Hydrologic effects from ecosystem responses to climate and land use changes

Network News Fall 2009, Vol. 22 No. 2
ASM Reports

A report of an ASM working group, September 2009

Of all the ecosystem services, a sustainable supply of high-quality water may be the most important. Streamflow from forests provides two-thirds of the water supply in the United States. Nationwide, high quality water supplies depend entirely on a range of natural ecosystem types. Climate change, drought, outbreaks of insects and pathogens, wildfire, and ecological succession are altering ecosystems' ability to provide abundant, clean water from the headwaters of our water-supply systems.

Simultaneously, public land management is changing in response to wildfire and endangered species concerns; private lands have undergone major changes in ownership and management; and many other ecosystems have declined in their capacities to sustain natural processes needed to maintain clean water due to conversion to exurban development.

While the near-term, local hydrologic effects of these land-use changes are relatively well understood, ongoing and cumulative climatic changes in ecosystems will likely have widely varying effects on sustainable water supplies to downstream areas. As a result, many regions face a difficult balancing act between additional development, flood protection, water supply for urban areas and agriculture, and water releases for endangered species protection.

A group of 65 LTER scientists representing 21 sites met at the All-Scientists' Meeting in September 2009 to discuss the question, "How will ongoing and projected changes in ecosystem conditions affect water supplies?" The participants concluded that a sustainable supply of clean water is one of the most important and fundamental ecosystem services on which humans depend, and that future water supplies are likely to change in ways that may be difficult to predict. Climate change is expected to increase periodic hydrologic drought, increase the intensity of floods, and compromise water quality. Predictions of future water supplies and water quality are highly uncertain, in part, because multiple ecosystem processes provide adaptation and resilience to climate variability.

Predictions of future water supplies also depend on historical land use, which established the expected water yields used for planning, even though land-use changes continue to modify ecosystems and water supplies. In addition, land-use change may exacerbate or mitigate adverse climate change effects on water supply. Experience from LTER sites over the past 30 or more years, recounted during the working group session, provides an initial assessment of current understanding about ecosystem adaptation and resilience to climate variability and change, and the effects of these processes on water supply.

However, more synthesis is clearly needed to further develop regional perspectives on this topic. Below we summarize the initial findings reported during the workshop and suggest a possible follow-up workshop to broaden and deepen the LTER network synthesis on how ecosystem adaptation and resilience may affect future freshwater supplies in response to climate change and land use change.

LTER sites are detecting a wide range of trends in streamflow. In Polar Regions, river flows have increased annually (ARC), increased in autumn (NWT), or flow variability is very high (MCM). Streamflow has increased in one forested site (HBR) and decreased only in spring in another (AND). In grassland sites (KNZ), streamflow has both increased and decreased.

Many distinctive trends in precipitation, temperature, and snow/ice have already been detected at LTER sites that are likely to affect future streamflow and water supply. Polar and alpine sites report that declining ice cover and permafrost melt has augmented streamflow at seasonal or interannual time scales (ARC, BNZ, NWT, MCM). Forested sites with annual snowpacks report declining duration and amounts of seasonal snow (AND, HBR, NWT). Precipitation has increased steadily over 45-100 years at several sites (HBR, HFR, KNZ, SEV, SGS), but has not changed at others (AND, ARC, CWT, NWT), and has reportedly decreased at one (MCM). Several sites in a range of climatic settings expect, or have detected, an increased frequency of extreme precipitation events (ARC, CAP, HBR, KNZ, LUQ, MCR). Some of these sites also report increased drought (LUQ, KNZ) or evidence of increased evapotranspiration and landscape drying (ARC). Climate at many sites is coupled with sea surface temperatures and ocean-atmosphere interactions, producing multi-year oscillations (the El Niño-Southern Oscillation) or decadal oscillations (the Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO)) in precipitation and/or temperature (AND, CAP, FCE, SEV, SGS).

Streamflow trends also may be attributed to changing land-use trends. LTER sites reported effects on water supply or water quality from land use changes, including urbanization and suburbanization (BES, CAP), forest cutting, replanting, and regeneration (AND, CWT, HBR, HFR), cultivation (KNZ), grazing (JRN, SEV), dams (PIE), and multiple concurrent changes (MCR). Land-cover changes were seen as especially likely to interact with climate change to create compound, synergistic, or antagonistic “multiple stressors” on water supplies. For example, many western, snowmelt-dependent regions where population has been increasing, especially in urban areas, may experience altered seasonality of runoff due to snowmelt timing or reduced snowpack, compounded with increased water demand.

Climate changes manifested in altered precipitation, temperature, and snow do not produce simple effects on water supplies because of the adaptability and resilience of ecosystems and socio-economic responses to climatic changes. In fact, LTER sites report a variety of vegetation adaptations to climate change or climate variability, including altered phenology, shifts in plant species composition, and tree mortality. Altered disturbance regimes, such as increased lightning storms and fires in the tundra (ARC) or tree mortality due to hemlock woolly adelgid (CWT), whose northern expansion is limited by minimum winter temperatures (HFR), may either mitigate or exacerbate climate change effects on water supplies. Disturbances, including deforestation, desertification, vegetation conversion, urbanization/suburbanization, fire-induced landscape changes, invasive species, and sea-level rise all potentially interact and contribute to changes in soil water, evapotranspiration, streamflow, and water quality, with likely implications for ecosystem resilience to these combined stressors.

Workshop organizers agreed that records and analyses at a subset of LTER sites would benefit from a continued, more focused effort to study this topic.