Michael J. Furniss, Leslie M. Reid, Brian Staab. Pacific Northwest, Pacific Southwest Research Stations, Pacific Northwest Region.
Clean, reliable water supplies have been identified as one of the most valuable products of our national forests, which currently supply drinking water for at least 60 million people in the United States. The utility and value of this water supply depend strongly not only on its volume and quality, but also on its timing. The existing network of water supply facilities is designed in part to take in water when it becomes available through rainfall or snowmelt, and to provide sufficient water storage to mete out the water when and where it is needed. A large part of the water storage system is natural: snowpacks store vast quantities through winter months, groundwater supplies streams with dependable base flows, and aquifers replenished by upland precipitation store water that can later be tapped by wells.
A warming planet and changing climate will alter the distribution, volume, timing, and type of precipitation, and will also modify the distribution and timing of water needs. The water supply and storage network designed for pre-change conditions will be less suitable for the new conditions. Meanwhile, increasing populations and warmer conditions will increase the demand for water. Climate change will also probably alter the quality of water, with effects on the ability of water supplies to support some uses. Watershed-based adaptations are needed if the flow of watershed-derived goods and services is to be maintained in the face of changing conditions.
Weather and climate are driven by solar radiation and the resulting temperature differentials, and the distribution of typical weather patterns will shift as global warming increases average temperatures. To the extent that new conditions differ from pre-change conditions, local ecosystems, landscape systems, and human communities will experience climatic stresses. The process of adaptation to the new conditions is likely to result in a period of increased disturbance to terrestrial ecosystems (e.g., increased wildfire severity, disease outbreaks, and species invasions), stream networks, and patterns of human use. Each of these changes holds strong implications for the supply and use of water from national forests and grasslands and the ability of these lands to support myriad aquatic species.
General patterns of climate change emerge from all predictive models: some areas are likely to receive more precipitation and some less. Warming temperatures will result in less precipitation falling as snow, smaller snowpacks, earlier snowmelt, increased incidence of rain-on-snow flooding, reduced dry-season streamflows, greater moisture stress on vegetation, and increased stress on aquatic ecosystems. Areas subject to increased climatic extremes are likely to experience more frequent and larger floods and more frequent and longer droughts. Warming conditions are likely to trigger more extensive and severe insect outbreaks and more frequent, larger, and more severe wildfires, contributing to reduced water quality through increased erosion. Clean water supplies will become increasingly scarce, and water-related ecosystem services will be at greater risk.
Climate change must be considered in context of the individual and cumulative effects of other stresses affecting watersheds. Climate change will compound the severity of other problems such as species extirpation, population pressures, and water scarcity. Aquatic habitats that are in marginal condition may be rendered unusable for some species by warming temperatures and reduced flows.
Options for Adapting to Likely Changes
Established principles of watershed management will remain a primary response to the increased demands and risks imposed by a warming planet. But because of increased climatic stresses, the consequences of inadequate watershed management will become more serious and more immediate. Watershed managers will need to carefully consider the many potential interactions between altered physical, biological, and social environments to ensure that management decisions are appropriate for likely future conditions.
Assessing watershed and aquatic ecosystem vulnerability and setting priorities for application of effective measures will be especially important. Land managers will need to be able to (1) set priorities for adaptive actions by identifying watersheds and amenities most vulnerable to climate change (2) construct scenarios for a range of plausible future climatic changes and assess the likely effects of each, and (3) select mitigations appropriate for the impacts likely in particular watersheds for each scenario. An approach is under development to assist managers in assessing risks, hazards, and impact mechanisms associated with climate change at the river basin and watershed scales.
Vegetation management measures such as large-scale thinning and riparian vegetation removal are often proposed to increase water yield from wildlands. Such proposals may be suggested in the future as an adaption to climate-induced changes in available water supply. However it is unlikely that vegetation management can produce significant long-term increases in downstream water flows at the scales of interest (Ziemer 1987, Kuhn et al. 2007), and short-term increases in flow can be negated by reductions in water quality owing to increased erosion and by reductions of flow below original levels during some stages of regrowth. A more effective adaptation strategy would focus on maintaining and restoring watershed health and resiliency, because such systems are more likely to provide a sustained flow of ecological services in face of ongoing and future disturbances, including those associated with climate change (Baron 2002). The types of actions that might be implemented will differ dramatically in different landscapes-they will depend on dominant watershed processes, key watershed services, and principal threats to those services. They could include:
- Protecting and restoring riparian forests to reduce stream temperatures and increase the quality of aquatic habitats.
- Improving or decommissioning roads to reduce erosion, increase flood plain connectivity, decrease peak flows, and reduce temperature impacts.
- Restoring meadows, wetlands, and flood plains to improve ecological continuity, increase water storage, reduce flood flows, increase local late-season summer low flows, and decrease stream temperatures. Restoring and maintaining persistently wet places in the terrestrial environment as "biological oases" for watershed resilience and to build a network of refugia.
- Maintaining and restoring environmental flows needed to support myriad stream processes in watersheds and aquatic ecosystems.
- Removing migration barriers and reestablishing habitat connectivity to help species adapt to changing conditions.
- Strategically reducing wildfire risks in watersheds vulnerable to excessive erosion, stream temperature increases, and other impacts.