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Mn/DOT
Agreement No. 74708
"Climatological
Characterization of Snowfall and Snowdrift in Minnesota"
Summary
Report for Task 3 of Project
Task 3: Tabler model performance
in Minnesota and agricultural impact assessment
Duration: 12 months
Deliverable: Task
summary report
Comment: This task
report addresses phase three of the project, which is to evaluate
snow transport models, evaluate the Tabler methodology for snow
fence design and assess agricultural impacts. It was agreed that
the report should include a summary of the methods used for calculating
snow transport and associated parameters following the Design Guidelines
for the Control of Blowing and Drifting Snow publication (Tabler
1994). This includes the calculation of potential snow transport,
relocation coefficient, mean seasonal snow transport and snow fence
storage capacity. It was further agreed that deliverables would
include a field assessment giving a detailed analysis of results
from the 2000 - 2001 field investigation at each of the three living
snow fence sites. Products include observed snow transport, snow
density, soil temperature, freezing depth, soil moisture and crop
yields. A second component of Task 3 is a case study analysis for
each field site. This product is Web-based and takes the user through
the steps necessary to design a living snow fence, while using a
combination of products developed in Task 2 with data collected
on-site. These steps follow those given in a training session conducted
by Dan Gullickson of Mn/DOT and Paul Flynn of the USDA/NRCS. As
with Task 2, these products were to be delivered in the form of
a CD so that mapped details of the analysis in color format would
be more easily interpreted and the case study could be facilitated
using an internet browser (but does not require Internet access).
Disclaimer: The
Minnesota Department of Transportation (Mn/DOT), in cooperation
with the University of Minnesota (UMN) and the Minnesota Department
of Natural Resources – State Climatology Office (SCO) provide these
data, and the associated maps, tables, and analyses for informational
purposes only. As with most government services, these data exist
in the public domain, but users are encouraged to pay special attention
to the conditions and provisions under which the data were collected,
as there were some variations in method of observation. The user
assumes the entire risk related to use of this database. Data are
provided on an "as is" basis and Mn/DOT, UMN, and the
SCO disclaims any and all warranties, whether express or implied,
including (without limitation) any implied warranties of merchantability
or fitness for a particular purpose. In no event will Mn/DOT, UMN,
or the SCO be liable to users or to any third party for any direct,
indirect, incidental, consequential, special or exemplary damages
or lost profit resulting from any use or misuse of this data.
Background:
Site Descriptions
A field investigation was conducted in southwest
Minnesota for the 2000 - 2001 winter season to test the applicability
of the Tabler model for snow transport and to assess the agricultural
implications of living snow fences. Three sites in southwest Minnesota
were used, located near the towns of Gaylord (Sibley Co.), Lamberton
(Redwood Co.) and Mountain Lake (Cottonwood Co.) (Figure
1), each site representing a different living snow fence
design. The first site was located 1.5 miles north of Gaylord,
MN, designed to protect an S-shaped, north-south oriented section
of trunk highway 22 (Image
1). It consisted of two 8-row strips of standing corn 2,150-ft
in length with 150-ft distance between the strips, and positioned
150-ft from the west side of the highway (Images
2, 3).
The height of the corn for the winter season ranged between 5 and
7-ft. Prior to snow deposition the corn was approximately 7-ft
(Image
4) and roughly 4-ft after snowmelt (Image
5). After snowmelt, the corn was hand-harvested by the
landowner to eliminate volunteer corn during the growing season.
For the 2001 season, corn was again planted. A second site located
two miles to the west of Lamberton, MN at the Southwest Research
and Outreach Center was established in collaboration with MN/DOT
and the Redwood Soil and Water Conservation District. It consists
of a 350-ft long east-west oriented twin-row planting established
in 1998, and placed 230-ft on the north side of Highway 14 (Image
6). Three varieties of honeysuckle (Arnold's Red, Freedom
and Hawkeye) were planted at a spacing of 4-ft and the rows spaced
4-ft on center (Image
7). At the time of planting, a 10-ft wide black geotextile
fabric (termed "EarthMat") was installed at the top of
the soil surface for permanent weed control. The height of the honeysuckle
for the winter season was also 6 - 7-ft tall. For the 2001 growing
season, the area north of the fence was left fallow and wheat was
planted on the south side of the fence. Finally, the third site
is located 8 miles north of Mountain Lake, MN on the north side
of Highway 30 (Image
8). The fence is a 1998 planting of alternating honeysuckle
and red cedar; however, the cedar is only 2 - 3-ft tall with the
honeysuckle at 8 - 9-ft tall, as of the 2000 winter season. The
position of the fence is 300-ft from the north side of the highway
and is 2000-ft in length. Soybean was planted in spring 2001 at
this site.
Winter Conditions
In determining the effectiveness of living fences
as snow barriers, it is necessary to report the meteorological conditions
so the results can be viewed in context. The seasonal snowfall
and historical ranking, precipitation, temperature, winds and soil
moisture conditions of the field sites are presented. Federal meteorological
observing stations are located at Gaylord and Lamberton; however
the closest station to the Mountain Lake site (by straight-line
distance) is in Springfield, MN, approximately 15 miles from the
field site. Data from these stations are used here because of the
observation frequency and instrument standardization.
Snowfall
Annual (July 2000 - June 2001) snowfall totals
for each location were 6-in to 38-in above the 30-year (1961 - 1990)
climatological mean, as seen in Table
1. The greatest positive departure
from normal for the year occurred in December, with monthly totals
of 22-in, 24-in and 20.6-in for Gaylord, Lamberton and Springfield,
respectively. When put in the context of the last 100 years, the
2000 - 01 winter season snowfall total (Figure
2) for southwest Minnesota ranks
in the 85th to 98th percentile, meaning that
85% to 98% of all observations are less than this total (Figure
3). This was also the case for much of the southern half
of Minnesota and along the north shore of Lake Superior.
Temperature
Both the maximum and minimum temperature data
show a two to three degree negative departure from the 30-year normal
for the 2000 - 01 season, with the greatest departure occurring
in December (Tables
2, 3).
Each of the three locations reported approximately 12°
F below normal for both the maximum and minimum temperature for
December and historically, these rank second for each station.
This also corresponds with the period of anomalous snowfall for
these locations.
Wind
Wind speed
and direction is essential when investigating snow transport and
blowing snow events. Hourly wind data were collected at the Gaylord
and Lamberton sites, but the Windom, MN airport observing station
was used to represent the Mountain Lake location (no wind observing
station was available for Springfield, as was used for the previous
parameters). Frequency distributions of the wind direction observations
for the dates within the snow accumulation season are divided into
the 16
cardinal directions for each location (Figs.
4, 5,
6).
The predominant wind directions for Gaylord, Lamberton and Windom
were 280°, 60°, and 335°, respectively for the 2000
- 01 winter season. However, this does not necessarily correspond
to the direction of snow transport, as will be discussed in the
results.
Precipitation and Soil Moisture
Finally, two crucial components to assessing
the agricultural implications of living snow fences are precipitation
for the pre- and post-winter season and soil moisture conditions.
Precipitation observations for Gaylord, Lamberton and Springfield
for the months of November and April illustrate a noticeable positive
departure from the 30-year normal (Table
4), specifically in April, which
set a record for Lamberton and Springfield. This anomalous rainfall
occurred at crucial stages, just prior to the onset of the snow
accumulation season and during and after snowmelt. The implications
of the November rain are that the soil goes into the winter season
with a high water content. Moreover, the excess rainfall during
snowmelt, coupled with the anomalously high snowfall for the season,
produced exceedingly wet soil conditions in the spring. Data from
Lamberton show that available soil water was above average from
the time of snowmelt until July (Fig.
7). Tile lines at the Southwest Research and Outreach Center
began running March 21, 2001 and ended on August 2, 2001.
Materials
and Methods:
Instrumentation
Automated meteorological observing stations
were deployed at the Gaylord and Lamberton living snow fence sites.
Each station was equipped with instrumentation to measure wind speed
and direction, air temperature and soil temperature. Wind speed
and direction was measured with an RM Young Wind Sentry for each
site (Model WS-03002, Campbell Scientific). Air temperature was
measured using a temperature probe and standard radiation shield
(Model HMP35C and 41002, Campbell Scientific). Thermocouples were
prepared using standard thermocouple wire (Type T, Omega Engineering)
and installed at a 4-in depth in the soil. Data from these sensors
were recorded using electronic data logging equipment at intervals
of one hour. At Gaylord, a 21X was used in conjunction with an
AM25T Multiplexer (Campbell Scientific). Similarly, data at Lamberton
were logged using a CR10 and AM416 Multiplexer (Campbell Scientific).
Snow stakes were installed at these two sites for manual snow depth
measurements, which consisted of a graduated wooden stick. Frost
tubes were installed at Lamberton for measuring the freezing depth
in the soil profile. The instruments were fabricated using a set
of plastic tubes, the inner tube a flexible vinyl half-inch diameter
tubing 3-ft in length, filled to 90% capacity with a dye solution
which changes colors when below 32° F (Image
9). The outside of this vinyl
tubing was graduated and inserted into a 2-in diameter, 8-ft long
PVC tube, capped at both ends. The cap on the top end of the outer
tube was attached by wire to the top of the inner tube such that
the zero line corresponded to the soil surface. Using a hydraulic
probe, a 2-in diameter core of soil was extracted such that the
frost tube could be inserted to a depth of 3-ft into the soil, with
5-ft above the soil surface.
Experimental Design
A transect approach was utilized at each field
site for the soil temperature, snow stake depth measurement and
frost tube instrumentation arrangement. The soil temperature sensors
were positioned in a line perpendicular to the Gaylord and Lamberton
fence lines at 20-ft intervals. For Lamberton, the sensors extended
100-ft from the leeward side of the fence line and 60-ft on the
windward side (Image
10), with a control site established
another 40-ft beyond that distance (Image
11). At Gaylord, soil temperature and snow stakes extended
between the two corn row strips and 25-ft past the outer rows of
each 8-row strip. Instruments were also placed 120-ft from the
windward corn rows for the purpose of control measurements (or not
under the influence of the snow fence). Correspondingly, snow stakes
were installed in the same manner. Frost tubes at the Lamberton
site were installed at 40-ft intervals, or at alternate soil temperature
and snow stake locations (Image
6). There was no automated instrumentation installed at
the Mountain Lake site; however, the snow depth measurements were
taken at 20-ft intervals in a transect perpendicular to the fence.
Summer Soil Temperature Measurements
Soil temperature measurements were also taken
under the geotextile fabric at the Lamberton planting site during
the summer of 2000 and 2001. Since the fabric is utilized by MN/DOT
to establish snow fence plantings, the objective of the measurements
was to compare the soil temperature at specific depths under the
fabric with those under bare soil. The fabric on the west end of
the fence line had no mature plantings; therefore the area was not
under the influence of vegetative cover (Image
12). Thermocouples were installed
at 2-in, 4-in, 8-in and 16-in depths under the fabric. Measurements
under bare soil were obtained from an automated weather station
located (distance?) at the Southwest Research and Outreach Center.
Daily maximum and minimum temperatures are compared at the two locations.
Snow Density
Late season snow density measurements were taken
to determine the density of the entire depth of the snowpack for
snow volume and snow transport estimates. To sample an entire snow
column, hollow 2-in diameter graduated PVC tubes were inserted into
the snowpack to the soil surface (Image
13). The snow depth was recorded and the snow inside the
tube was then bagged, sealed, and later melted and the volume was
recorded (Image
14). In this regard, the volume of snow
could be compared to the corresponding volume of water. The same
transect approach was used for these measurements as previously
mentioned; however the interval was 10-ft (rather than 20) in areas
where there was a strong height gradient. For each location along
the sampling transect, a pair of measurements was taken to increase
the sample size. These measurements were taken at each of the three
sites and two times during the season, March 2-3 (Image
15) and March 30, 2001 (Image
16), with the first date prior to the onset of snowmelt.
Soil Moisture
After the completion of snowmelt, gravimetric
soil moisture samples were taken for each site using the same transect
approach. The samples were taken at locations where the thermocouples
were installed, in a line perpendicular to each fence and at each
control site (upwind of the fence). A composite of 4 samples were
taken to determine soil moisture in the top 0 - 18-in of the soil
profile at each sampling location. Once extracted, the soil was
placed in plastic bags and sealed while transported to the laboratory.
Four samples (weighing approximately 25 grams) from each sampling
bag were weighed to obtain a wet weight, dried in an oven at 105°
F for 24 hours and weighed again for a dry weight. With this data,
the moisture content was then calculated.
Crop Yields
Areas adjacent to the snow fence at Lamberton
and Mountain Lake were harvested using yield equipment from the
Southwest Research and Outreach Center. Yields were determined
using a weigh wagon to weigh samples taken from transects parallel
to the fence line at both sites. At Lamberton, five 20-ft wide
and 900-ft long transects were run on the downwind side of the fence.
While at Mountain Lake, three 30-ft wide and 500-ft long transects
were obtained from each side of the fence.
Results:
Snow Deposition
Given the type of land use in the areas of Minnesota
where snow fences are utilized, it is important to determine the
amount of snow, and water, captured at these locations. Two important
parameters to consider for snow deposition are fence porosity and
height. Porosity of living snow fences is difficult to estimate,
as opposed to structural fences, because it is not a stable characteristic
of the vegetation. Living fences are 3-dimensional, affected by
the wind, and change over time as the plants mature. The configuration
at Gaylord, with 8-row strips of standing corn, function as a fence
with 50% porosity (Gullickson 1999, p. 47). Studies using structural
fences show that this porosity allows for the maximum snow storage
capacity, and is therefore the most effective (Gullickson 1999,
p. 34). The porosity of a double shrub row fence is estimated
to be 27.5%, at maturity. Through visual inspection of the Lamberton
fence (twin row honeysuckle), it was determined that the porosity
is much higher, and a value of 70% is more representative (Image
7). For the Mountain Lake fence design, the porosity
of the single row honeysuckle is estimated at 20% (Image
8).
It is important to note that with an increase
or decrease in porosity from 50%, the effectiveness of any fence
for a given height will be reduced. In other words, the fence storage
capacity decreases with an increase or decrease in porosity from
50%. The following empirical equation has been developed from snow
deposition observations for structural snow fences,
Qc = H2.2(3 + 4P
+ 44P2 - 60P3)
[1]
where Qc is the storage capacity
[t/m], H = fence height [m] and P = porosity [decimal].
Using the porosity and height values for each fence, the storage
capacity has been computed at the three field sites. The storage
capacity at Gaylord is found to be the highest, at 38.2 t/m for
each 8-row strip. Again, this is because of its porosity (0.5)
and it consists of two fence rows, with eight rows of corn each.
For the Lamberton fence, the storage capacity is, 30.8 t/m and for
the Mountain Lake fence design the capacity is 41.2 t/m.
In figures
8, 9,
and
10, a profile view of snow depth is illustrated for the
Gaylord, Lamberton and Mountain Lake sites, respectively. The snow
depth measurements were taken March 2 - 3, 2001, which was late
in the snow accumulation season and prior to the onset of snowmelt.
At Gaylord, approximately 300 lineal feet (perpendicular to the
cornrows) were under the influence of the fence with an increased
snowpack. This extended approximately 80-ft upwind of the first
set of rows and roughly 100-ft downwind of the second set. For
Lamberton and Mountain Lake, the lineal distance affected was roughly
120-ft and 150-ft, respectively. For all three fences, the snow
depth profile showed greater deposition on the leeward, or downwind
side of the fence than the windward, or upwind side, meaning a larger
portion of the transported snow was deposited on the downwind side.
Since the snowpack is stored on agricultural
land, an important result of the snowpack is the additional amount
of water that accumulates over the winter. The following table
illustrates differences in the amount of snow captured at each site,
in water equivalence.
|
Location
|
Water from snowpack
|
|
Gaylord
|
10.8 in / acre
|
|
Lamberton
|
10.3 in / acre
|
|
Mountain Lake
|
17.9 in / acre
|
The greatest amount of snow (water) over a unit
area was captured at the Mountain Lake site (Image
19). The Gaylord and Lamberton fences did not capture
as much snow in comparison (Image
17, 18).
It should be noted that these units are indicative of water from
the snowpack over a unit area. Due to the snow fence design, the
snow deposition at the Gaylord site covers a larger area (18.1 acres)
than the Lamberton and Mountain Lake sites (1.5 and 6.9 acres, respectively),
therefore overall, the Gaylord site captured the greatest volume
of water (or snow). This will also be evident when looking at the
observed snow transport for each site, as will be described in the
snow transport section.
Snowpack Density
The snowpack density measurements taken at each
site were compiled into two data sets (divided by date) and compared
to model estimates of snow density for the range of snow depths
observed. The empirical snow densification equation used by Tabler
(1994, p. 57) is represented by the following,
ps = 522 - (304/1.485h)(1
- e-1.485h)
[2]
where Ps = density (kg/m3)
and h = snow depth (m).
As can be seen from Figure
11, the observations of snow density are generally greater
than the model predictions. Both the observed and modeled density
show increasing density with depth, which is attributed to a rearrangement
and compression of snow particles from overburden pressure (Tabler
1994, p. 57). However, the underestimation of the model from our
observations could be attributed to the difference in location of
where the model was developed (Wyoming) versus where the observations
were taken. The arid climate of Wyoming results in lower snow water
equivalent (SWE) values than Minnesota for fresh snowfall, which
can have an impact on snow densification. The measurements taken
on March 30, 2001 show high densities (~ 0.6) at snow depths less
than 1-m (3.28-ft). This value is often the assumed density of
an actively melting snowpack, and by this time in the season, snowmelt
was in progress at each site (Image
16).
Snow Transport
Observed
The snow depth and corresponding density measurements
taken at each field site allowed for computing the total snow transport
over the snow accumulation season. The methodology used for calculating
the observed snow transport followed previous work conducted in
southern Minnesota by Tabler (1997, p. 109). Table
5 illustrates the differences in total snow transport for
the Gaylord, Lamberton and Mountain Lake sites. The snow fence
design at Gaylord (two 8-row strips of standing corn) was the most
effective of the three, capturing the most snow at 30, 30.5 t/m.
As mentioned previously, with a porosity of approximately 50%, studies
have concluded that this allows for maximum snow deposition (Gullickson
1999, p. 34). The design at Lamberton (twin-row honeysuckle) captured
approximately half of the Gaylord total with 16.6 t/m; however,
note that this design was a single fence line rather than two.
Also, the porosity was higher at Lamberton, which has an effect
on snow deposition. Finally, the honeysuckle/red cedar at Mountain
Lake captured slightly more than the Lamberton total with 18.6 t/m.
For this fence, the lower porosity also decreased the efficiency
of the fence.
Comparing observed snow storage with storage
capacity, the Gaylord fence captured 40% of capacity, Lamberton
was at 55% capacity and Mountain Lake was at 45% capacity. It is
important to note that the empirical equation for snow storage capacity
was developed using 50% porous structural snow fences, rather than
living. In addition, the living snow fence designs in this field
study did not have a bottom gap, or a section of fence just above
the surface with 100% porosity. Without the presence of an open
area at the bottom of a snow fence, it will become buried with snow
during the course of a winter season. If this occurs, as was the
case for this study, the snow trapping efficiency will be reduced.
This can also result in damage to a living fence due to heavy snow
loads. Given the snow deposition profile for each fence design (Fig.
8, Fig.
9, Fig.
10), the fences appeared to be near equilibrium and therefore
unable to further capture a significant amount of snow (see Gullickson
et al., p. 36).
Nonetheless, given the 2000-01 winter snowfall
(>90th percentile snowfall), the design of each fence
was more than sufficient to capture the windblown snow. This is
important from the standpoint of the design year and will be discussed
further in the mean seasonal section under snow transport. In each
case, a significant amount of snow was stored upwind of a roadway,
helping to mitigate hazardous blowing snow events and increasing
safety for travelers.
Potential
Previous studies have shown that snow transport
in the first 16-ft above the surface is proportional to the wind
speed raised to a power, assuming a constant snowcover (Tabler 1994,
p. 37). The potential amount of transported snow for a specific
area can be calculated using the following empirically-developed
equation,
Qupot = (ui3.8/233847)(f)(86400)(n)
[3]
where Qupot is the potential
snow transport downwind from an infinite fetch with unlimited snow
cover [kg/m], ui is the midpoint of the 10-m wind
speed [m/s], f is the frequency of observations in the ui
wind speed class over a snow accumulation season with n days.
The potential snow transport (Qupot) was calculated
from wind data for the 86 locations given in Task 2, using only
the days within the snow accumulation season for each location.
The calculations were restricted to hours when the air temperature
was below 32˚ F (using hourly temperature observations) and
to wind speeds greater than 11 kts. Above the freezing point and
at low wind speeds, it has been shown that snow transport is negligible.
One use of the potential snow transport calculation
is to determine the prevailing direction of snow transport, as seen
in Table
6 (1000 kg/m = 1 t/m). Note that the direction of greatest
snow transport is in the range of 270° - 300°
for Gaylord, 300° to 330° for Lamberton, and 300°
to 330° for Mountain Lake. Note that the snow transport for Lamberton
is significantly different from the wind direction frequency distribution
(Figure
5). From the standpoint of snow fence design criteria,
this information is needed for calculation of the attack angle.
Relocation coefficient
A second use of the potential snow transport
equation is for determining the correct relocation coefficient,
which is used in the mean seasonal snow transport equation based
on snowfall according to
Qt = 1500 (Swe)(r)(1
- 0.14F/3000)
[4]
where Qt is the mean seasonal
snow transport [t/m], Swe is the water equivalent
of the total mean (1971 - 2000) snowfall over the accumulation season
[m], r is the relocation factor and F is fetch distance
[m]. Previous studies in Wyoming, U.S. and Siberia have shown the
relocation factor to be no higher than 0.7, meaning that no more
than 70% of the snowfall is relocated over the winter, therefore
this number can be taken as the extreme case.
The relocation coefficient is defined as the
proportion of winter snowfall water equivalent that is relocated
by the wind. To calculate this, a ratio is taken of the potential
transport by wind (Qupot) to the seasonal snow transport
based on an unlimited fetch. The latter can be represented by the
following,
Q = 1500 r (Swe)
[5]
where Q is the mean seasonal snow transport
[t/m], Swe is the water equivalent of the total
mean (1971 - 2000) snowfall over the accumulation season [m] and
r is the relocation coefficient that is solved
for. Through personal communication with Tabler (2001), it is expected
that this value will range from 0.3 to 0.5 for Minnesota, whereas
in the northeast US, the range is 0.2 to 0.3 (Tabler, 1994, p. 44).
From Figure
12, it is evident that there is a geographic variability
to the relocation coefficient. The topography and land-use characteristics
of western Minnesota are such that this area experiences a greater
frequency of higher wind speeds, yielding a higher total potential
snow transport (Qupot), and therefore a higher relocation
coefficient. Conversely, south-central portions and sites in the
forested north have comparatively lower coefficients. Averaging
all locations, the relocation factor is 0.35.
Mean seasonal
With the calculated relocation factor for southern
Minnesota, a better estimate of seasonal snow transport can be calculated
using equation 3. For each field site, the relocation coefficients
of 0.25, 0.50 and 0.63 were used for Gaylord, Lamberton and Mountain
Lake, respectively. The average snow water equivalent for November
through March was obtained using Figure
13 and is 0.09 for Gaylord, Lamberton and Mountain Lake.
Mean snowfall (1971 - 2000) for each site was obtained using Figure
14. To obtain fetch distance, aerial photographs and topographic
maps were used, in addition to visual inspection of each site.
For Gaylord, Lamberton and Mountain Lake, the fetch distances were
found to be 1280m, 640m and 700m, respectively. Using these variables,
the mean seasonal snow transport was calculated to be 14.6 t/m,
16.4 t/m, and 22.3 t/m, respectively. As can be seen, snow transport
calculated using equation 3 yields an underestimate for Gaylord
and Lamberton and a slight overestimate for Mountain Lake in relation
to the observed snow transport (~15.3 t/m per 8-row strip, 16.6
t/m and 18.6 t/m). Given the historical ranking of the 2000 – 01
winter snowfall, this field investigation is more representative
of an extreme case. Using the mean seasonal snowfall in Eq. [4]
for design purposes, rather than the 2000-01 annual total, or a
99th percentile ranking, would be satisfactory for the fences to
function properly.
Soil Temperature
Winter
The substantial snowpack that was sustained
over the snow accumulation season provided effective insulation
for the underlying soil. The frost tube data at Lamberton and the
4-in soil temperatures at Lamberton and Gaylord illustrate the influence
of snowcover on the depth of the freezing layer. For the control
site at Lamberton (100-ft upwind of the fence) the freezing layer
penetrated to a depth of 35-in, which was reached March 15 (Figure
15). The rate of change for this 32° F isotherm
was relatively constant at approximately 4.5 inches per week from
November 15 to March 15. At this time, the freezing depth remained
constant for roughly 2 weeks until it began moving toward the surface
in response to warming temperatures.
Data from all frost tubes showed the same approximate
temperature initially, after installation at the beginning of November.
However, as the snow depth increased at the 4 frost tube locations
in close proximity to the fence, the insulation provided by the
snowpack resulted in a warming of the soil. The frost tube located
20-ft downwind of the fence was under the influence of a relatively
deep snowpack from the end of November through the end of the snow
season. This resulted in soil temperatures beneath the snowpack
that remained above freezing until snowmelt. Similarly, the instrument
located 10-ft from the upwind side showed a slight decrease in the
freezing layer from mid-December (3-in) through March (1-in). Also,
the frost tube 40-ft from the downwind side reported an increase
in the freezing depth from 2-in to 8-in from installation to mid-December.
This was followed by a warming, and a decrease in the freezing depth
to 5-in by the end of March. Again, the snowpack provided sufficient
insulation to impede further frost penetration in the soil profile.
The 4-in soil temperature data also show a dependence
on associated snow cover and respond to the diurnal changes in air
temperature (Figure
16, see Figure
8 for soil temperature locations). At the locations under
a deep snowpack (greater than 3-ft) the 4-in soil temperatures were
in the range of 32° - 34° F, while temperatures
in the absence of a snowpack were in the range of 20°
- 28° F. The diurnal temperature range varied as well,
with insulated locations having a 1° - 2°
F range and unprotected locations in the 2° - 5°
F range. This is most evident in the December - January observations
and by March, the difference in soil temperature between locations
is negligible (Figure
17). By April, the effect of snowmelt is evident in the
soil temperatures in which locations where the temperature sensors
are no longer covered by snow show a marked increase in temperature
and a large diurnal variation (Figure
18). At this point, soil temperature is largely a function
of air temperature, solar radiation and soil water content.
In conclusion, the insulating properties of
the snowpack helped to mediate the underlying soil temperature such
that when snowmelt began, the meltwater was able to infiltrate the
unfrozen soil. In unprotected areas, the soil is frozen much deeper
resulting in meltwater runoff, unable to penetrate the frozen soil.
Summer
A comparison of the daily soil temperature measurements
in summer under the fabric and bare soil at the 2-in depth revealed
a consistently higher maximum temperature under bare soil (Figure
19). This was also the case for the 4-in and 8-in as maximum
temperatures as well (Figs
20, 21).
However, the minimum temperatures under the fabric showed higher
values under the fabric, rather than bare soil. The thermal conductivity
of the fabric could be such that less heat is allowed to escape
from the soil surface. During nighttime conditions in summer, sensible
heat flux is generally away from the surface such that thermal energy
is transferred from the warm soil to the cooler air. However, with
the fabric covering the soil surface, the heat loss is shown to
be less. Deeper in the soil profile, however, both the maximum
and minimum daily soil temperature under the fabric were consistently
higher than under bare soil (Figure
22). This could be attributed to the decreased heat loss
under the fabric, such that more thermal energy is stored in the
soil profile. In summary, despite the presence of the black fabric,
surface temperatures are no higher than temperatures under bare
soil with the same ambient conditions. Therefore, by utilizing
the fabric to establish living snow fence plantings, the survival
will not be hindered by unusually high daily maximum temperature
conditions.
Soil Moisture
For April, the precipitation departure from
normal was 4 to 6-in, which historically ranks 1st for
Lamberton and Mountain Lake and 2nd for Gaylord. Data
collection was delayed due to the abundant rainfall received in
the spring of 2001 (Table
4). Samples were collected April 20 at Lamberton and April
27 at Gaylord and Mountain Lake. The soil moisture measurements
show no significant difference based on proximity to fence (and
over-winter snowpack) (Figs.
23, 24,
25).
The values ranged from 23% to 32% at Gaylord, 25% to 31% at Lamberton
and 33% to 41% at Mountain Lake. From speaking with the agricultural
producers at each site, it was learned that planting was not delayed
due to the deep snowpack and allowing time for snowmelt. Rather,
the abundant rainfall resulted in late planting, both at these locations
and for much of southern Minnesota.
Crop Yields
Samples taken from the south side of the fence
at Lamberton show an increase in yield with increasing distance
from the fence (Figure
26). Of the five samples, the lowest yield was in the first
20-ft south of the fence at 39.5 bu/A. This corresponds to the
area of deepest over-winter snowpack (~ 4.5-ft). The yields from
40- to 100-ft away from the fence showed little variation at 53.6,
53.2 and 54.1 Bu/A. Recall from Figure
9 that the snowpack height decreased rapidly at approximately
40-ft on the south side of the fence, such that snow depths were
1 - 2-ft. In the area directly south of the fence, there appeared
to be foxtail growing in the wheat, resulting in a competition problem.
The wheat may have been planted when the soil conditions were too
wet resulting in an emergence problem, which allowed the foxtail
to develop.
A similar correlation between snow depth and
crop yields was found for the Mountain Lake site. From Figure
27, it is evident that yields closer to the fence, and in
areas sustaining a deep over-winter snowpack, were lower than areas
further from the fence (see Fig.
10 for snow depth). Yield for the 30 - 60-ft transect downwind
of the fence had slightly lower yields than the area directly south
of the fence (the area influenced by the deepest snowpack). However,
this is thought to be due the fact that 2 rows were knocked down
for various lengths due to prior damage from fertilizer equipment.
Also, in past years the land north, or upwind, of the fence, has
yielded higher than the land south, or downwind, of the fence.
These results confirm this observation and coincide with the wheat
yield variation as described previously.
At Gaylord, yield samples were not taken due to several factors.
The two 8-row strips planted in 2000 followed the contour of the
roadway, having an S-shaped curvature. However, for the 2001 growing
season, corn was planted in a north-south straight-line orientation;
therefore only a relatively short distance of the corn rows could
be harvested for comparable yield observations. In addition, much
of the crop experienced lodging due to severe winds just prior to
harvest in early November. Although no quantitative measurements
were taken at this site, the landowner thought yields for the 2001
growing season were down from previous years. This was the case
across the entire field rather than just in the area affected by
increased snow deposition.
In summary, yields in the direct vicinity of
the Lamberton and Mountain Lake living snow fence were hindered,
which appears to be the result of conditions associated with the
over-winter snowpack. However, these results are only indicative
of one season and should be regarded in such a manner, and in the
context of the 2000 - 01 environmental conditions, which is on the
extreme end of the distribution. It should also be noted that previous
studies have shown that living windbreaks alter the microclimate
of the environment in the vicinity of the fence (Brandle and Hintz
1987, Kort 1988, NRCS). Such influences include reduced wind speed,
higher relative humidity and temperature and reductions in soil
moisture that often result in a yield decrease adjacent to the fence.
References:
(1) Brandle, J. and Hintz, D. 1987. An ill wind meets a windbreak.
Science of Food and Agricuture, Volume 5, Number 4.
(2) Kort, J. 1988. Benefits of windbreaks to forage and field crops.
Agricuture Ecosystems and Environment. 22/23: 65-190.
(3) Gullickson, Dan et al., 1999. Catching
the Snow with Living Snow Fences. MN/DOT Office
of Environmental Services and University of Minnesota Extension
Service (MI-7311-S) 140 pp.
(4) Tabler, Ronald D., 1994.
"Design Guidelines for Control of Blowing and Drifting Snow",
SHRP-H-381, Strategic Highway Research Program, National Research
Council, Washington, D.C., 364 pp.
(5) Tabler, Ronald D., 1997. "Recommended
Drift Control Measures for Selected Sites in Southern Minnesota",
Prepared for MN/DOT by Tabler and Associates, Niwot, Colorado, 111
pp.
(6) Windbreaks for Conservation, Natural Resources Conservation
Service Agriculture Information Bulletin 339.
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