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.
IntroductionA)
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 22.5km2 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 predevelopment 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 GaugesA 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.
The recently installed gauges consist of two major components, a tipping bucket
mechanism and a digital HOBO® Event data logger, produced by the Onset Computer
Corporation. The gauges are mast mounted 1-1.5 meters from the ground surface (see
Photo 2). They are installed in canopy clearings with a minimum clearance allowance of
45 degrees from the vertical; sighted from the gauge orifice. Data must be downloaded
manually, and weekly network maintenance visits make this study quite exercise
intensive. 10 inches of PVC piping (Photo 3) were attached to the top of some of the
gauges to continue monitoring as fall precipitation began to fall as mixed snow, sleet and
rain at the upper elevations. The gauges could then collect up to 12 inches of frozen
precipitation and record the melted equivalent if temperatures allowed.
Snow GaugesDue to the funnel/tipping bucket mechanisms, these gauge designs are not suitable
for collecting frozen precipitation and thus must be disabled for the winter seasons. To
account for this problem, two antifreeze snow adapters were purchased through an
undergraduate funding award. The adapters are reservoirs designed to sit atop the tipping
bucket rain gauges (Photo 4). The reservoirs are half filled with 2.5 gallons of a
glycol/ethanol antifreeze solution. A thin skin of mineral oil prevents evaporative loss of
the solution. Frozen precipitation melts as it accumulates in the reservoir and the volume
displaced is released into and recorded by the tipping bucket gauge. The discharged
solution is intercepted by a container, packed out of the watershed, and disposed of
appropriately.
These ‘winterized’ gauges supplement the two pre-existing heated precipitation
gauges in the study area. The USGS currently plans to install a third heated precipitation
gauge for the 2003-2004 winter season. This will bring the total number of winteroperating gauges to 5, and the summer total to 17, all within 21.6 km2; a uniquely small
study area.
B)
Gauge Transects
The network of precipitation gauges recently installed throughout the two
watersheds was designed to create three transects of individual mountain slopes (see
Figures 8a,b, and c). Two transects share the same East-West orientation on East-facing
slopes, which is necessary for proper data comparison. These transects are the Ranch
Brook transect, which consists of four gauges placed up the middle of the Ranch Valley
to a 820m col in the ridgeline, and the Gondola transect, which consists of four gauges
and follows the ski trail beneath Stowe’s eight-passenger gondola to the Gondola station
at 1136m. The third transect is of Spruce Peak. The Spruce transect is composed of four
gauges with a North-South orientation on a South-facing slope. All three transects share
a fifth gauge maintained by the USGS located at the 430m West Branch stream gauge
station. A single rain gauge (atop the resort’s Octagon Building, see Photo 5), separate
of the three transects, is located at 1114m at the top of Stowe Resort’s quad lift. This
single gauge is located along the Toll Road ridge, the watershed boundary separating
Ranch Brook and West Branch watersheds. This is a possible location for a future fourth
transect once a permanent heated gauge replaces the current seasonal Octagon gauge,
which could then be moved to a lower elevation along the Toll Road.
These transects are designed to compare the precipitation trends of the two
different watersheds. The gauges are spaced at relatively equal intervals to obtain a
consistent spread of elevation coverage. This provides the most accurate data for
determining precipitation/elevation regressions. Figure 8 is a series of three graphs
illustrating the relatively equal elevation spread of the gauges for each transect. This
allows for precipitation comparisons at uniform elevations between the watersheds.
Winter Gauge TransectThe network of winterized gauges was sparse for the 2002-2003 winter season;
the study’s first winter. The presence of two pre-existing, four-season monitoring
stations was used to our advantage. These gauges were located at the base of the
mountain, maintained by the USGS, and near the summit, maintained by the NWS. This
left a gap in elevation that was satisfied by installing two snow gauges at 432m and 706m
in the Ranch Brook watershed (Figure 7). While the density of gauges of the summer
network allowed for data between watersheds to be compared, the minor network of
winterized gauges narrowed observances solely to precipitation/elevation relationships.
C)
Snowpack Analysis
In most northern and alpine environments, snowmelt runoff is responsible for
both the annual maximum instantaneous discharge and a major portion of the annual flow
(Woo, 1985). Snowpack within the study area constitutes a means of hydrologic storage
generally from late October to May (SkiVT-L, 2003). An analysis of winter data must be
included in an effort to understand the region’s annual hydrologic transfers and
variability.
The snowpack in the Ranch Brook watershed was monitored for depth and snow
water equivalent from December 4, 2002 through April 4, 2003. These data are
compared to winter precipitation data measured from the two snow gauges in Ranch
Valley and NWS precipitation data recorded at 1204m. Monitoring precipitation,
snowpack dynamics, and streamflow helps create a solid understanding of the winter
hydrologic cycle. These variables are interdependent. Precipitation and temperature
govern the accumulation and storage of a region’s snowpack, which, in turn, directly
affects springtime flow volumes. By monitoring the volume of water held within the
snowpack as a function of elevation, a regression equation can be derived to estimate the
total water volume stored within the snowpack of the basin. This estimate can then be
compared to actual volumes of spring melt runoff and corrections can be made to
promote model accuracy.
Snow data from the 2002-2003 winter season were complimented by repeated
snowpack analyses using coring techniques to determine the amount of water contained
in the snowpack. A course of five survey locations, each preformed at different
elevations, was conducted in Ranch Valley. Two of the five survey locations were also at
snow gauge sites. Figure 7 shows the snow core locations within the Ranch Brook
watershed.
By plunging PVC pipe into the snowpack and weighing the resulting core, the
retained amount of water known as snow water equivalent (SWE) and expressed in
length units, is determined. The dynamics of the SWE held within the snowpack of a
basin over time can be modeled by obtaining these data throughout the snow season.
Five snow surveys of Ranch Brook watershed were completed from December 4, 2002
through April 4, 2003.
These data, coupled with precipitation and stream gauge data, provide a unique
opportunity to closely monitor the hydrologic cycle of a small mountainous watershed.
Mapping precipitation and stream flow help to better understand, and make it possible to
predict, changes in the natural environment. Monitoring variables such as SWE can also
provide insight into the degree of snowpack storage potential, which is important in the
evaluation of flood hazards.
While the 2002-2003 winter season was spent working out the bugs inherent to
any new system, subsequent winters will see comprehensive annual monitoring of Ranch
Brook’s, and possibly West Branch’s, hydrological dynamics. These data will then blend
seamlessly with similar spring, summer, and autumn data.
Discussion and Data Presentation-
A)
Correlations of Precipitation and Elevation
This study used a couple differing approaches in an attempt to understand the
relationship between precipitation and elevation within the Mount Mansfield watersheds.
The large range in elevation covered by the precipitation gauge network facilitated the
construction of precipitation/elevation regression relationships. The limiting factor
became the duration of the period of record, which ranged from three months to less than
one month (see Table 1). The window of observation, when all 13 gauges were
simultaneously operational, was limited. Regressions were created for a variety of time
periods and from different gauge locations to correct for this inadequacy. These time
periods ranged from individual storms to months. Correlations between elevation and
precipitation were determined from network-wide data, individual basin data, and
transect-specific data.
A comparison of network-wide data from a September 22 rainstorm shows that
totals across the study area ranged from 33mm (1.3 inches) recorded at the NWS summit
station, to 14mm (0.55 inches) recorded at the lowest elevation gauge in Ranch Valley
(Figure 9). Data from the thirteen gauges that recorded this storm event showed a linear
increase in precipitation with elevation. The data suggest a 1cm/650m (0.4in/2100ft)
positive linear relationship. By comparing data recorded along a specific slope a more
accurate precipitation/elevation relationship may be obtained. Data were collected along
the Gondola transect during the largest 24-hour storm event of the collection period,
which occurred on September 11, 2002. 80mm (3.15in) fell at 1204m in just over 12
hours. A linear relationship of 1cm/450m (0.4in/1500ft) was documented (Figure 10).
These two slopes derived using network-wide (1/650), and transect-specific (1/450) data
appear similar, but observation over a longer period of time might generate statistically
different regressions.
Analyzing single-storm precipitation totals for elevation regressions introduces
some degree of error when attempting to determine a fitting average equation to represent
precipitation distribution trends throughout the research area. The single-storm approach
assumes that the storm event is representative of the typical, or average, precipitation
storm event for the region. Given the diversity of Vermont’s weather and climate, it
would be impetuous to use a single-storm event to represent an average annual
precipitation/elevation correlation. Instead, a larger time scale on the order of years
would be preferable. Considering the constraints discussed above, and for the purpose of
this preliminary report, one to two month time periods will be used experimentally to
represent the average elevation distribution of precipitation for the Mount Mansfield area.
Precipitation totals for the period of August 10th- September 25th 2002 were
analyzed (Figure 11a). The data are from the six network-wide gauges operational
during that time period (refer to Table 1). The precipitation/elevation regression was
positive and linear; 1cm/200m (0.4in/660ft). Varying regressions can be obtained by
adjusting the period of observation to include or exclude storm events. The period of
August 10th-October 5th, 2002 yielded a lesser slope of 0.5cm/800m (Figure 11b).
Using this method of network-wide gauge locations and one to two months of data, the
average precipitation/elevation relationship was 1cm/245m (0.4in/800ft).
This approach to obtaining an average linear regression from network-wide gauge
data assumes the distribution of rainfall with elevation is equal across all of the study
area. Correlations between precipitation and elevation should be examined for individual
watersheds, and more specifically, individual mountain slopes to further remove
erroneous data owing to factors of variable precipitation. As expected, data comparisons
done for the longest period of observation and of data from individual slopes produce the
least number of outliers and tend to be more statistically sound (higher R2 values).
Three different graphs were created using data from individual mountain slopes.
The period of observation was from September 18 – October 17, 2002. Each graph was
constructed from transect-specific precipitation data. Regressions were obtained from the
Gondola, Spruce, and Ranch Brook Transects (Figures 12a,b, and c). All transects
consisted of data from five gauges except for the Spruce Peak Transect, which had only
three gauges due to wind damage to the summit gauge. The Gondola and Ranch Brook
Transects shared data from the NWS summit station since both slopes end at a ridgeline
of similar aspect and elevation to the location of the NWS station. The Gondola transect
showed a 1.1cm/200m (0.43in/650ft) increase in precipitation with elevation. The Spruce
Peak and Ranch Brook transects both had precipitation/elevation relationships of
approximately 1.1cm/125m (0.43in/410ft). Because the Spruce and Gondola transects lie
within the same watershed, their regressions are averaged. A statistical comparison of
the averaged West Branch regressions and the Ranch regression yields a significance
value of 0.507, suggesting that the slopes are parallel. The parallelism means that
precipitation increases with elevation at a steady rate, and that this rate is uniform at
1.5cm/200m for the entire study area (Figure 12d).
It is important to note the difference in the Y-intercept values between the two
regression equations. For the period of September 18 – October 17, 2002 the Y-intercept
value from the West Branch (Gondola) regression was 96.4, while the Y - intercept of the
Ranch regression was 57.6 (Figure 12a and c). This value reflects the amount of
precipitation that falls in each respective watershed. This is direct evidence of spatial
variability of precipitation.
B)
Spatial Variability of Precipitation
This study was specifically designed to monitor the variability of snowfall and
rainfall on two watersheds of the eastern slopes of Mount Mansfield. The density of the
network and the placement of individual gauges in valleys and on promontories provide
an appropriate framework for a study of this scale. The purpose of this study was not to
qualitatively explain observed variability, but rather to recognize and document it.
The greatest variability was observed as increased precipitation on summits and in
close proximity to ridgelines. This was determined by analyzing precipitation data along
individual transects. In nearly every case scenario, the NWS gauge located on the
ridgeline recorded far greater precipitation then the other gauges, located at lower
elevations East of the ridge (Figures 9, 10, 11c, and 13). This high elevation variability
often poses as statistical outliers and makes precipitation/elevation correlations difficult.
The best approach to demonstrating the presence of spatial variability of
precipitation within the study area is to compare precipitation totals from similar
locations. Rainfall on the two eastern aspect slopes, the Ranch Brook and Gondola
transects, was compared because of the slopes’ aforementioned physical similarities and
the observed discharge discrepancy between the two watersheds. Similar elevations from
the two watersheds were compared for both single storm events and longer periods. Data
collected over the span of a month from two gauges at an average elevation of about
530m were compared (Figure 13a). During this event the Gondola location received 9%
more rainfall than the Ranch location. For the same period, the Gondola location
received 14% more precipitation at 800m than the neighboring Ranch location (Figure
13b). Upon closer inspection of these data, it appears that a single September 27-28th
storm event introduced a majority of the discrepancy observed for this time period.
This
discrepancy in rainfall between the Ranch and Gondola slopes is not observable at the
upper elevations of both watersheds. The Octagon gauge at 1,114m is used to record
Ranch Brook’s upper elevation precipitation. These data are compared to the data
retrieved from the Gondola summit station (Figure 14). The Gondola gauge received
slightly less than 8% more precipitation for this storm event. Regardless of elevation, the
Gondola transect received more rainfall than fell on the Ranch Brook watershed, but this
difference was greater at middle and lower elevations.
Individual transect data, recorded at similar elevations, for the same late
September storm event was compared. This type of comparison displays the spatial
variability of precipitation in the study area by minimizing elevation affects on
precipitation. Precipitation was collected from each of the three transects at an average
elevation of about 530m (Figure 15a). Rainfall totals at approximately 530m were;
Spruce: 3.8cm, Ranch: 4.1cm, and Gondola: 4.8cm. Rainfall totals at approximately
800m were also compared (Figure 15b). The variability between Ranch Brook and West
Branch watersheds at this elevation was significant. The Spruce Peak and Gondola
locations received nearly the same amount of rain; 5.2cm. Ranch Brook received far less
precipitation at 820m; 3.8cm, 35% less rain than fell at this elevation on the West Branch
watershed.
C)
Volume Differences Between the Watersheds
Comparisons of rainfall totals from single storm events may provide a glimpse of
what occurs on the larger time scale, but it may reflect an abnormality. To avoid the
latter possibility it is important to compare precipitation totals from the two watersheds
over the course of the longest period of observation. The data presented in this report
prove that greater precipitation falls on the West Branch watershed than the Ranch Brook
watershed. The degree of variability varies with elevation and location. For that reason,
an average regression for each watershed was constructed (Figures 12a,b, and c). These
regressions were used to estimate the total volume of precipitation that fell within each
watershed during the same period for which the regression equations were derived. The
slope of the regressions used was the average slope (0.0755x) of precipitation/elevation,
determined to be statistically parallel (Figure 12d). A digital elevation model (DEM)
was developed of the study area for the purpose of extrapolating these regressions across
the watersheds. The regression applied to the 11.7km2 West Branch watershed was an
average of the two regressions derived for the Spruce and Gondola transects. This
average regression and the one applied to the 9.6km2 Ranch Brook (Figure 12c) were
determined from data recorded 9/18/2002 - 10/17/2002.
A hypsometry graph was created, using a DEM, to calculate a volume of
precipitation that fell within the watershed boundaries for the month. This volume, in
cubic meters, was divided by basin area, in square meters, to create a comparable length
value (Table 2).
Table 2
Land Area (m2)
Precipitation (m3)
(9/18/2002 - 10/17/2002)
Precipitation Depth (m)
(Volume/Area)
Precipitation Depth (mm)
Runoff (mm)
(Volume/Area)
Average Runoff (mm)
West
Branch
Ranch
Brook
Ratio
(West/Ranch)
1.18x107
1.53x106
9.84x106
1.13x106
1.20
1.35
0.1293
0.1145
1.13
129.3
94.05
114.5
69.39
1.35
108.3
76.3
1.42
Table 2 caption: Precipitation and area values determined from DEM
derived basin hypsometry. Stream runoff volumes from USGS data.
The West Branch watershed is determined to have received 127.9mm of rainfall.
The Ranch Brook watershed received 115.1mm of rain. Using the regressions,
determined from a one-month period of record, the West Branch watershed was
calculated to have received 11.1% more precipitation than the neighboring Ranch Brook
watershed. This value can be compared to the runoff discrepancy observed for the same
period of time. The West Branch stream gauge recorded 35.5% greater runoff for this
month. The precipitation difference does not entirely account for the discharge
discrepancy observed between the two watersheds. The variability of precipitation
between the two watersheds is certainly appreciable and is a likely factor in the observed
runoff discrepancy.
D)
Snowpack Dynamics within Ranch Brook Watershed
Unfortunately for the preliminary study, over three months of SWE data are
unavailable due to a January snowboarding accident that kept me from extensive field
work through mid March. Also, winter precipitation data within Ranch Brook are limited
due to equipment failure, partially as a result of the field hiatus. At the time of
publishing, USGS streamflow data was available through November, 2002, and NWS
precipitation databases were only updated through 12/31/02. This limited comparison
opportunities that will accompany future studies of the watersheds. A graph created by
the University of Vermont of daily reports from the National Weather Service displays
the precipitation occurring at 1204m over the course of the snow season (Figure 16a).
This data source is unofficial, and is not considered reliable for the purpose of this report,
but shows most precipitation activity occurred early in the 2002-2003 snow season, and
then again in late March. Engineers working atop The Nose at the WCAX television
transmitter monitor the snowpack depth throughout the winter and UVM records and
archives these data. The peak of the snowpack depth, recorded at 1,190m, occurred
around the first week of March 2003 (Figure 16b).
The SWE data were obtained by completing five snow courses (Figure 17). The
snowpack at upper elevations tended to accumulate much faster and SWE peaked
significantly higher than lower elevations. This difference was due in part to the greater
likelihood of early season precipitation falling as snow at upper elevations, and in part to
greater precipitation falling at higher elevations.
The largest change in SWE was observed from December 18 to January 4 at
1082m (Table 3). During this period, temperatures at the summit reached a maximum of
7°C (44°F), and 3.43cm (1.35in) of rain and 40.6cm of snow were recorded at 1204m.
These variable conditions caused the snowpack to respond differently at each elevation.
3.73cm of precipitation fell at 432m, but the SWE of the snowpack at this low elevation
only increased by 0.62cm, which suggests melting occurred. The depth of the snowpack
did not change significantly despite the occurrence of both rain-on-snow and
accumulating snow events. Ranch 706m received slightly less precipitation for this
period of time, experienced very little change in SWE, and lost 4cm of snow depth.
These data suggest that the lower elevations underwent melting that was then balanced by
accumulating snowfall. The 1082m snowpack did not experience melting; the snowpack
absorbed the rainfall, increasing the SWE and subsequent snow accumulations increased
the snow depth.
Table 3
Period 12/18 – 1/3
Ranch 432m
Ranch 706m
Ranch 810m
Ranch 1082m
NWS 1204m
Precipitation (cm)
3.73
3.15
N.A.
N.A.
8.97
∆ SWE (cm)
+0.62
-0.05
-0.28
+10.9
N.A.
∆ Snow Depth (cm)
+1
-4
-6
+22
+20
Table 3 caption:
Ranch Brook precipitation data, and observed changes in
SWE and snowpack depth for the period of December 18th, 2002 – January 3rd, 2003.
Over the course of the winter, the snowpack at lower and middle elevations did
not gain SWE at the same rate as the upper elevations. This is likely due to the
precipitation/elevation correlation discussed in previous sections, as well as melting
conditions experienced at lower elevations and not at upper elevations. Above freezing
middle and lower elevation temperatures occurred during December 11-13th and then
again during December 18-21st (NWS Data, 2003). This warm up did not significantly
affect the upper elevation snowpack due to the colder conditions on the upper mountain.
Because of the thinner early season snowpack at middle and lower elevations, such a
short period of above freezing temperatures would have the ability to completely saturate
and begin to the melt the snow. These conditions would limit increases in SWE. The
rates of depletion of SWE depend upon such factors as degree of saturation before
exposure to melting conditions, and the type and degree of melting conditions, i.e. sun
exposure, above freezing temperatures, or rain-on-snow events.
An example of the likelihood of lower elevations experiencing melt conditions
was observed early in the season, along the Ranch gauge transect during November 1st –
November 9th, 2002. This period was below freezing and 15 inches of snow were
recorded over five days at the NWS summit station. 10 inches fell on lower elevations.
These were the first significant snowfalls of the season and the rain gauge network in
Ranch Brook was still recording data. The 10-inch PVC extensions on the gauges
allowed for the snowfall to be captured, allowing the gauges to retain up to 12 inches of
snow without loss. A significant warm-up accompanied a rainstorm on November 10th,
during which all the snow was melted and recorded (along with the liquid precipitation)
by the gauges (Figure 18a). This melting and rainfall created the most statistically
accurate precipitation/elevation regression (R2=0.996) of the observation period (Figure
18b). Melting conditions such as these, especially in thin snowpack situations, are
responsible for limiting seasonal SWE accumulation. The highest elevations gain SWE
and avoid melting because these elevations more likely to remain colder and the deeper
snowpack can retain more water before melting.
Further research could be done monitoring stream runoff vs. SWE. An observed
increase in streamflow during the winter would suggest a ripening snowpack, a snow
course would determine the extent of saturation with elevation, and meteorological
parameters would indicate the degree of melting to be expected. Coupling stream,
precipitation, temperature, and SWE data in this manner would be important in
evaluating regional flood hazards and the timing of peak flows.
The March 23rd 2003 snow course recorded 49.6cm of water stored in the
snowpack at 1082m, 26.7cm at 880m, 18.3cm at 706m, and 13.2cm recorded at 432m
(Figure 19). These were the maximum SWE values of the winter. A linear regression is
formed to model the distribution of water within the watershed by creating a graph of
SWE recorded during peak storage vs. elevation (Figure 19). This regression is then
used to estimate the total water volume stored within the snowpack of the Ranch Brook
watershed by running the regression equation through the Ranch Brook digital elevation
model. The snowpack of the Ranch Brook watershed, at the seasonal peak, was
determined to contain over 225 million cubic meters of water. This volume, divided by
watershed area, yields a linear depth of 22cm of SWE. This value is 27.3% of Ranch
Brook’s annual runoff.
SummaryRainfall occurring within the Ranch Brook and West Branch upper drainage
basins was monitored from August 10th, 2002 through October 30, 2002. This study
obtained large quantities of precipitation data (15 stations) over a 22.5-km2 study area.
Snowfall and snowpack dynamics within the Ranch Brook watershed were monitored for
the 2002-2003 winter season using a network of four snow gauges and repeated
snowpack analyses. Data were analyzed for trends of spatial variability and correlations
between precipitation and elevation. The data presented and discussed in this report
support a linear trend of increased precipitation with elevation. This trend was consistent
for both eastern watersheds of Mount Mansfield. Precipitation was determined to be
slightly higher on summits and in close proximity to prominent ridges. Before the recent
installation of the 13 rain gauges, researchers only had two precipitation gauge locations,
one at the lowest elevation and the other on the upper ridgeline. This study serves as
proof that, while these two locations may collect the lowest and highest respective
precipitation totals, the exact spread of precipitation between the two locations is better
represented with a locally derived linear regression.
Basin-wide precipitation for each watershed was estimated using basin
hypsometry data derived from a digital elevation model. Linear regressions determined
from a month of observation were applied to the hypsometry data to estimate
precipitation volumes for both watersheds. The volume calculations determined that the
West Branch watershed received greater precipitation per land area than the Ranch Brook
watershed for this single month. The USGS stream gauge on West Branch recorded 50%
greater runoff per basin area observed for the 2001 water year. While the precipitation
difference is lower than the average runoff discrepancy, it is likely a major contributing
factor and helps explain the difference in annual runoff.
Conclusions-
Projections for Future Project Development-
Support for Project-
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