Declining Winter Snowpack May Not Dramatically Impact Upland Ecosystems

Global warming is reducing snowpacks across the western United States, with potentially far-reaching implications for downstream water resources. In the studies, Boise State researchers wanted to know how changing snowpacks will impact upland ecosystems in areas of the mountains that are not near streams.

The Boise State geoscientists found the benefit of winter snow accumulation to high-elevation ecosystems is limited by the soil’s ability to store water. While mountain snowpacks are important natural reservoirs extending spring and summer water delivery to downstream users and ecosystems, the study found that the coarse-grained, shallow soils can store only a fraction of the snowmelt into the summer when the water is needed. This means that declines in snowpack may have a minimal impact on summer water availability in these locations.

In a related study, researchers found a large difference in the capacity of the soil to store water on north-and south-facing slopes. The study found soils on north-facing slopes in a semi-arid mountain region in Idaho can hold up to 50 percent more water than soils on south-facing slopes. Because south-facing slopes dry out faster, the ability of vegetation to survive the dry summers is limited. The study also found that the more heavily vegetated north-facing slopes have the capacity to store more water because of finer-grained and deeper soils, which in turn produce drastically different soil water retention capacity. The researchers concluded that these differences are driven by various levels of solar radiation; the south-facing soils receive considerably more light and energy from the sun than their north-facing counterparts.

Researchers say these studies do not suggest that upland ecosystems will be less sensitive to climate change, but rather that changes to winter snowpack may not be the primary reason for the impact.

Both studies appear online in the journal Hydrological Processes.

“What is interesting about these studies is they suggest that the soils might be more sensitive to changes in precipitation timing rather than amount,” said studies’ coauthor Jim McNamara, professor of geosciences. “The limited ability of soils to store water from snowmelt highlights the potential importance of spring and early summer precipitation, and changes in spring precipitation may have a profound impact on upland water availability.”

The researchers took weather data and soil samples from nearly a dozen sites in the Dry Creek Watershed outside Boise at various times throughout a year. The study sites were located at four elevations ranging from 2,000 feet to 5,000 feet, with two sites on north and south-facing slopes at each specific elevation. Estimates of mean annual precipitation and mean annual air temperature were calculated for the elevation of each site and soil moisture sensors were installed at multiple depths.

The scientists found soil porosity, soil organic matter and silt content all were greater on the north-facing slopes and each contributed to higher water retention in the soil. These results, along with the observation that soils on north-facing slopes tend to be deeper, indicate that north-facing slopes in this region can store more water from the wet winter months into the dry summer.