Dennis W. Hallema

Regional patterns of postwildfire streamflow response in the Western United States: The importance of scale-specific connectivity

Wildfires can impact streamflow by modifying net precipitation, infiltration, evapotranspiration, snowmelt, and hillslope run-off pathways. Regional differences in fire trends and postwildfire streamflow responses across the conterminous United States have spurred concerns about the impact on streamflow in forests that serve as water resource areas. This is notably the case for the Western United States, where fire activity and burn severity have increased in conjunction with climate change and increased forest density due to human fire suppression. In this review, we discuss the effects of wildfire on hydrological processes with a special focus on regional differences in postwildfire streamflow responses in forests. Postwildfire peak flows and annual water yields are generally higher in regions with a Mediterranean or semi-arid climate (Southern California and the Southwest) compared to the highlands (Rocky Mountains and the Pacific Northwest), where fire-induced changes in hydraulic connectivity along the hillslope results in the delivery of more water, more rapidly to streams. No clear streamflow response patterns have been identified in the humid subtropical Southeastern United States, where most fires are prescribed fires with a low burn severity, and more research is needed in that region. Improved assessment of postwildfire streamflow relies on quantitative spatial knowledge of landscape variables such as prestorm soil moisture, burn severity and correlations with soil surface sealing, water repellency, and ash deposition. The latest studies furthermore emphasize that understanding the effects of hydrological processes on postwildfire dynamic hydraulic connectivity, notably at the hillslope and watershed scales, and the relationship between overlapping disturbances including those other than wildfire is necessary for the development of risk assessment tools.

Assessment of wildland fire impacts on watershed annual water yield: Analytical framework and case studies in the United States

Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Raleigh, North Carolina 27606, USA Oak Ridge Institute for Science and Education, U.S. Department of Energy, Oak Ridge, Tennessee 37830, USA Coweeta Hydrologic Laboratory, Southern Research Station, U.S. Department of Agriculture Forest Service, Otto, North Carolina 28763, USA Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Asheville, North Carolina 28804, USA Center for Forest Disturbance Science, Southern Research Station, U.S. Department of Agriculture Forest Service, Athens, Georgia 30602, USA Oak Ridge National Laboratory, U.S. Department of Energy, Grand Rapids, Minnesota 55744, USA Correspondence Dennis W. Hallema, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, 920 Main Campus Dr. Suite 300, Raleigh, NC 27606, USA. Email: dwhallem@ncsu.edu Funding information Joint Fire Science Program, U.S. Department of Agriculture Forest Service Southern Research Station.

Surface storm flow prediction on hillslopes based on topography and hydrologic connectivity

BackgroundHillslopes provide critical watershed ecosystem services such as soil erosion control and storm flow regulation through collecting, storing, and releasing rain water. During intense rainstorms, rainfall intensity and infiltration capacity on the hillslope control Hortonian runoff while the topographic attributes of the hillslope (e.g., slope, aspect, curvature) and the channel network define the structural hydraulic connectivity that determines how rapidly excess water is transferred. This paper discusses literature on the link between topographic attributes and hydrologic connectivity and demonstrates how this link can be used to define a parsimonious model for predicting surface runoff during high intensity rainfall.Main textFirst, we provide a topographic characterization of the hillslope necessary to determine the structural hydrologic connectivity of surface flow based on existing literature. Subsequently, we demonstrate a hydrologic surface response model that routes the geomorphologic unit hydrograph (GIUH) through a spatial domain of representative elementary hillslopes reflecting the structural hydrologic connectivity. Topographic attributes impact flow and travel time distributions by affecting gravitational acceleration of overland flow and channel, solar irradiance, flow deceleration by vegetation, and flow divergence/convergence.ConclusionsWe show with an example where we apply the GIUH-based model to hypothetical hillslopes that the spatial organization of the channel network is critical in the flow and travel time distribution, and that topographic attributes are key in obtaining simple yet accurate representations of hydrologic connectivity. Parsimonious GIUH models of surface runoff that use this hydrologic connectivity have the advantage of low data requirements, being scalable and applicable regardless of the spatial complexity of the hillslope, and have the potential to fundamentally improve flood forecasting tools used in the assessment of ecosystem services.

Ecohydrological processes and ecosystem services in the Anthropocene: a review

The framework for ecosystem services has been increasingly used in integrated watershed ecosystem management practices that involve scientists, engineers, managers, and policy makers. The objective of this review is to explore the intimate connections between ecohydrological processes and water-related ecosystem services in human-dominated ecosystems in the Anthropocene. We synthesize current literature to illustrate the importance of understanding the ecohydrological processes for accurately quantifying ecosystem services under different environmental and socioeconomic settings and scales. Our synthesis focuses on managed ecosystems that are dominated by humans and explores how ecological processes affect the tradeoffs and synergies of multiple ecosystem services. We identify research gaps in studying ecological processes mainly including energy, carbon, water, and nutrient balances to better assess and quantify ecosystem services that are critical for sustaining natural resources for future generations. To better assess ecosystem services, future ecohydrological studies need to better account for the scaling effects of natural and anthropogenic stressors exerted on evapotranspiration and other water supply and demand processes. Future studies should focus on the bidirectional interactions between hydrological functions and services and human actions to solve real world problems such as water shortages, ecological degradation, and climate change adaptation.

Preface for the article collection “Ecohydrological Processes and Ecosystem Services”

Water is essential to life on Earth. Ecohydrology, the study of interactions between ecological and hydrological processes, is fundamental to our understanding and quantification of services provided by ecosystems. Our knowledge of ecohydrology is incomplete due to the complex nature of ecosystems, which are constantly changing under multiple stresses from air pollution to climate change, from deforestation to urbanization, and from soil erosion to soil pollution. Ecosystem services, the goods and services that ecosystems provide for human wellbeing, are increasingly adopted as a framework worldwide for ecological restoration and conservation, watershed management, and sustainable development policy making. Therefore, linking ecohydrological processes (e.g., evapotranspiration, streamflow, groundwater recharge) to ecosystem services (e.g., carbon sequestration, water quality improvement, biodiversity conservation, heat island mitigation) is critical to the advancement of ecological science and ecosystem restoration at multiple scales, from a single species to the entire globe. This special issue includes ten articles that aim at addressing questions in the following research areas:

Reframing the Challenge of Global Wildfire Threats to Water Supplies

The timing, extent, and severity of forest wildfires have increased in many parts of the world in recent decades. These wildfires can have substantial and devastating impacts on water supply, ecohydrological systems, and sociohydrosystems. Existing frameworks to assess the magnitude and spatial extent of these effects generally focus on local processes or services and are not readily transferable to other regions. However, there is a growing need for regional, continental, and global scale indices to assess the potential effect of wildfires on freshwater availability and water supply resilience. Such indices must consider both the individual and compound effects of wildfires. In so doing, this will enable comprehensive insights on the water security paradigm and the value of hydrological services in fire-affected areas around the globe. Plain Language Summary The number of large forest fires and the length of the wildfire season have both increased globally in the past few decades. Wildfire trends are expected to continue due to increasing occurrence of drought and denser forests associated with historical forest management and fire suppression. This development has raised concerns for water supplies because most water used for irrigation, industry, hydropower, recreation, and community drinking water comes from rivers draining watersheds that are prone to wildfires. As such, it is critical to improve our understanding of the capacity of watersheds and downstream communities to absorb or mitigate fire impacts. In this commentary, we emphasize the need for new continental and global scale indices to assess the full range of wildfire hazards to water supply and society. This will ultimately contribute to sustainable policies and land management plans for safeguarding water supplies and community health.