Watershed Hydrology
The Mt. Mansfield Paired Watersheds Study
Since September 2000, the U.S. Geological Survey has been continuously operating stream gages at Ranch Brook and West Branch near Stowe, Vermont (Wemple et al., 2007). The gaging was designed as a paired watershed study, with Ranch Brook (9.6 km2) as the forested control watershed, and West Branch (11.7 km2) as the developed watershed. The West Branch watershed contains nearly the entire extent of the four-season Stowe Mountain Resort. In the classic paired watershed approach, monitoring would be conducted prior to any development, but the resort was established long before the study began.
However, the resort underwent a significant expansion during the course of the study, so the study design is appropriate to assess the effect of the expansion. This report on the Mt. Mansfield gaging is for Water Year (WY) 2016 (October 2015 through September 2016). The report interprets the WY16 streamflow's in the context of the full 16-year record. Historic and near real-time flow data are available on the USGS website (links provided in Additional Resources).
In WY2016, the gages were jointly funded through a cooperative agreement between the USGS, the Vermont Department of Environmental Conservation and the Forest Ecosystem Monitoring Cooperative. The gages provide valuable information on mountain hydrology in Vermont, and how mountain landscapes respond to development and extreme events. To our knowledge, these are still the only gaged watersheds at a ski resort. The gages have supported projects on snow hydrology and water quality by the University of Vermont, Sterling College, Vermont Agency of Natural Resources, and others. In particular, Beverley Wemple and students at University of Vermont have used the gages as a base for student projects and hands-on learning, and to attract additional funding for value-added research.
The Data
Stream gages on Ranch Brook and West Branch provide continuous monitoring of stream water heights (stage), which are related to discharge (flow) by an empirical rating based on frequent discharge measurements. This information provides a basis for monitoring of long-term hydrology patterns and water quality trends including: baseline conditions, trends in stream acid/base status, cations (Ca, K, Mg, Na, and Si), anions (Cl, NO3, SO4) suspended sediment output, snowpack and snowmelt and extreme climate events. These gaging stations provide a watershed framework for other FEMC efforts including nutrient cycling, forest health assessments, forest fragmentation and biological monitoring.
Discharge vs. runoff
Streamflow, or discharge, is measured in volume per unit time – in the U.S., typically as cubic feet per second, or cfs (Figure 30). Throughout this report, we use runoff rather than flow. Runoff is discharge divided by watershed area, and allows for direct comparison of flow in basins of different size. For example, if one basin is double the size of another and has double the flow, runoff would be the same. The dimensions of runoff are depth per unit time, i.e. the same as precipitation, thus allowing runoff to be directly compared to precipitation. For example, if a watershed receives 1500 mm/yr. of precipitation and has 1000 mm/yr. of runoff, that means 500 mm/yr. was lost to evapotranspiration plus or minus a change in the amount of water stored in the watershed, e.g. in soils.
2016 in Summary
Relative to the 16-year record, WY2016 had below average runoff for the third consecutive year. WY16 (start of water year is in October) featured a low runoff fall and winter, then an above normal spring due to a very steep warmup in March, then a very dry summer and early fall with droughty areas around the state by July and persisting into the fall. The dry conditions resulted in ground fires (where roots and organic matter catch fire) in Chittenden County, and in Lake Champlain, water levels were extremely low causing hazardous navigation in some areas.
Overall, runoff was less than the long-term average (Figure 32). As in Water Year 2015, the cumulative runoff in Water Year 2016 ran below average for the most of the year except the spring when runoff was either at or slightly above the long-term (Figure 32). By May, both sites were well below the long-term average. Cumulative runoff at both sites continued to be well below average for the rest of the water year. The relative runoff patterns at the two sites in Water Year 2016 were similar to the long-term patterns (Figure 31), with both streams generating similar runoff until part way into the spring snowmelt, when West Branch consistently generated greater runoff. Part of the greater snowmelt runoff was from melting of machine-made snow. (Water for snowmaking comes from West Branch upstream of the gage, so it is not double-counted). Runoff at West Branch continued to exceed that at Ranch Brook through the summer due to higher sustained base flow (Figure 31).
The most notable aspect of WY 2016 at both sites was the lower cumulative runoff compared with the long term record. This time period also had consistently higer than normal temperatures (Burlington NWS data, link in additional resources) with the exception of October 2015, the start of the water year, and April 2016, where the monthly average temperatures were slightly below long-term normal temperature.
Long Term Trends
As noted in previous reports, West Branch has consistently yielded higher runoff (flow normalized to watershed area) than Ranch Brook (Wemple et al., 2007) (Figure 30 and Figure 33). Over the long-term, the average difference has been 21% greater runoff at West Branch. The Water Year 2015 differential was 17%, below the long-term average (Figure 33). Greater runoff at West Branch is what we would expect from the creation of open land and development; but that the high magnitude of the differential suggests that some part of the difference may be natural. In previous reports, we noted the extreme variability of large summer storms; these may preferentially impact West Branch. FEMC cooperators are currently investigating the role of local meteorology on the flow regimes.
In a first step to assess the hydrologic impact of the resort expansion, we constructed flow duration curves for two three-year periods of approximately equal precipitation, from before and after the construction period (Figure 35). Preliminary analysis suggests that the resort build-out had no clear impact on the hydrology, except for the low-flow regime. Construction of a new snowmaking pond with greater storage has lessened the need to draw water directly from the stream at low flows, thus enabling a higher sustained baseflow in late fall and winter.
Implications
Mountain ecosystems worldwide are increasingly stressed by development of year-round recreational venues, tourism and other development such as communication towers and wind farms. Climate change disproportionally affects these ecosystems with warming temperatures and fewer more intense precipitation events, which are increasingly in the form of rain rather than snow. Plants and animals adapted to live at high altitudes suffer. Ski areas with no snowmaking capacity are almost unheard of and certainly not viable and all areas are moving toward becoming year-round operations that rely on golf courses, waterparks, mountain bike trails and other recreational activities that do not require snow. As these build-outs progress there are more impervious surfaces – parking lots, condominiums, and tennis courts which will alter the patterns, volume, velocity and chemical make-up of runoff.
Climate models predict more extreme precipitation events (already evident) that can potentially flood mountain streams leading to erosion, loss of stream bank cover and scouring of stream bottoms causing major disruptions to fish and macroinvertebrate habitat, increased sedimentation and water temperature (if cover is lost) and changes in essential stream nutrient and oxygen concentrations. Conversely, extended periods of low flows (drought conditions), whether naturally occurring or human induced (e.g. water for hotels and residences and snowmaking) can also adversely affect both aquatic and riparian animal and plant communities.
This study provides valuable information quantifying differences in overall streamflow volumes, peak flows, minimum flows, and timing and duration of each in both an undeveloped and a developed watershed at high elevation. This project has, and will continue, to produce real-world data needed by State regulatory agencies to make data-driven, environmentally sound decisions about development at Vermont's high elevation sites. Without proper regulatory oversight, safeguards and controls, alterations in streamflow (quantity, velocities, timing, and water quality) can potentially have devastating impacts on aquatic and riparian community's down-stream of highly developed sites.
Vermont's high elevation areas have the potential to be heavily impacted as the result of increased annual use and changing climatic conditions.
References
- Wemple, B., J. Shanley, J. Denner, D. Ross and K. Mills. 2007. Hydrology and water quality in two mountain basins of the northeastern US: assessing baseline conditions and effects of ski area development. Hydrological Processes 21(12): 1639-1650.
Additional Resources
- Burlington National Weather Service data accessible at: http://w2.weather.gov/climate/xmacis.php?wfo=btv.
- West Branch data are accessible at: http://waterdata.usgs.gov/vt/nwis/uv?site_no=04288225.
- Ranch Brook data are accessible at: http://waterdata.usgs.gov/vt/nwis/uv?site_no=04288230.
- Paired Watershed Study on the East Slope of Mount Mansfield: https://www.uvm.edu/femc/project/paired-watershed-study-east-slope-mount