. INTRODUCTION Wildland fire has the potential to significantly impact hydrologic processes such as surface nmoff, sediment yield, and sediment and nutrient transport to streams. The magnitude and duration of watershed responses to fue depends on the interactions among burn severity, post fire precipitation regime, topography, soil characteristics, and vegetative recovery rate. The typical impact of fire is an immediate change in vegetative cover, forest floor surfhce, physical properties of the soil, followed by midand long term changes in biological pools and nutrient cycling processes. Vegetation and litter protect the soil against the forces of erosion by maintaining high infiltration rates and low levels of overland flow (Covert and others 2005) Vegetative cover and forest floor are the primary drivers of sediment responses to fire. Large reductions of vegetative cover, particularly the ground vegetation and forest floor, leave the soil prone to raindrop impact and reduce rainfall infiltration and storage so that erosive overland flow tends to occur more readily (Shakesby and Doerr 2005) Nutrient responses are also impacted by changes in vegetation and forest floor, as well as changes in biological processes that regulate cycling processes. The magnitude and duration of hydrologic and water quality responses vary greatly across ecosystem types in the continental U.S. As such, it has been difficult to generalize response or apply knowledge derived fiom one region of the U.S. to any other. Over the past several years, a growing body of research has provided hydrologyfwater quality response data across a range of ecosystems, fire types, soils, and climate regimes. In this paper, we synthesize current knowledge on fixtors regulating water quality in contrasting ecosystems. Defining Fire Severity Fire severity depends on the interaction between fire intensity (rate at which thermal energy is produced) and duration (length of time burning occurs at a particular point) and describes the magnitude of the disturbance and reflects the degree of change in ecosystem processes (Neary and others 2005) Fire severity is a qualitative measure of the effects of fire on site and soil resources; it can occur along a spectrum fiom high to low or can be described as a patchwork, mosaic, matrix or mixed severity event. Debano and others (1998) describe a light severity bum as one that bums only surface fuels, leaves the soil covered with partially charred organic material, and little to no duff consumption (fermentation (Oe) humus (Oa) layers) A moderate severity burn results fiom a large proportion of the organic material burned away fiom the surface of the soil and the remaining fuel is deeply charred. A highseverity burn results fiom all of the organic matexial burned away from the soil surfiice, organic material below the surface is consumed or charred. Fire severity has been assessed by numerous methods such as degree of destruction of aboveground live and dead biomass (Neary and others ZOOS) amount of forest floor consumed, particularly the duff layer, or heat penetration into the mineral soil (Swift and others 1993) Precipitation Regime After fire, rainfall intensity and duration can influence the amount of sediment delivered to a stream channel. The detachment of soil particles by rainsplash or overland flow and their transfer downslope are sensitive to modifications in land surface properties caused by fire (Johansen and others 2001, Sakesby and Doerr 2006) In low rainfall ecosystems, surface runoff and erosion may not be observed if there is a long period of post fire recovery before the first rainfall event. Even in ecosystems with low mean annual rainfall, a high intensity rainstorm immediately after wildand fire can create runoff that alters the topography of the hillslope, which subsequently impacts stream channels. Rainstorm events need to have enough energy to transport sediment. Swift and others (1993) determined that rainfall events of >50 rnrn hr were required to transport material after a fell and burn prescribed fire in the southern Appalachians. Sediment yields are typically higher in the first year after burning, especially when the burned watershed has been exposed to high intensity rainfall events immediately after the fire has exposed the soil swfhce. Some of the largest increases in surface runoff have been observed where shortduration, high intensity convective rainstorms occur. For example, after the 1996 Buffalo Creek Fire in Colorado, two short duration, high intensity rainstorms 90 mm hr I) removed ash from the hillslopes, rilled the hillslope surfiices, channelized subtle drainages, which led to a headward extension of the channel network, and deposited sediment in stream channels (Moody and Kinner 2006) Kunze and Stednick (2006) found that rainfall intensity explained more than 80% of the variability in sediment yields. After the 2000 Bobcat Fire in Colorado, a single storm with 30 min rainfall intensity of 42 mm hr" resulted in 370 kg ha and 950 kg ha' sediment yields, on treated (erosion control with contour log felling, grass seeding, and mulching) and untreated watersheds, respectively (Kunze and Stednick 2006) Vegetation Recovery Post fire soil erosion amounts vary not only with rainfall but also with bum severity, topography, soil characteristics and amount of vegetative recovery. Under moderate to severe fire severity that removes vegetation and forest floor cover, transpiration, interception and surfice storage capacity for rain are temporarily reduced. Conversely, any fireinduced alterations to storage capacity and water repellency will decline as vegetation and ground litter recover. Ground cover protects the soil from raindrop impact and offers resistance to overland flow. Vegetation recovery rates are strongly affected by fire size and severity, post fire erosion events and vary by climate and geographic area. Rapid vegetation establishment has been regarded as the most cost effective method to promote water infiltration and reduce hillslope erosion (Robichaud 2005) In the western U.S. land management agencies have spent tens of millions of dollars on post fire emergency watershed stabilization measures to minimize flood runoff, onsite erosion, offsite sedimentation, mud and debris flows, and other hydrologic damage to natural habitats (Robichaud 2005) Post fire hillslope rehabilitation treatments include seeding for vegetative re growth, ground covers or mulches, and barrier and trenches that physically hold runoff and sediment In the eastem U.S. such costly and dramatic post fire rehabilitation efforts are typically not required. Even after severe fire, recovery rates of southern Appalachian watersheds are much faster than western forests due to rapid vegetative re growth (Clinton and Vose 2000, Elliott and others 1999) Low severity burning, such as prescribed fires, can promote a herbaceous flora (Elliott and others 1999, Gilliarn 1988, Hutchinson and others 2005) increase plant available nutrients (Elliott and others 2004) and thin fiom below over crowded forests. While large, severe fires can cause changes in successional rates, alter species composition, generate volatilization of nutrients and ash entrainment in smoke columns, produce rapid or decreased soil mineralization rates, and result in subsequent nutrient losses through accelerated erosion (Neary and others 1999) Surfirce Runoff and Erosion Wildland fires are often landscape scale disturbances that can alter the hydrologic and erosion responses of catchments. Erosion can occur when ground cover is reduced or consumed, and subsequently infiltration rates are reduced, i.e. water repellency is high. Fire induced or enhanced soil water repellency (hydrophobicity) is commonly viewed as a key contributor to the substantial increases in hillslope runoff and erosion observed following severe wildfire (Debano and others 2005, Doerr and others 2006, Hufhan and others 2001, particularly in the western U.S. Soils do not all exhibit the same degree of water repellency; a water repellent soil is classified as one on which a drop of water will not spontaneously penetrate. Water drop penetration time (WDPT) has been used extensively to characterize soil water repellency (Letey 200 1) Several factors associated with fire, such as removal of surface litter and higher raindrop impact, would produce higher runoff and erosion fiom burned compared with unburned catchments, independent of water repel