ANALYSIS OF SPATIAL VARIABILITY OF PRECIPITATION AND …



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.

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.

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 Gauges-

Due 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 winter-operating 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 Transect-

The 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-

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

| |West Branch |Ranch |Ratio |

| | |Brook |(West/Ranch) |

|Land Area (m2) |1.18x107 |9.84x106 |1.20 |

|Precipitation (m3) |1.53x106 |1.13x106 |1.35 |

|(9/18/2002 - 10/17/2002) | | | |

|Precipitation Depth (m) |0.1293 |0.1145 |1.13 |

|(Volume/Area) | | | |

|Precipitation Depth (mm) |129.3 |114.5 |- |

|Runoff (mm) |94.05 |69.39 |1.35 |

|(Volume/Area) | | | |

|Average Runoff (mm) |108.3 |76.3 |1.42 |

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 |Precipitation (cm) |∆ SWE (cm) |∆ Snow Depth (cm) |

|Ranch 432m |3.73 |+0.62 |+1 |

|Ranch 706m |3.15 |-0.05 |-4 |

|Ranch 810m |N.A. |-0.28 |-6 |

|Ranch 1082m |N.A. |+10.9 |+22 |

|NWS 1204m |8.97 |N.A. |+20 |

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.

Summary-

Rainfall 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-

References-

Dunne, T., Leopold, L. B., 1978: Water in Environmental Planning. W.H. Freeman, NY, NY.,

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Taylor, G. H., C. Daly, and W. P. Gibson, 1993: Development of a new Oregon annual precipitation map using the PRISM model. The State Climatologist, 17(2), 1-4.

Daly, C., R. P. Neilson, and D. L. Phillips, 1994: A statistical-topographic model for mapping climatological precipitation over mountainous terrain. J. Appl. Meteor., 33, 140-158.

Daly, C., G. Taylor, and W. Gibson, 1997, The PRISM Approach to Mapping Precipitation and Temperature, 10th Conf. on Applied Climatology, Reno,NV, Amer. Meteor. Soc., 10-12.

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Taylor, G.H., C. Daly and W.P. Gibson, 1995, Development of a Model for Use in Estimating the Spatial Distribution of Precipitation, 9th Conf. on Applied Climatology, Dallas, TX, Amer. Meteor. Soc., 92-93.

Junker, N. W., 1992: Heavy Rain Forecasting Manual, National Weather Service Training Center. 91 pp.

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Woo, M.K. (ed.), 1985. Focus: Hydrology of snow and ice. The Canadian Geographer, 29, 173-183.

National Weather Service. Record of Climatologic Data. Lamoille County, Co-op Identification Number: 435416. Date of record: July 1, 2002 – December 31, 2002.

Dupigny-Giroux, Vermont State Climatologist. Website created 1998: Accessed 2003.

Mount Washington Observatory. Weather Archive: Accessed 2003.

Green Mountain Club, 2000: Long Trail Guide, 24th edition. The Leahy Press Inc., Montpelier, VT.

USGS data…..

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Table 2 caption: Precipitation and area values determined from DEM derived basin hypsometry. Stream runoff volumes from USGS data.

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.

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