Forecast: Hotter, Drier, between Floods

Oregon’s Climate Researchers Pessimistic

Recent climate changes have caused the seasonal cycles of river and stream flows to be different than what they have been. As temperatures in the winter warm up, more rain falls and less snow falls in the winter than it did before. So less snow accumulates in the winter snow pack, and stream flow increases in the winter but decreases in the summer. The peak flow in rivers occurs earlier in the spring, and the magnitude of peak flow has changed.

There are other significant impacts. Climate change in Oregon has affected the hydrology of a watershed, the demand for water, and the size and thickness of glaciers.

Changes in availability and flow of fresh water
One of the unique features of fresh water supplies in the Pacific Northwest is that most of the water falls as snow in the winter, remains stored in the snow pack for many months, and then is released to the rivers in the summer. As the climate changes and regional temperatures rise, less snow falls, and it falls in different months of the year than it did before. In the winter, melting snow and more frequent rain are contributing to wintertime flooding, which is now more common. In the summer, there is less water in the rivers, because more water ran off earlier in the year; this leads to shortages of water (Hamlet et al 2001).  In basins where rain supplies most of the water throughout the year, an increase of precipitation affects the seasonal pattern of runoff into the rivers (Franczyk and Chang 2009a).

But in high-elevation basins where snow predominates over rain, higher temperatures by themselves change the timing of runoff, even when the amount of precipitation stays the same (Graves and Chang 2007). In contrast, runoff of water may be sustained through the summer in the High Cascades, where slopes are gentle and the rocks are young and volcanic. However, groundwater supplies a larger percentage of the seasonal water flows in the High Cascades, so it’s uncertain how we should forecast water flows in that high-elevation region (Chang and Jung).

In the Willamette River basin, the complex topography and geology also affect the way that each sub-basin responds to a changing climate. In the Western Cascades, the amount of water stored seasonally in snow (known as “snow-water equivalent”) will decline and most of the runoff of water will occur earlier in the year, by the year 2080. Such changes in the timing and location of river flows will significantly affect water users and the economy, especially in the Willamette basin (Franczyk and Chang 2007). Current patterns of water use vary from place to place, and these variations need to be considered in the future management of water as climate changes (Franczyk 2009b). Land use will also change, which will affect future river runoff. All of these trends need to be included in studies of the impacts of climate change (Praskievicz and Chang 2009b).

Recent research, using sophisticated observations, climate models and Northwest US hydrological models, indicates that as much as 60% of these changes in the water cycle result from human activities. The chances for a water crisis are high in Oregon.

Changes in stream water quality
Changes in the volume of flowing water and in the temperature of the air above the stream will alter the chemistry and biology of the stream. This affects the concentration of various nutrients.
Warmer water along with diminished stream flow harms aquatic life in the river. In the Tualatin River, water temperatures have increased significantly in the last 20 years (Chang and Block 2005). While the daily high temperature of the air is still the most important thing that affects the temperature of a stream, the local geology, condition of the stream bed, size of the basin, and variations in the flow all affect the water temperature.

Changes in extreme hydrologic events
Even though the intensity of precipitation in the Willamette River basin has not changed in the winter season since 1972 (Praskievicz and Chang 2009b), the 2007 Report of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2007) cautioned that extreme events, such as floods and droughts, are increasingly likely because of climate change. Urban areas are particularly vulnerable to flooding because the large area of paved surfaces does not absorb rain water, and a large number of buildings are vulnerable to high waters. In a study in Portland (Chang et al 2009), if climate does change as expected in the future, there will be more frequent major storms that now occur every 25 years or less. Flooding is likely to be more common at those road intersections with a history of chronic flooding.

Changes in water demand
Water consumption in the suburban city of Hillsboro, Oregon, was not affected by drought conditions in one study (House-Peters et al 2009). But when rainfall was low and daytime temperatures were above normal, the summertime water consumption depended more on physical qualities of the property than on socio-economic qualities of the residents. This suggests that smart urban planning could reduce water demand in seasons that are warmer than usual.

Snow and ice
Warmer temperatures have led to rapid retreat or thinning of glaciers on Mt. Hood and other places in the Pacific Northwest (Fountain 2007). The seven glaciers on Mt. Hood lost an average of 34% of their surface area between the years 1907 and 2004 (Jackson and Fountain 2007). Rising temperatures and decreased snow pack will alter the seasonal availability of water for hydroelectric power (IPCC 2007). Changing patterns of water flow throughout the year, and changing levels of lakes and reservoirs, also affect life in rivers and streams (USEPA 1998).
Availability of water from the winter snow pack, as measured by the “Spring Snow Water Equivalent,” has been declining since 1925, and especially since 1950. The Oregon Cascade mountains have experienced the largest losses of available water in the West (Mote et al 2005) as precipitation has decreased at the same time as the region has warmed.

Summary from the Oregon Climate Change Resource Institute.  Full source citations at OCCRI site.