Analysis of Spatial Variability of Precipitation and Snow Accumulation on Mount Mansfield,
Stowe, Vermont
An Undergraduate Senior Research Project
By:
Keith N. Musselman
Submitted in partial fulfillment
of the requirements for the degree of
Bachelor of Science
Department of Geology
University of Vermont
March 31, 2003
Abstract-
Recent research on two upper watersheds of the West Branch Little River (West Branch) and Ranch Brook, both located on the eastern side of Mount Mansfield in the town of Stowe, Vermont, indicates substantial differences in unit area runoff between the two basins. These disparities may be explained in part by spatial variability of precipitation inputs. This study seeks to better understand the microclimatology of these upper elevation watersheds. The costs and difficulties in maintaining an adequate network of upper high elevation weather stations have limited research focused on spatial precipitation patterns. Forecasting these small-scale precipitation patterns in mountainous regions is a difficult undertaking due to the insufficient density of recording stations and the variable effects of terrain and elevation on storm behavior. This study obtains large quantities of precipitation data (15 stations) over a 22.5-km2 study area. Rainfall occurring between August 10th and October 30th, 2002 was documented using a network of thirteen automated recording rain gauges recently installed throughout the two watersheds. Snowfall from December 12, 2002 through the end of the 2003 snow season was monitored along the Ranch Brook Transect with a network of three snow gauges and NWS station data from the summit. These snow data were complimented by repeated snow pack analyses using coring techniques, conducted along the Ranch Brook transect.
These precipitation data are used to map, document, and increase understanding of small-scale precipitation trends in the region. They are analyzed using average elevation/precipitation regressions. The study is geared toward proving a direct correlation between increases in precipitation with elevation as well as understanding the effect of azimuth and large topographic features such as ridgelines and prominent summits. Preliminary results of this study suggest an average positive linear precipitation/elevation relationship of 2cm/250m derived from one month of data. A significant increase in precipitation is also observed in close proximity to major ridgelines and summits. Using regressions determined from one month of observation, the West Branch watershed was calculated to have received 129.3mm (volume/area) of precipitation and the Ranch Brook watershed was calculated to have received 114.5mm (volume/area). Storms during this period of time loaded the West Branch watershed with 13% more rain than the neighboring Ranch Brook watershed. These findings help explain the runoff discrepancy observed between the upper high elevation watersheds of Ranch Brook and West Branch of the Little River.
1. Introduction-
A) Background-
Precipitation is fundamental to the hydrologic cycle. Much of the ecology, geography, and land use of a region depend upon water. Precipitation provides both constraints and opportunities in land and water management (Dunne, 1978). Resource management is of special interest to this project since the Stowe Mountain Resort is located within the study area. The ski resort depends upon spring snowmelt as well as summer and fall rainstorms to fill its snowmaking reservoir, necessary to remain competitive in an industry which needs to compensate for a poor snow season with a man-made base. Thin soils and steep slopes leave these watersheds susceptible to erosion and impervious surfaces augment peak flows and diminish natural flood buffers (VMC, 1996). Monitoring precipitation occurring on specific watersheds and the flow volumes of local rivers is pertinent to understanding the region’s water budget. With these basic, but very valuable data it is possible to make knowledgeable decisions concerning such topics as appropriate snowmaking water budgets or determining the effects that clearing land for trails and roads may have on erosion, stream sediment load, and surface runoff volumes.
B) Precipitation Patterns
Many professionals concerned with planning and management rely on meteorological forecasts to determine how much precipitation will fall where and when. Forecasting precipitation events over mountainous regions is a difficult undertaking due to the variable effect of terrain and elevation on storm behavior (Gibson et al. 1997). While substantial research has been done on spatial precipitation patterns in the Intermountain West (Taylor et al, 1993, Daly et al, 1994, Ralph et al, 1999), very little detailed documentation has been done in Northeastern North America.
Daly et al. [1994] have determined that terrain dominates the spatial patterns of precipitation in mountainous regions. The effectiveness of a terrain feature in amplifying precipitation depends on its ability to block and uplift moisture-bearing air. This ability is determined mainly by the profile the feature presents to on-coming air flow (Daly et al., 1997). Small-scale precipitation patterns are often not reflected in weather service data because recording stations are not of sufficient density in rough terrain (Gibson et al, 1997). This study obtains large quantities of precipitation data (15 stations) over a 22.5-km2 study area. With such an extensive data set, I am able to describe localized spatial variability of precipitation over large elevation ranges and short distances.
2. Setting-
A) The Watersheds
The two watersheds involved in this study are upper-elevation watersheds located on the eastern slopes of Mount Mansfield, in the town of Stowe, Vermont (Figure 1). The majority of the land in both drainage basins is owned by the State of Vermont. The watersheds are designated by two stream gauge stations, installed in 2000, on both the West Branch of the Little River (West Branch) and Ranch Brook for the purpose of an on-going paired watershed study conducted by the United States Geological Survey (USGS), researchers at the University of Vermont (UVM), in conjunction with the Vermont Monitoring Cooperative (VMC). The study uses a comparative approach to understand the effects of land development on stream water quality and quantity (Wemple et al., 200???). These effects are of special concern to this area because of the recently received Act 250 approval for a major expansion of the Stowe Mountain Resort, including a new 18-hole “Wilderness” golf course, up to 400 housing units, and a commercial plaza (Stowe Mountain Resort, 2003).
The 9.6 km2 Ranch Brook watershed, located within Ranch Valley, is forested and undisturbed, except for a small network of narrow cross-country ski trails and an unpaved, low elevation, gated access road. It ranges in elevation from 400m at the stream gauge station to 1,204m at The Nose, one of Mansfield’s prominent summits. It has an easterly drainage pattern, and is bordered on three sides by ridges. The watershed’s western perimeter runs 4km along the spine of the Green Mountains. The northern boundary lies along a ridge, dividing the two watersheds. The Stowe Mountain Resort maintains a summer auto road (Toll Road) that winds along this divide toward The Nose. The southern perimeter of the watershed runs the length of the 4.5km Sky Top Ridge.
The West Branch watershed is 11.7 km2 and includes the Stowe Mountain Resort. It ranges in elevation from 430m at the West Branch stream gauge station, to 1,339m at the summit of Mount Mansfield, Vermont’s highest peak. The watershed contains over 63 km of cleared trails, paved and unpaved roads, 6.5 km of Vermont Highway 108, all comprising a total of over 2 km2 of cleared land (Stowe Mountain Resort, 2002). It has an easterly to southeasterly drainage pattern. 4.6km of the western perimeter of this drainage basin is above 1160m (3,800ft), the longest sustained high-elevation ridgeline in the Green Mountains. Half of Mansfield’s 101 hectares (250 acres) of regionally rare, alpine tundra exists within the West Branch watershed boundary (GMC, 2000). In the shadows of this ridge and its prominent summits, are 43 alpine trails and 5 aerial lifts of the Mansfield side of Stowe Mountain Resort (Stowe Mountain Resort, 2003). The northern boundary of this watershed includes Smuggler’s Notch and its cliffs. Above the Notch’s eastern cliffs (Photo 1) is the 1,012m (3,320ft) summit of Spruce Peak, with its thin soils and Stowe’s network of 13 south facing alpine ski trails serviced by four aerial lifts (Stowe Mountain Resort, 2003).
Both watersheds are adjacent, and similar in size, geology, soil, and aspect. The primary differences are land use and topographic relief. While land use is known to play a major role in the fate and environmental interactions of fallen precipitation (Makino, 1999, Troendle & Meiman, 1984), it seems to have limited effects on the spatial variability of falling precipitation. Relief and topography are of specific interest to this study because they are significant contributing factors to spatially variable precipitation (Taylor et al., 1995).
B) Current Hydrologic Research
The West Branch and Ranch Brook watersheds are currently involved in a paired watershed study being conducted and overseen by Beverley Wemple; UVM Geography Department, Donald Ross; UVM Department of Plant and Soil Sciences, and Jamie Shanley; USGS NH/VT District. This team of researchers and scientists has developed a scientific approach to understanding environmental impacts of Vermont ski resorts. The two drainage basins offer highly favorable conditions for a paired watershed study. The Ranch Brook watershed is an undeveloped, forested “control” basin, representing pre-development conditions. The West Branch watershed is the “treatment” basin, encompassing the ski resort, VT 108, and residential properties. The watersheds share similarities discussed in the previous section. These similarities allow for comparative assessments of hydrologic differences resulting from land cover variability and development in the two basins. Two streamgage stations were installed in October 2000 on the West Branch and Ranch Brook and have since provided a continuous data record. Water is sampled automatically at the stations and is tested for total suspended solids. These measurements are used to analyze water, sediment, and chemical fluxes from the individual watersheds (Wemple et al., 200????).
Preliminary data from the watersheds indicate a 40 - 50% greater annual water yield from the West Branch watershed. Wemple and Shanley suggest this discrepancy indicates unresolved differences in precipitation capture of the West Branch watershed. My precipitation study serves as a satellite project of this cooperative research. An extensive record of high elevation precipitation data helps create an understanding of the trends and spatial precipitation variability within the watersheds. Such knowledge will support a correlation between the observed discharge discrepancy and differences in precipitation capture of the watersheds.
C) Regional Climate
As the old New England saying goes, “If you don’t like the weather, wait awhile.” This adage holds true for the irregular and often unpredictable weather and climate in Vermont. Variations in diurnal and annual temperatures, differences in the same season from year to year, and variability in weather from place to place, characterize Vermont as having a dynamic climate (Dupigny-Giroux, 1998). Vermont and much of New England inherit their dynamic climates from the convergence of multiple storm tracks directly overhead (Mount Washington Observatory, 2003). Local factors also have a significant effect on the climate. Factors such as elevation differences, terrain, and proximity to water bodies such as Lake Champlain are all major players (Dupigny-Giroux, 1998). The northwestern corner of the state experiences effects from all of these weather factors.
No published work exists on orographically induced precipitation in Northern New England. A qualitative regional analysis of the factors behind orographic precipitation is beyond the scope of this report, but a definition of this process is important. The development of heavy precipitation depends upon adequate moisture and upward motion (Junker, 1999). Air masses are orographically uplifted as they come against mountainous topography. This uplift of an air mass causes saturation and can result in increased precipitation across high terrain. Figure 2 displays a simplified sketch of this localized process. Orographic precipitation occurs not only during the summer, but also during all months of the year as storms are lifted up and over the Greens.
Regional evidence for this orographic effect is that higher elevations in Vermont receive more precipitation than neighboring lower elevations (Figure 3). More evidence for this locally occurring process is observed by comparing National Weather Service precipitation data measured in Burlington, Vermont (elevation 104m) to those measured on the summit of Mount Mansfield (1,204m) for the same period of time (Figure 4). This comparison reveals that Mount Mansfield often receives precipitation, while Burlington (100km west) remains dry. The majority of precipitation events for this period were recorded on Mount Mansfield while significantly less to none fell in Burlington.
D) Microclimate of the Watersheds
The watersheds of interest experience weather that is perhaps some of the most variable in Vermont, and even New England, mostly because of their mountainous, upper-elevations. This study will consider 1,000m and higher (3,280ft+) to be upper elevation terrain, where the greatest precipitation could be expected. Only 10% of the Ranch Brook watershed is located above 1,000m. This is in contrast to 20% of the larger West Branch watershed above 1,000m (see Figure 5) (Wemple et al, 200???). West Branch contains 2.36km2 of upper elevation, while Ranch Brook contains 0.98km2. Since the West Branch watershed has nearly two and a half times more upper elevation land area than the Ranch Brook watershed, and it is known that precipitation increases with elevation, it can be expected that greater precipitation would fall within the West Branch watershed boundary. This is proven in the Data section of this report.
The relief and North-South aspect of the Mount Mansfield ridge establishes a means of orographic blocking, causing oncoming air masses to rise. This process causes heavier precipitation localized to areas in close proximity to prominent summits and North-South oriented ridgelines. The western half of the West Branch watershed is in one of these reoccurring areas of increased precipitation. In contrast, only a small segment of the northwestern quadrant of the Ranch Brook watershed has considerable relief capable of significant orographic uplift. The relief of Mansfield’s ridgeline is best viewed from the West. This ridgeline forms the western perimeters of the watershed divides (Figure 6).
3. Methodology-
A) The Precipitation Gauges
Rain Gauges-
A network of 13 automated recording rain gauges, installed throughout the two watersheds in late summer of 2002, was added to three pre-existing precipitation gauges in the study area. Table 1 lists the period of record for each gauge. Of the pre-existing gauges, two are heated, year-round stations. One is maintained by the USGS at elevation 430m, and the other by the National Weather Service at elevation 1204m. The third gauge is maintained through the summer months by UVM researchers at elevation 884m. Figure 7 is a topographic map of the watersheds and the precipitation gauge network.