Passive wick fluxmeters: Design considerations and field applications

Abstract

Optimization of water use in agriculture and quantification of percolation from landfills and watersheds require reliable estimates of vadose zone water fluxes. Current technology is limited primarily to lysimeters, which directly measure water flux but are expensive and may in some way disrupt flow, causing errors in the measured drainage. We report on design considerations and field tests of an alternative approach, passive wick fluxmeters, which use a control tube to minimize convergent or divergent flow. Design calculations with a quasi-three-dimensional model illustrate how convergence and divergence can be minimized for a range of soil and climatic conditions under steady state and transient fluxes using control tubes of varying heights. There exists a critical recharge rate for a given wick length, where the fluxmeter collection efficiency is 100% regardless of the height of the control tube. Otherwise, convergent or divergent flow will occur, especially when the control tube height is small. While divergence is eliminated in coarse soils using control tubes, it is reduced but not eliminated in finer soils, particularly for fluxes 60% annual precipitation) from gravel surfaces and least (no drainage) from silt loam soils. In Oregon and New Mexico, United States, and in New Zealand, drainage showed substantial spatial variability. The New Mexico tests were located in semiarid canyon bottom terraces, with flash flood prone locations having extremely high drainage/precipitation ratios. In the wettest environments, drainage was found to be closely linked to the rate and duration of precipitation events.

Full text

Passive wick fluxmeters: Design considerations
and field applications
G. W. Gee,
1
B. D. Newman,
2,3
S. R. Green,
4
R. Meissner,
5
H. Rupp,
5
Z. F. Zhang,
1
J. M. Keller,
1,6
W. J. Waugh,
7
M. van der Velde,
8
and J. Salazar
2
Received 11 April 2008; revised 25 December 2008; accepted 2 February 2009; published 22 April 2009.
[1] Optimization of water use in agriculture and quantification of percolation from
landfills and watersheds require reliable estimates of vadose zone water fluxes. Current
technology is limited primarily to lysimeters, which directly measure water flux but are
expensive and may in some way disrupt flow, causing errors in the measured drainage.
We report on design considerations and field tests of an alternative approach, passive wick
fluxmeters, which use a control tube to minimize convergent or divergent flow. Design
calculations with a quasi-three-dimensional model illustrate how convergence and
divergence can be minimized for a range of soil and climatic conditions under steady
state and transient fluxes using control tubes of varying heights. There exists a critical
recharge rate for a given wick length, where the fluxmeter collection efficiency is 100%
regardless of the height of the control tube. Otherwise, convergent or divergent flow will
occur, especially when the control tube height is small. While divergence is eliminated
in coarse soils using control tubes, it is reduced but not eliminated in finer soils,
particularly for fluxes <100 mm/a. Passive wick fluxmeters were tested in soils ranging
from nonvegetated semiarid settings in the United States to grasslands in Germany and
rain-fed crops in New Zealand and the South Pacific. Where side-by-side comparisons of
drainage were made between passive wick fluxmeters and conventional lysimeters in
the United States and Germany, agreement was very good. In semiarid settings, drainage
was found to depend upon precipitation distribution, surface soil, topographic relief, and
the type and amount of vegetation. In Washington State, United States, soil texture
dominated all factors controlling drainage from test landfill covers. As expected, drainage
was greatest (>60% annual precipitation) from gravel surfaces and least (no drainage)
from silt loam soils. In Oregon and New Mexico, United States, and in New Zealand,
drainage showed substantial spatial variability. The New Mexico tests were located in
semiarid canyon bottom terraces, with flash flood prone locations having extremely high
drainage/precipitation ratios. In the wettest environments, drainage was found to be
closely linked to the rate and duration of precipitation events.
Citation: Gee, G. W., B. D. Newman, S. R. Green, R. Meissner, H. Rupp, Z. F. Zhang, J. M. Keller, W. J. Waugh, M. van der Velde,
and J. Salazar (2009), Passive wick fluxmeters: Design considerations and field applications, Water Resour. Res., 45, W04420,
doi:10.1029/2008WR007088.
1. Introduction
[2] It is difficult to measure vadose zone water flow rates
for at least three reasons. First, vadose zone flow rates span
over 4 orders of magnitude, from less than 1 mm/a to more
than 10,000 mm/a; second, spatial distribution of water
fluxes are often highly variable over short distances making
the measurements scale variant and often hard to interpret;
and third, the placement of water flux sensors can disrupt
the flow, causing either convergent or divergent flow with
resultant inaccuracies in water flux estimates. At present,
there is no standard method available for measuring soil
water flux.
[
3] Estimates of water flux are best derived from direct
measurements. The most basic approach is the use of
lysimetry [Allen et al., 1991], where a quantity of drainage
water is captured in a buried container, and in some fashion,
drainage volumes are measured over time. A wide range of
lysimeters has been employed, including pan lysimeters,
equilibrium tension lysimeters and wick lysimeters, each
with their own advantages and disadvantages.
1
Hydrology Group, Energy and Environmental Division, Pacific North-
west National Laboratory, Richland, Washington, USA.
2
Earth and Environmental Sciences Division, Los Alamos National
Laboratory, Los Alamos, New Mexico, USA.
3
Now at Isotope Hydrology Section, International Atomic Energy
Agency, Vienna, Austria.
4
Sustainable Land Use, HortResearch, Palmerston North, New Zealand.
5
Department of Soil Physics, Helmholtz Centre for Environmental
Research, Falkenberg, Germany.
6
Now at GeoSystems Analysis, Inc., Hood River, Oregon, USA.
7
S. M. Stoller Corporation, Grand Junction, Colorado, USA.
8
Joint Research Center of the European Commission Institute for
Environment and Sustainability, Ispra, Italy.
Copyright 2009 by the American Geophysical Union.
0043-1397/09/2008WR007088$09.00
W04420
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[4] Pan lysimeters collect only free drainage water and
are sometimes called ‘‘zero tension’’ lysimeters, while
tension lysimeters and wick lysimeters collect water under
tension, either through a vacuum control system or applied
via a hanging water column (i.e., wick). Pan lysimeters
[Rich ards, 1950; Jemison and Fox, 1992; Chiu and
Shackelford, 2000; Zhu et al., 2002] are the least expensive
system and have been used extensively over the years to
assess drainage water quality and to a lesser extent to
estimate water flux. Their major drawback is divergence,
due to the fact that unless the soil is very coarse, or the pan
is very large and the flow correspondingly high, flow
around the pan can be significant, resulting in large under-
estimates of flow.
[
5] The ‘‘equilibrium tension’’ lysimeter was originally
conceived over 72 years ago by Ivie and Richards [1937],
further developed by others [ Cary, 1968; Dirksen, 1974;
van Grinsven et al., 1988, Brye et al., 1999, 2001; Barzegar
et al., 2004; Kosugi and Katsuyama, 2004] and recently
automated by Masarik et al. [2004]. The basic feature of the
equilibrium tension lysimeter is the collection of drainage
water from a buried pressure plate or membrane using
vacuum, controlled at soil water tensions close to those
found in the surrounding soil. This method is likely the
most accurate water flux method but it is also the most
expensive and most difficult to operate and maintain.
Concerns about the reliability of tension control when near
saturation [Gee, 2005; Morari, 2006] and possible effects of
plate resistance on measured fluxes [Kasteel et al., 2007] as
well as interactions due to meter placement in the soil
profile [Mertens et al., 2005] add uncertainties to flux
measurements when using the equilibrium tension lysimeter
method.
[
6] As a water flux meter, the passive wick lysimeter is a
compromise between the complications and expense of
equilibrium tension lysimeters and the simplicity of the less
accurate pan lysimeter. Passive wick lysimeters maintain
tension on the soil using an inert wicking material, such as
fiberglass [Holder et al. , 1991] or rock wool [Ben-Gal and
Shani, 2002]. A hanging water column is created , and
drainage water is pulled out of the lysimeter while the lower
soil boundary is ‘‘passively’’ maintained at a pressure less
than atmospheric, so that the soil at the bottom of the
lysimeter stays u nsaturated. The deg ree of unsaturation
depends upon the length of the wick and its hydraulic
properties, the water flux, and the soil type [Holder et al.,
1991; Boll et al., 1992; Knutson and Selker, 1994; Rimmer
et al., 1995; Zhu et a l., 2002]. For typical wick-type
fluxmeters, the wick material is highly conductive and the
wick area sufficiently large that flow in and through the
lysimeter is not restricted. No external controller is used to
maintain pressure; rather the lysimeter relies on the nearly
static pressure created by the hanging water column (wick).
The pressure in a wick lysimeter is always less (more
negative) than that found in a pan lysimeter. Where direct
comparisons have been made, passive wick lysimeters have
generally outperformed pan lysimeters in their ability to
capture drainage water [Zhu et al., 2002] though not always
[Boll et al., 1997]. In extensive field testing, collection
efficiency (CE) has been shown to equal or exceed 100%
for passive wick lysimeters [Louie et al., 2000] while
average CE values for pan lysimeters were found to be less
than half that amount [Zhu et al., 2002]. Adaptation of the
passive wick lysimeter concept has been made by Gee et al.
[2002, 2003] who designed and laboratory tested what we
call here a passive wick fluxmeter. In this design, drainage
flux is measured automatically with a tipping spoon that
collects water draining from a fiberglass wick. The wick
‘‘passively’’ controls the pressure head in the soil at a value
that ca n approach the length of the wick. A soil -filled
control tube is placed directly above the wick to minimize
divergent or convergent flow. The major disadvantage of the
passive wick fluxmeter is the lack of precision control of the
tension, relying on the wick and control tube to approximate
the range of tensions which dominate when drainage occurs,
thus limiting the wick units generally to coarser-textured
soils. The adva ntage of the passive wick unit over the
vacuum controlled lysimeter is robustness and reliability
for long-term measurements.
[
7] In this paper, we investigate the use of passive wick
fluxmeters as drainage monitors for a number of soil types
and flux conditions. Specifically, we evaluate their perfor-
mance for sand, loamy sand, structured clay, and silt loam
soils, for fluxes from 1 to 10,000 mm/a, through modeling
and then evaluate their field performance over a range of
soils and climatic conditions. This paper reports a collection
of studies, each with specific objectives, as described later.
The overall objective was to assess the use of passive wick
fluxmeters as reliable drainage monitors. The field study
discussions highlight the broad range of conditions where
fluxmeters can be applied.
2. Materials and Methods
2.1. Fluxmeter Design
[
8] An objective in our design has been to make the
passive wick fluxmeter robust enough to operate for mul-
tiple years without repair, recalibration or replacement. In
the following, we describe passive wick fluxmeters that
were specifically designed for long-term (multiple-year) use
and at the s ame time were relatively easy to construct and
install.
[
9] The passive wick fluxmeter monitors drainage from a
soil-filled funnel. The soil captures flow from a predeter-
mined area where it drains into the funnel neck occupied by
a conductive material capable of applying a capillary
pressure to the overlying soil. Water flux is measured
directly by placing a water monitoring device (e.g., minia-
ture tipping bucket or recording autosiphon ) below the
lower end of the wick (Figure 1).
[
10] The funnel neck is 2.5 cm in diameter and is filled
with a fiberglass wick material that creates a hanging water
column. In some of our tests (Figure 2, right), we used two
intertwined fiberglass ropes (Pepperell Braiding Company,
Pepperell, M assachusetts), each having a diameter of
12.7 mm. In other tests (Figure 2, middle), we used larger
diameter (2.5 cm) wick material (Amatex, Norristown,
Pennsylvania). The ropes were kiln dried at 400°Cfor3h
to remove glue and other organic materials, as recommended
by Knutson et al. [1993]. The top 15 cm of the wick
material was separated into single strands, which wer e used
to line the interior of the funnel. To prevent soil from
filtering through the funnel and the wick, a thin layer of
diatomaceous earth was placed in the bottom of the funnel
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W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS W04420

above the wick. Diatomaceous e arth material is highly
conductive and does not restrict flow within the fluxmeter.
The wick extended vertically below the soil-filled funnel.
Passive wick fluxmeters incorporate a method to control
convergence or divergence of flow. The control tube con-
sists of a plastic or galvanized pipe, about the same diameter
as the top end of the funnel and extends from the funnel top
up to a height of 60 cm. For the tests reported here, the
inside diameter of the control tube ranged from 20 to 21 cm
with corresponding surface area ranging from 314 cm
2
and
340 cm
2
. Note that larger dimensions for wick and control
tube can be accommodated in the design and have been
used by others [Jabro et al., 2008]. The K
sat
of the wick
is extre mely high and under normal flow conditions
(<10,000 mm/a) offers little resistance to the overall flow
in the water fluxmeter.
[
11] Water was collected from the wick in two ways. The
first used a miniature rain gauge (Rain-O-Matic, Pronamic
Co. Ltd., Silkeborg, Denmark) that consists of a reed switch
and a small plastic spoon to which a magnet is attached
(Figure 2, right). The tipping spoon is positioned in a
10.2 cm diameter. PVC plastic tube designed to isolate the
wick from the surrounding soil. As the spoon fills and
Figure 1. Schematic of a passive wick fluxmeter [after
Gee et al., 2002].
Figure 2. Schematic of three water fluxmeters tested in the same field in Tongatapu, Tonga. Shown from
left to right are a pan-type (Z-WFM) fluxmeter, passive wick fluxmeter with-capacitance sensor (C-WFM),
and passive wick fluxmeter with tipping bucket (T-WFM) [after van der Velde et al., 2005]. Cups shown at
the bottom of each fluxmeter represent the collection zones for water samples. The Gee wick unit uses a
tipping spoon, while the Decagon unit uses an autosiphon and capacitance probe detector.
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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empties, the magnet moves past the reed switch, causing an
electrical pulse to be counted on an event recorder. Because
the tipping spoon is enclosed, there is no evaporation from
it, and even when the soil drains and dries, the humidity
near the tipping spoon typically remains at 100%. All
exposed components of the buried gauge are potted and
sealed so that they do not corrode in the high humidity [Gee
et al., 2002]. A number of these tipping spoon units have
been in the ground and operationa l now for over 6 years.
For our fluxmeter design, a water flow rate of 0.6 mL/min
(about 8 min per tip) was near the upper range of interest
(i.e., 10,000 mm/a). The lower range of interest is less
than 1 mm/a, which is achievable because the resolution of
one tip is equivalent to 0.15 mm water. The second
collection method (Figure 2, middle) uses an ECHO-type
capacitance probe (Decagon Devices, Pullman, Washing-
ton) in a manner similar to that reported by Masarik et al.
[2004] with the following modifications. The capacitance
probe is placed in the center of a water reservoir (60 mL
capacity) and as the water fills the reservoir, corresponding
electrical capacitance changes are recorded. As the capacity
of the water collection chamber is approached, an autosi-
phon discharges the reservoir (40 mL), and the process is
repeated. Data loggers can be programmed to capture either
the dis charge or the stage as indicted by the changing
electrical capacitance reading of the probe [van der Velde
et al., 2005]. Passive wick fluxmeters also have two other
useful design features. The first feature is a sample tube that
extends from the reservoir at the bottom of the meter to
above ground. By inserting a syringe at the end of the
sample tube, water samples can be extracted for analysis of
leachate chemistry (Figure 1). The second feature is that a
similar tube can be used to check the water measurement
device calibration in situ (Figure 1). This verification is
accomplished by injecting a known volume of water into the
tube which then flows directly to the measurement system
(not through the wick). One can then compare the amount of
water injected with the amount measured.
2.2. Numerical Simulations
[
12] Because of the mismatch that can occur between the
soil water pressures inside and outside of the fluxmeter,
convergent or divergent flow can occur but may be mini-
mized by properly selecting the height of the control tube or
adjusting the length of the wick. There are obvious limi-
tations in changing these parameters and in preliminary tests
we found that a 60-wick length and 60-cm control tube
(Figures 1 and 2) appeared to work reasonably well for
relatively coarse soil conditions (e.g., gravels, sands, etc.) of
interest in agricultural and waste management applications.
We did observe convergent flow in a well-aggregated clay
soil under tropical rainfall conditions [van der Velde et al.,
2005] and were curious to know what the optimal design
might be for this somewhat extreme situation. We were also
interested in knowing under what conditions passive wick
fluxmeters will work in finer soils for a range of water
fluxes. To assist in this assessment, we ran a series of
numerical simulations for a series of soils and fluxmeter
dimensions.
[
13] Flow was simulated using the Subsurface Transport
Over Multiple Phases (STOMP) simulator [W hite and
Oostrom, 2006], which is designed to solve a variety of
nonlinear, multiple-phase, multidimensional flow and trans-
port problems for unsaturated porous media. A cylindrical
coordinate system was used, and only one vertical slice of
the cylinder was used in the simulations. Because the flow
through a water flux meter is axisymmetric, the simulation
was equivalent to a three-dimensional simulation. The
simulation domain was subdivided into a grid with variable
spacing steps (Dx and Dz). The minimum value of Dx was
1 mm, which was at two locations where the bottom of the
funnel and the wall of the fluxmeter reside. The minimum
value of Dz was 2 mm, which was where the funnel was
located. The modeling domain was 1 m horizontally and 2 m
vertically and was discretized into 104  128 nodes
(Figure 3).
[
14] Both steady state and transient simulations were
carried out. For the steady state simulations, the upper
boundary conditions were set as a constant flux of 1, 10,
100, 1000, and 10,000 mm/a. The lower boundary outside
the fluxmeter was set as a unit gradient condition and inside
the fluxmeter, at the bottom of the fluxmeter, was set as a
constant head of 60 cm or 0 cm for the case without a
wick. The control tube length was varied in the modeling
from 0 to 100 cm. The wall of the fluxmeter was treated as
being impermeable. The differences between steady state
and transient simulations were that for the transient cases,
the upper boundary conditions were set as variable flux.
Simulations were carried out for soils with four different
textures: sand, loamy sand, structured clay, and silt loam
soil. The hydraulic parameters are summarized in Table 1.
2.3. Field Tests
[
15] As part of an informal collaborat ive program to
evaluate methods for water flux ( drainage) monitoring,
passive wick fluxmeters were tested over the past 6 years
at a number of sites throughout the world. We report results
from eight sites, each with differing soils and climate. These
include semiarid sites near Richland, Washington; Lake-
view, Oregon; and Los Alamos, New Mexico, United
States, and at semihumid and humid sites, i ncluding a
grassland site in Germany, a squash plantation on the island
of Tongatapu, in Tonga (South Sea Islands), and a potato
field and a pasture site in New Zealand. At two locations
(Richland and Germany), direct comparisons were made
between drainage from large lysimeters and fluxmeters. At
two humid sites (Tonga and New Zealand), drainage was
estimated from water balance considerations. At the remain-
ing two sites (Lakeview and Los Alamos) we looked at the
utility of the fluxmeters to quantify subsurface flow and
drainage using multiple fluxmeters, placed in locations
where flows were expected to be highly variable. Additional
details for each site are provided in the section 3.
3. Results and Discussion
3.1. Modeling
[
16] For the flux analysis, we calculated the flux recovery
or collection efficiency (CE = J
m
/J
a
), expressed as a per-
centage of the measured flux, J
m,
to the actual flux, J
a
,
where the actual flux is the applied water flux incident on
the meter. Values of CE > 100% indicate convergence while
values less than 100% indicate divergence. Using the
STOMP simulator and the soil characteristics for the four
soils tested, we calculated the CE for passive wick flux-
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W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS W04420

meters that have varying control tube heights (from zero to
100 cm). The calculated CEs as a function of control tube
height are shown in Figure 4 for the four soil types. There
exists a critical recharge rate, q
c
, under which the soil water
suction equals the length of the wick (60 cm for our cases).
Particular q
c
values were 0.08, 935, 149, and 6644 mm/a for
the sand, loamy sand, structured clay, and silt loam, respec-
tively. If q = q
c
, the CE was 100% regardless of height of
the control tube. Otherwise, convergent (for q > q
c
)or
divergent (for q < q
c
) flow was predicted, especially when
the control tube height was too small. The use of a control
tube will reduce the magnitude of both convergence and
divergence errors. In all case s, the collection efficiency
approaches 100% when the control tube height becomes
very large. For a given soil with an annual average suction
h (under a certain recharge rate, q), the sum of wick length,
L
w
, and the control tube height, L
c
, must be much larger
than h, i.e., L
w
+L
c
 h. Otherwise, no or very little
drainage will occur. For example, for the loamy sand soil
under a 100 mm/a recharge condition, the corresponding
soil suction, h = 101 cm. Hence, L
w
+L
c
 101 cm so that
significant amount of drainage can occur. Unfortunately,
there does not exist a critical value of L
w
+L
c
such that the
collect ion efficiency will be 100%. On the basis of the
simulation results when using a 60-cm-long wick and under
a 100 mm/a recharge condition, extending the control tube
height to 80 cm (L
w
+L
c
 h = 40 cm) results in an CE value
of 80%. For many applications agreement between mea-
sured and actual drainage within 20% may be satisfactory.
[
17] For coarse soil (e.g., sand, Figure 4a) passive wick
fluxmeters with 60-cm-long wicks operate satisfactorily
over the range from 1 to 10,000 mm/a. As soils become
finer (Figures 4b and 4c), passive wick fluxmeters become
less effective at low fluxes, but begin to perform satisfac-
torily at higher fluxes (>100 mm/a). In the silt loam soil
(Figure 4d), divergence is still evident at fluxes >1000 mm/a.
The impact of soil aggregation or macropores is evident in
Figure 4c (structured clay), where collection efficiencies are
higher than those found in loamy sand (Figure 4b) for
devices having similar control tube heights. This result
illustrates how both soil texture and structure help determine
the range over which the fluxmeter is most effective. Where
abundant macropores are present in soils, it is expected that
CEs will be higher than where macropores are less abundant
or absent.
[
18] Transient inputs had less effect on flux efficiencies of
passive wick fluxmeters than expected. We simulated tran-
sient inputs on the structured clay soil (Table 2). Cases were
Table 1. Soil Hydraulic Properties
a
Soil K
s
(m s
1
) a (m
1
)Nq
s
(m
3
m
3
) q
r
(m
3
m
3
)
Sand 2.92  10
4
8.05 4.81 0.31 0.093
Loamy sand 2.00  10
5
5.00 2.00 0.44 0.03
Structured clay 1.97  10
6
5.00 1.49 0.59 0.49
Silt loam 1.00  10
5
1.78 1.34 0.50 0.00
a
Properties are for van Genuchten [1980] – type parameterization. Data
for the sand, loamy sand, and silt loam soils taken from properties used for
Hanford soils [Gee et al., 2004]. Structure clay soil properties are for
structured clay soil at the Tonga site [van der Velde et al., 2005].
Figure 3. (a) Grid for STOMP simulations of passive wick fluxmeter with a 60-cm wick and a 60-cm
control tube height. (b) Blowup of Figure 3a. The model assumes that the soil inside the fluxmeter is the
same as the soil outside the fluxmeter.
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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W04420

run for fluxmeters with 60-cm-long wicks and with control
tube heights of 0, 20, and 60 cm. Case TR1 applied water as
follows: 12 events over 60 days, with rain occurring on
every fifth day (i.e., on every fifth day it rained 17.5 mm,
then no rain until the next fifth day). The rain was spread
evenly over each rain day. Case TR2 applied four moderate
rain events (22.5 mm/d every 12th day, i.e., on days 12, 24,
48 and 60), plus an extreme, 120 mm/d event on day 36.
Differences in collection efficiencies between steady state
and transient cases for the passive wick fluxmeter are
relatively small, with differences varying from 5 to 15%
over all test cases (Table 2). The effect of control tube
height on drainage was far more dramatic. Without a control
tube (H = 0), the CE value of the fluxmeter was excessive,
with convergence causing as much as a threefold (300%)
error in the measured drainage. In contrast, with a 60-cm
control tube, the CE value was just over 100%, resulting in
an error of less than 10% in the measured drainage for both
the steady state and transient cases. In effect, the control
tube offsets the impact of excessive wick length, particu-
larly under conditions of relatively high water influxes (e.g.,
1278 mm/a). This is an important design consideration and
suggests that a passive wick fluxmeter with a sufficiently
long control tube behaves much like an ‘‘infinitely long
column,’’ making the wick length less important in affecting
the pressure at the top of the fluxmeter, thus minimizing
convergence or divergence. In our case, for the modeled soil
(structured clay) and climate (210 mm infiltration in
60 days), the 60-cm-long control tube is not quite long
enough to prevent some convergence. However, in terms of
practical design it appears to be well within a reasonable
error limit (<10%), and most of the field applications
described later used a 60-c m-long control tube. These
model results are comparable to those of Gooijer [2007],
who simulated drainage from five soils types ranging from
coarse sands to loams and reported that under steady state
conditions coarse sands showed much less divergence than
loam-textured soils. Our modeling results also agree with
observations from laboratory tests, where fine soils show
appreciable divergence and low collection efficiencies
[Kohl and Carlson, 1997; Gee et al., 2002]. The modeling
conducted to date is not exhaustive. It is limited to just a few
soil types and only one wick type with one fluxmeter area
(340 cm
2
) with varying control tube heights. However, the
modeling does illustrate the general performance of this
kind of passive wick fluxmeter and how divergence and
Figure 4. Simulated collection efficiency as a percentage (J
m
/J
a
= 100X measured/actual) for selected
soils under a variety of steady flux conditions and control tube heights. These results are for passive wick
fluxmeters with 60-cm-long wicks. When flux rates were 0.08, 935, 149, and 6644 mm/a for the sand,
loamy sand, structured clay, and silt loam, the collection efficiency was 100% regardless of the control
tube height.
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W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS W04420

convergence can be minimized. With further effort, nomo-
grams of expected collection efficiencies could be devel-
oped for all combinations of soils and drainage rates of
interest. Such an approach could lead to custom designing
fluxmeters for site specific soil and climatic conditions.
However, practical limitations of device size (length, area,
etc.) and cost will most likely dictate an optimal design for a
specific site. In future modeling, a more accurate method
will require using actual dimensions and hydraulic proper-
ties of the wick and not assume a control of 60 cm at the
bottom soil boundary. The present modeling assumes an
ideal wick with no flow restrictions, hence the divergence
(and convergence) we have estimated lies somew here
between that for an ideal wick and that expected for a pan
lysimeter.
[
19] Mertens et al. [2007] have used modeling to opti-
mize the design of conventional passive wick samplers
[e.g., Brandi-Dohrn et al., 1996] for year-round assessment
of drainage and chemical fluxes. Mertens et al. [2007]
concluded that a wick with optimal hydraul ic properties,
but with a variable length, would minimize drainage errors
by better matching the expected soil water pressure con-
ditions, which vary with season at a proposed test site.
However, their model did not include any assessment of
control tubes to minimize the effect of seasonal pressure
changes on drainage rates. Although aware of control tube
technology, Morari [2006] and Mertens et al. [2007]
expressed concerns that there might be preferential flow
along the walls of the control tube which would compro-
mise drainage estimates. While this may be a concern under
saturated flow conditions, preferential flow along the walls
of the control tube is normally restricted, if not eliminated,
when the soil remains unsaturated and the top of the control
tube is below ground surface . In our simulations, we
assumed unsaturated flow occurred i n all cases, so the
impact of wall flow was not modeled explicitly.
[
20] Another problem that can occur under ponded con-
ditions is that they can give rise to perched water tables,
particularly where subsoils are much less permeable than
surface soils and rapid drainage is impeded. In fact, the
issue of elevated water tables (ponding) in early spring is
common at some locations and must be dealt with if year-
round operation of fluxmeters is desired. Preferred sites for
fluxmeter placement are those with permanently deep water
tables (i.e., water tables that never rise above the bottom of
the lysimeter or fluxmeter). In our design, this suggests that
water tables should remain at least 1.2 m below ground
surface. Other issues, such a s soil disturbance during
fluxmeter placement (leading to modification of hydraulic
properties because of changes in texture, bulk density, layer
sequences, etc.) and changes in natural preferential flow
paths, including the impacts of biotic intrusion, such as
roots, worms, ants, small animals, etc., should be recog-
nized and addressed for each site. For cropped soils, surface
land disturbance is a frequent practice, so fluxmeter place-
ment via auger hole (or similar access) using disturbed soil
in the control tube should not seriously compromise the
fluxmeter performance, partic ularly f or coarser-textured
soils in agricultural settings. Similarly, fluxmeters with
disturbed soil columns placed in engineered soils of landfill
covers should also be acceptable because the land surface
has already been highly disturbed. In some cases, flux-
meters may be built directly into a landfill cover (i.e., placed
during construction of the landfill cover). Some complica-
tions may be encountered in areas with minimally disturbed
landscapes and watersheds, but these are areas where
localized drainage rates are generally of less interest than
for agricultural lands or waste disposal sites. Mertens et al.
[2005, 2007] and Morari [2006] have used modeling to
show that placement and spacing of fluxmeters in the soil
can give rise to measurement errors. Ensuring year-round
performance of fluxmeters can present some challenges. For
one thing, the pressure in the soil water typically changes
with season. For nonirrigated or deficit irrigated cropland,
summer typically produces lower (more negative) soil
pressures and the fluxes are correspondingly low, making
only a minor contribution to the annual drainage. It is when
the soil wets up in the spring that it is most important for the
fluxmeter to perform accurately to capture the bulk of the
‘‘first-drainage’’ events. For a silt loam soil, Morari [2006]
has shown that errors of more than 30% occurred under
high draina ge c onditions, when the pressures were not
properly matched inside and outside the lysimeter.
[
21] While optimal designs of passive wick fluxmeters
are not finalized, the simul ations reported here provide
general guidance for using our fluxmeter design (e.g.,
60-cm wick length, 60-cm control tube height, with a 300 to
340 cm
2
cross-sectional area). The modeling suggests that if
soils are coarse textured (i.e., gravels, sands, structured clay,
etc.), then our design should be a suitable tool for measuring
drainage over the entire range of climatic conditions, from
arid to humid sites. For finer soils, drainage monitoring with
the passive wick fluxmeter is more restrictive and will most
likely work best under irrigated conditions where large
(>100 mm/a) drainage rates are anticipated. With this
guidance in mind, we next provide field results that show
just how well passive wick fluxmeters have performed in
time over a range of soil and climate conditions.
3.2. Field Results
[
22] The field study discussions below provide compar-
isons between fluxmeter results and other measures of
Table 2. Simulated Water Flow Inside and Outside a Passive Wick
Fluxmeter and the Resultant Collection Efficiencies for a 60-cm
Wick and Control Tube Heights
a
Case H = 0 cm H = 20 cm H = 60 cm
Cumulative Flux Outside of Fluxmeter (mm)
SS 3.5 mm/d 210.1 210.1 210.1
TR-1, 12 events 210.5 210.5 210.5
TR-2, 5 events 209.8 209.8 209.8
Cumulative Flux Inside of Fluxmeter (mm)
SS 3.5 mm/d 633.4 247.5 214.2
TR-1, 12 events 659.8 258.2 224.3
TR-2, 5 events 563.9 239.4 216.1
Collection Efficiency (%)
SS 3.5 mm/d 301.5 117.8 101.9
TR-1, 12 events 313.4 122.7 106.5
TR-2, 5 events 268.8 114.1 103.0
a
Control heights, H, are 0, 20, and 60 cm. Results are for steady state (SS =
3.5 mm/d) and for two transient cases (TR-1, 12 events and TR-2, 5 events)
run for 60 days with a total of 210 mm applied for all cases.
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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drainage that help demonstrate the performance of flux-
meters under a variety of conditions. In addition, they
describe the kinds of problems that can be investigated with
fluxmeters, and some of the important issues that need to be
considered during experimental d esign. A summary of
passive wick fluxmeter data for the eight field sites with a
range of soils and climate is provided in Table 3.
3.2.1. Measurement Comparisons: Passive Wick
Fluxmeters Versus Lysimeters or Water Balance
Estimates
3.2.1.1. Washington State Site
[
23] The objective of this study was to compare drainage
measurements from passive wick fluxmeters with existing
deep drainage lysimetry. This site is located in south central
Washington State on the U.S. Department of Energy’s
Hanford Site, which has a semiarid climate (cool wet
winters, hot dry summers) with an average annual precip-
itation of about 180 mm/a. Drainage data were collected
from passive wick fluxmeters for multiple years at three
locations at this sit e. At location 1, two passi ve wick
fluxmeters of standard design (60 cm wick, 60 cm control
tube) were installed in a 7.6-m-deep drainage lysimeter
[Gee et al., 2005] and monitored for a period of 4.4 years.
During this time the coarse sand surface of the lysimeter
was kept free of vegetation. The drainage response of the
fluxmeters was largely in response to winter rain and
snowmelt as nearly half (348 mm) of the total (786 mm)
precipitation drained (Figure 5). At location 2, a fluxmeter
was installed in barren gravelly sand adjacent to a 1.5 m
deep lysimeter containing the same gravelly sand material
and was monitored for 4.1 years. At location 3, a fluxmeter
was installed in barren silt loam adjacent to a 1.5 m deep
lysimeter with similar silt loam texture [Fayer and Gee,
2006] and was monitored for 4.2 years. In all three cases,
the fluxmeters and lysimeters were kept vegetation free
during the test period. Drainage from water fluxmeters were
compared between shallow placement of water fluxmeters
and deep drainage from lysimeters with corresponding
treatments. Annual drainage rates for the two paired flux-
meters at location 1 were comparable, and agreed to within
15% of t he drainage observ ed from the deep (7.6 m)
lysimeter in which the fluxmeters were placed (Figure 5).
Water pressure heads were found to range from 45 to
60 cm (as measured with tensiometers) throughout the
drainage period. This was comparable to the length of the
wicks in the fluxmeters. At locations 2 and 3 at Hanford,
under identical climate, the drainage rates varied according
to surface texture. As expected, coarse gravel yielded the
largest drainage and silt loam yielded the least drainage
(Table 3 and Figure 6). Coarse gravels drained more than
half the annual precipitation, which typically resulted from
winter rain and snow. In contrast, silt loam soil stored a
larger fraction of the winter precipitation and subsequently
reduced the w inter drainage. It s is recognized that the
nondraining conditions observed in both the fluxmeter and
lysimeter at Hanford are likely to be biased low because of
the zero-tension lower boundary condition in each device.
These data better represent the performance of a capillary
barrier (with silt loam soil over coarse sand or gravel, where
the silt loam is 1.5 m deep for the silt loam soil and effectively
1.2 m deep for the fluxmeter). The data confirmed that
drainage events are linked closely to surface soil texture
and that passive wick fluxmeters can successfully measure
multiyear drainage rates ranging from zero to >100 mm/a, in
close agreement with values obtained from large lysimeters at
the Hanford Site.
3.2.1.2. German Site
[
24] The objective of this study was to compare passive
wick lysimetry with data collected from large weighing
lysimeters. This study site is located at a lysimeter complex
in a small catchment area approximately 10 km south of
Falkenberg, Germany. The soil has a sand texture and drains
readily. Two passive wick fluxmeters of standard design
were installed adjacent to a large weighing lysimeter (1.13 m
diameter and 2 m deep [Meissner et al., 2007]). Drainage
data were collected for 26 months (2.2 years) and the results
of the fluxmeters were compared to the lysimeter drainage
for the same period (Table 3 and Figure 7). Precipitation
during the test period was 1240 mm while drainage from
this grass covered sand was about 250 mm. Agreement
between the lysimeter and passive wick fluxmeters was
within 5% (240 mm versus 250 mm). In addition to
providing accurate measures of drainage, fluxmeters with
long-term, maintenance-free performance are preferred over
those that must be serviced frequently. Both at the Hanford
and German sites, the passive wick units were easy to
Table 3. Drainage Data Related to Surface Soil and Climate Obtained From Passive Wick Lysimeters From Six
Locations
a
Location
Surface
Soil
Precipitation,
mm
Water Fluxmeter,
mm
Actual Drainage,
mm
Hanford site location 1 sand 420 270 275
Hanford site location 2 gravel 450 180 190
Hanford site location 3 silt loam 450 0 0
Germany (grass meadow) sand 1280 250 240
Lakeview, Oregon,
United States
silt loam 410 600 – 996 NA
Los Alamos, New Mexico,
United States
loam/sand 914 0 – 4148 NA
New Zealand (potato crop) gritty silt loam 1150 742 700
b
New Zealand (pasture) silt loam 1265 384 359
b
Tonga (squash plantation) clay (aggregate) 350 540 250
b
a
New Zealand and Tonga Site drainage data estimated by water balance; all other drainage data from large lysimeters. The
Hanford site is in Washington, United States. NA means not available.
b
Water balance estimate.
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maintain and required little or no service during their
operation.
3.2.1.3. Tonga Site
[
25] The objective of this study was to compare drainage
results from three different passive wick fluxmeters with
drainage estimated from site water balance considerations.
This study was conducted at a squash plantation on the
island of Tongatapu in Tonga and is described in detail by
van der Velde et al. [2005]. The soil is a structured, oxidized
clay (volcanic ash) and is well drained. Three sets of
fluxmeters (a total of six units) were installed at this site.
Two sets were passive wick type, essentially the same
Figure 5. Hanford site location 1. Cumulative drainage from a bare sand surface by paired passive wick
fluxmeters (WFM-1 and WFM-2) compared to measured precipitation (mm). Drainage occurs typically
in winter months in response to rainfall and snowmelt events when evaporative demand is very
low. Agreement between fluxmeter results for the 4.4-year test period is excellent. Fluxmeter drainage
(360 mm) is nearly half of the cumulative precipitation and within 15% of that measured independently
by the 7.6-m-deep lysimeter.
Figure 6. Drainage records from conventional lysimeters compared to passive wick fluxmeters for two
soil types (gravelly sand and silt loam) at a semiarid site in Richland, Washington (United States).
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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dimensions (60-cm control tube and 60-cm-long wicks) and
one fluxmeter set was wickless, but with a 20-cm-long
control tube (Figure 2). These units were installed in the soil
in July 2003, immediately after the field had been ploughed
and prepared for planting. Data were collected for a period
of 60 days, during which rainfall totaled 340 mm. The site is
relatively flat and no runoff was observed during the test. A
simple water balance (i.e., drainage equals rainfall minus
evapotranspiration) as described by van der Velde et al.
[2006]wasusedtoestimatedrainagetobe217mm.
Drainage measure d from the passive wick fluxmeters aver-
aged 482 mm, yielding a collection efficiency of 223%. In
comparison, the pair of wickless fluxmeters, each with a
20-cm-high control tube, collected an average of 240 mm
drainage, yielding a collection efficiency of 110%. The
high (>100%) collection efficiency from all fluxmeters is
attributed to convergent flow. While the wickless units
yielded drainage estimates close to that of the simple water
balance, the passive wick units produced excessive drainage
(collection efficiencies >200%). Soil water contents were
measured during the tests but there were no direct measure-
ments of soil water pressures or other hydraulic parameters
(e.g., infiltration or field estimates of saturated hydraulic
conductivity). The pressure conditions estimated by modeling
with laboratory-determined hydraulic properties indicated that
soil water pressure heads in the field could have been in the
range of 10 to 20 cm (pressure head) during drainage
events. It also should be pointed out that field-measured
water contents were lower than those modeled by van der
Velde et al. [2005], which suggests that actual field pressure
heads could have been lower (more negative ) than estimated
from t he simulations. This also implies that the actual
drainage losses from the field may have been lower than
estimated by the simple water balance calculations. However,
the fact that drainage exceeded precipitation is compelling
evidence that the passive wick units collected convergent
flow during the 60-day test on this structured clay soil.
[
26] Using STOMP, we simulated drainage from struc-
tured clay (Table 1) and found that for high drainage
conditions (rates >1000 mm/a), collection efficiency should
be 100% for a fluxmeter with a 20-cm-high control tube
and no wick, in agreement with what was found in the field.
Additional simulations also showed that for this soil, under
high drainage conditions, wicks up to 20 cm in length with
less than 20 cm high control tubes would work to minimize
convergence errors. So it appears that the wick length of
60 cm used on our field test was not well matched to the soil
and rainfall conditions. With hindsight, this mismatch might
have been avoided if field measurements of the soil’s
hydraulic properties, and climate data had been used to
premodel the site to determine the optimum device design
(as suggested by Mertens et al. [2007]).
[
27] It should be noted that our design model (Figure 2)
using soil properties similar to those provided by van der
Velde et al. [2005] showed that convergent flow could occur
with passive wick fluxmeters, but not to the degree observed
in the field. This could be the result of imperfect matching
of the soil characteristics used in the model with actual field
conditions. There is the possibility of localized ponding
during heavy rains which could lead to excessive preferen-
tial flow not captured in the model. While we cannot rule
this out, it seems unlikely that such phenomena would affect
only the passive wick units and not the wickless units since
they were all installed in the same pit and backfilled in a
similar manner [van der Velde et al., 2005]. On the other
hand, there was some indirect evidence of local variations in
water flow at the site. One of the passive wick fluxmeters
quit working shortly after a heavy rain; then began working
again as the soil drained, suggesting that there may have
been temporary ponding around this fluxmeter. None of the
other units experienced this problem. The tops of the
passive wick units were located closer to the surface than
the wickless fluxmeters (Figure 2), so the passive wick units
were more accessible to near-surface ponding which could
Figure 7. German site (grassland on a sandy soil) passive wick fluxmeters results compared to
lysimeter results.
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W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS W04420

have occurred during intense rain storms. In a drainage
study at a humid site (>2000 mm/a) in Sri Lanka, also with a
well drained, structured clay soil, Gee et al. [2004] did not
observe convergent flow using passive wick fluxmeters
with the standard design (60-cm wick, 60-cm control
tube). So generalizat ions about successful placement in
structured clay soil cannot be made. From these Tongan
tests it appears that for structured clay soils subject to high-
flow rates that shorter wicks might have worked better than
those used in the standard passive wick design. More site
detail, such as field-measured pressure heads, and premod-
eling of the site, could help t o better guide fluxmeter
configurations in such soils.
3.2.1.4. New Zealand Site (Potato Field)
[
28] The objective of this study was to evaluate the use of
multiple passive wick fluxmeters for estimating the range of
drainage from a New Zealand potato field. Fluxmeter results
were compared to a simple water balance estimate. Six
recording and six nonrecording passive wick fluxmeters
were installed at a depth of 60 cm under a rain-fed potato
crop near Matamata, New Zealand. The soil was a free
draining Waihou gritty silt loam (typic orthic allophanic
soil) with hydraulic and physical properties described in the
New Zealand Soils Database (entry SB10113, Landc are
Resear ch, New Ze aland). The fluxmeters were installed
immediately after the crop was planted in November
2005. The soil surface was mounded into 30 cm high ridges
at 90 cm spacings, and the passive wick fluxmeters were
installed approximately halfway between the ridge and the
furrow. During the first 2 weeks of the experiment there was
enough rainfall, some 156 mm, to initiate drainage events
through all of the passive wick fluxmeters (Figure 8).
Thereafter, for the next 4 months cumulative evapotranspi-
ration losses exceeded the total rainfall and so, as expected,
the devices did not record any more drainage over the
relatively dry summer period. Drainage recommenced in
mid April 2006, coinciding with a die off of the crop
canopy, and a rewetting of the soil profile following the
early onset of autumnal rains. Drainage events were
recorded often, throughout the winter period, usually fol-
lowing rainfall events > 5 mm/d. Over the 10 months of this
experiment the fluxmeters drained about 65% of the total
rainfall, on average (Table 3). However, there was a very
wide scatter in measured drainage from the twelve passive
wick fluxmeters (Table 4). Nine of the devices recorded less
drainage than the total rainfall (we will refer to these as the
‘‘nonponded’’ set of passive wick fluxmeters). The other
three devices recorded substantially more drainage than
rainfall ( we will refer to these as the ‘‘ponded’’ set of
passive wick fluxmeters).
Figure 8. Water balance calculations of drainage losses from a potato field near Matamata, New
Zealand (solid line). The open circles are measured drainage fluxes from nine passive wick fluxmeters
that recorded less than the total rainfall. The solid circles are data from three other passive wick
fluxmeters that recorded substantially more drainage than the total rainfall.
Table 4. Fluxmeter Results From the Potato Field Near Matatama,
New Zealand
Fluxmeter
a
Fluxmeter Drainage (mm) Drain/Precip
b
Auto-1 612 0.54
Auto-2 205 0.18
Auto-3 149 0.13
Auto-4 743 0.65
Auto-5 363 0.32
Auto-6 1501 1.31
2
Man-1 155 0.14
Man-2 1097 0.96
Man-3 267 0.23
Man-4 1909 1.67
c
Man-5 1568 1.37
c
Man-6 427 0.37
a
Auto refers to the six recording flux meters, and Man refers to the six
nonrecording (manual) flux meters.
b
Drain/Precip is the ratio of total drainage divided by total precipitation.
c
Passive wick fluxmeters that were located in lower parts of the field
where runoff and ponding most likely contributed to additional drainage
volumes.
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[29] The three ponded p assive wick fluxmeters were
located in lower parts of the potato field where runoff water
and surface ponding was often observed following heavy
rainfall events. This ponding most likely contributed to the
additional drainage volumes recorded by these ‘‘ponded’’
devices. The tops of the control tubes were located at a depth
of just 30 cm below the base of the furrows. Furthermore, the
imprecise location of the passive wick fluxmeters in relation
to the ridges and furrows, whose shape changed over the
course of the growing season as the ploughe d t opsoil
packed down, increases the likelihood that some of the
passive wick fluxmeters would eventually be closer to the
center of the furrow and therefore capture more percolation
water that runs off the ridges. The converse is also likely,
that some of the passive wick fluxmeters would measure
lower drainage volumes if they were located closer to the
tops of the ridges.
[
30] If we average the drainage volumes from the nine
‘‘nonponded’’ passive wick fluxmeters then the measured
drainage rates closely match model simulations of the site
water balance that were performed using a simple water
balance calculation [Green et al., 1999]. The mean drainage
of 446 mm was consistent with the estimated drainage of
456 mm obtained from the crop model calculator and
overall water balance considerations. However, this may
be fortuitous, because the coefficient of variation of the
measured drainage rates from 12 lysimeters, over the whole
growing season, was close to 80%. Averaging all twelve
passive wick fluxmeters yielded about 60% more drainage
than the model predicted.
[
31] The performance of the passive wick fluxmeters in
this well-drained New Zealand s oil, was modeled by
assuming the soil acts as a structured clay (Figure 4) and
that the drainage rates were in the range from 100 to
1000 mm/a. Model output indicated that the standard design
(60-cm wick and 60-cm control tube) would work for this site
and should control both divergence and convergence. Care
was taken to install all the fluxmeters in a consistent manner
in the soil profile, so collection efficiencies should have been
consistent among all units with the drainage variations
reflecting the real variability in the field caused by microrelief
and natural variability in soil transport properties. Unlike the
Tongan test, the majority of the passive wick fluxmeters
showed no evidence of convergent flow. Three fluxmeters
did show excessive drainage bu t these results could be
explained by observed localized ponding and topographic
variations in the rows and furrows of the potato field. These
results indicate that a larger number of lysimeters may be
required to obtain a representative value for the average
drainage losses under highly structured cropped soils.
[
32] One would expect large variability in localized
drainage rates in the case of cropped soils, where the field
is ploughed and made ready for planting each year. In that
case a hard pan may develop in some, but not all, places in
the field. The experience with the NZ potato study is one
example where additional variability in drainage would be
expected because some furrows were observed to pond
under heavy rainfall while others did not. This ponding
leads to a redistribution of surface waters that eventually
find their way into the soil, down the pathway of least
resistance [Deurer et al., 2003]. The New Zealand data
clearly suggest that multiple measurements of drainage are
required to capt ure the high vari ability that occurs i n
subsurface fluxes. Zhu et al. [2002] and Morari [2006],
among others, have described a statistical procedure for
assessing how many drainage meters might be required to
obtain a preestablished level of accuracy (e.g., a 20% error
margin). For the New Zealand potato field, it is apparent
that more than 12 fluxmeters would be needed to achieve a
20% error margin. Practical limits in terms of initial cost and
maintenance will preclude the installation of many more
fluxmeters for any given test. The total number of passive
wick fluxmeters will in part depend on how the data will be
used. If spatial statistics were available for the site, optimi-
zation routines that place the meters at expected highest
drainage locations might be employed, so that fewer meters
could be used and yet allow the overall field-scale drainage
to be assessed. Such an evaluation is yet to be completed at
this site. There was a much smaller variation across the field
in the nitrate concentrations of the drainage waters (leaching
losses of nitrate will be reported elsewhere in relation to a
nitrogen budget for this field crop).
3.2.1.5. New Zealand Site (Dry Land Pasture)
[
33] The objective of this study was to evaluate the
performance of multiple passive wick fluxmeters with
undisturbed soil cores in assessing the range of expected
drainage from a New Zealand dry-land pasture. Fluxmeter
results were also compared to a s imple water balance
estimate. Twenty four recording passive wick fluxmeters
were installed at a depth of 40 cm under a dry-land pasture
on a flat site near Palmerston North, New Zealand. The soil
was a poorly drained Tokomaru silt loam (argillic-fragic
perch gley pallic soil) with hydraulic and physical proper-
ties described in the New Zealand Soils Database (entry
SB09559, Landcare Research, New Zealand). The profile
consisted of 20 cm of silt loam on 20 cm of silty clay loam
with 20–40 cm of clay loam. The experimental site was part
of a dairy farm and it was mole drained to a depth of 40 cm.
[
34] For this experiment we used intact soil columns, with
wicks shortened to just 10 cm to simulate drainage into the
moles. Soil columns (20 cm diameter and 40 cm depth)
were driven into ground that had been irrigated the previous
evening using 20–30 mm of water. Each column was then
excavated using a tractor-mounted back hoe, and placed
immediately on top of a passive wick fluxmeter. A thin
layer (1 cm) of sand was used to establish good contact
between the soil column and the fluxmeter device. The
whole device was then placed into the ground such that the
upper surface matched the surrounding pasture and the top
1–2 cm of the control tube extended above the soil surface
to prevent runoff (or run on) of water. A weather station was
located next to the fluxmeters to enable a calculation of
potential ET from the pasture. TDR probes were also
installed into the pasture, next to the passive wick flux-
meters, to record changes in soil water content from the top
0.4 m of the profile. The TDR probes and the weather
station shared the same logger that also recorded drainage
from the 24 passive wick fluxmeters.
[
35] The flux meters were installed at the end of a very dry
summer, and monitoring continue d for the next 17 months
(Figure 9). Over the course of the experiment the pasture
received no irrigation and all stock was excluded from the
site. The grass was mowed to a height of 3 cm, once every
1–2 weeks in the summer and less frequently over the
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W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS W04420

winter. During the first winter (May to September ) we
recorded about 100 mm of drainage, on average, following
some 400 mm of rainfall. The soil was initially quite dry
(about 50% of field capacity) and it took about 3 months of
winter rainfall before the soil reached field capacity. About
25% of the winter rainfall ended up as drainage. A large
spring (September) rainfall in the first year helped to rewet
the soil profile but it did no t generate any significant
drainage events. Similarly, a large rainfall event in mid
summer (December) also did not yield any significant
drainage. This was because the soil profile remained below
field capacity (about 40% in the to p 40 cm ) on both
occasions. More drainage occurred during the second winter
(200 mm) because the soil remained wetter for a longer time
because of more rainfall, and so a much larger fraction of
rainfall (85%) drained below a depth of 0.4 m. There was
always a good correspondence between drainage events and
times when the soil was close to or exceeded field capacity.
We observed a moderate scatter (CV = 30%) in cumula-
tive drainage as recorded by 18 of the passive wick
fluxmeters. Unfortunately, three of the devices failed to
record automatically (possibly because of a programming
error with the data logger) and another two devices failed
halfway through the experiment because of problems with
the tipping spoon mechanism. Nonetheless, all drainage
water was collected by siphoning volumes out of the bottom
of the passive wick fluxmeters devices, so that cumulative
drainage losses were still able to be measured in each case.
[
36] The performance of the passive wick fluxmeters in
this poorly drained soil appears to be very good. Total
rainfall (1265 mm) exceeded the pasture ET (906 mm) as
calculated using the FAO-56 Penman-Monteith formula
assuming a stress point of 30% (L/L) and a wilting point
of 16% (L/L). Measured drainage rates (385 mm ± 116 mm
standard deviation) were quite close to that expected from a
simple water balance.
3.2.2. Additional Fluxmeter Studies Focusing
on Spatial Variation of Drainage
3.2.2.1. Oregon Site
[
37] The objective of this study was to determine if
drainage was occurring through a shallow cover soil placed
over uranium tailings. Three passive wick fluxmeters were
installed in 2005 in the top slope of a landfill cover at a
uranium mill tailings disposal site near Lakeview, Oregon
(United States).
[
38] The placement of the fluxmeters in the cover is
shown in Figure 10. The tops of the fluxmeters are located
in the tailings layer just below the clay layer that was
intended to act as a radon gas barrier and a water drainage
barrier. The passive wick fluxmeters began recording drain-
age in mid-November 2005, just 1 week after the start of a
prolonged precipitation event, and continued to drain until
early June 2006. Instantaneous drainage flux during this
period ranged between 9,780 mm/a and 26,800 mm/a.
Cumulative drainage values from the three fluxmeters are
shown in Figure 11.
[
39] The cumula tive drainage was greater tha n total
precipitation during the test period (Table 3). This is partly
attributed to water harvesting, which occurs at this site as
water that infiltrates the topsoil is diverted laterally down-
slope by gravity through an underlying coarse filter layer
located just above the compacted clay layer. We measured
the bulk density and moisture content of the clay layer as we
excavated. The soil was then recompacted to match predis-
turbance conditions after placing the fluxmeters in the
tailings. A falling-he ad technique was used to compare
the saturated hydraulic condu ctivity (K
S
) of the reco m-
pacted clay layer with undisturbed compacted soil nearby.
The K
S
values were comparable suggesting that the recom-
paction had negligible effect on hydraulic properties inside
the fluxmeter (W. J. Waugh et al., Performance evaluation
of the engineered cover at the Lakeview, Oregon, Uranium
Figure 9. Time series of rainfall (black line), soil water content (open triangles), and drainage (gray
line) under a dry-land pasture on a poorly drained Tokomaru silt loam near Palmerston North, New
Zealand. Drainage data represent the mean and standard deviation of the cumulative drainage as
measured from 16 passive wick fluxmeters.
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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Mill Tailings Site, paper presented at Waste Management
2007 Symposium, WMSymposia, Inc., Phoenix, Arizona).
[
40] Snowmelt and rainfall in early spring produced
significant lateral flow in the gravel drainage layer of the
cover as it encountered lower-permeability clay liner caus-
ing the excess drainage water to move laterally downslope.
Observations at this site indicated that sufficient water
drained laterally across the top slope of the cover causing
ponding of water and a high percolation flux at the low end
of the top slope where the fluxmeters were located. Even
though a high bulk density was achieved during construc-
tion of the radon barrier, root growth patterns and dye traces
(observed during air entry permeameter tests) suggest that
Figure 10. A cross section of the Lakeview site with an expanded view of the cover side slope and top
slope. Fluxmeters were placed at the low end of the top slope and immediately below the compacted silt
and clay layer shown in the expanded view.
Figure 11. Cumulative precipitation and drainage from three passive wick fluxmeters at the Oregon
site. The lower line is cumulative precipitation and the other three lines are fluxmeter drainage results.
Drainage exceeds precipitation at all monitoring sites and is attributed to lateral flow of winter
precipitation that infiltrates the landfill cover and then moves downslope over the clay liner.
14 of 18
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preferential flow occurs through the clay layer (Waugh et
al., presented paper, 2007).
[
41] At all monitoring points, drainage was found to
exceed the total precipitation. This suggests that in winter
and early spring lateral flow is a prominent feature of the
site water balance. Fluxmeters were helpful in determining
that the landfill cover did not prevent drainage under these
semiarid conditions.
3.2.2.2. Los Alamos Site
[
42] The Los Alamos study was different to the other
studies reported here in that the objective was to quantify
transient drainage in a semiarid canyon floor with ephem-
eral flow. A specific objective was to examine the role of the
terraces/floodplains in the canyon floor for generating deep
drainage/potential recharge. Studies by Constantz et al.
[2002, 2003], Good rich et al. [2004], and others have
shown that the channels in semiarid canyons and arroyos
are locations where transient deep drainage events occur,
and these locations can be important contributors to ground-
water recharge. However, there has been little research on
drainage in the terraces and floodplains adjacent to the
stream channel under semiarid ephemeral flow conditions
(see Newman et al. [2006] for a discussion of riparian zone
drainage and lateral subsurface flow issues in semiarid
drainages). The fluxmeter approach appeared to be a
potentially useful way of monitoring transient drainage
events, so a seri es of standard passive wick fluxmeters (with
60 cm wick and 60 cm control tube) was installed in middle
to lower Mortandad canyon at Los Alamos National Labo-
ratory, in north-central New Mexico, United States.
[
43] Mortandad canyon contains nitrate, perchlorate, and
radionuclide contaminated sediments and groundwaters as a
result of former releases from activities at Los Alamos
National Laboratory (see Los Alamos National Laboratory
(LANL) [2006] for more specifics). Because contaminants
reside within and below sediments on the canyon floor, it is
important to understand how significant any transient drain-
age events might be. Twelve fluxmeters were installed at ten
terrace locations along the canyon floor including on both
sides of the channel and at two in-channel locations. This
arrangement allowed us to evaluate spatial variability along
approximately 3.2 km of the middle and lower canyon. The
area spanned ponderosa pine forest (Pinus ponderosa) and
pin˜on-juniper woodland (Pinus edulis and Juniperus mono-
sperma) vegetation types and included a variety of topo-
graphic features.
[
44] Fluxmeters were installed during late April through
early May 2005 to monitor drainage from the upper 1 m of
the canyon soils/sediments. The fluxmeters were installed in
intercanopy spaces (outside of the drip line of any nearby
trees). Some of the locations had thin (<30 cm) loam
topsoils, but all the profiles were dominated by sand (often
coarse sand). Given the sandy nature of the canyon sedi-
ments, the previously discussed problems with divergence
should be relatively minor. During installation, the upper
few centimeters of sod/topsoil was removed and carefully
set aside so that it could be replaced over the fluxmeter in as
intact a condition as possible. A power auger was then used
to excavate a hole to sufficient depth for installation. The
fluxmeters were lowered into the hole and the topsoil was
then repacked on top of the fluxmeters in the same order as
the in situ condition. Lifts were tamped every few cm to
approximate the natural bulk density. The sands were not
cohesive and typically had little horizonation, so the repack
effect is expected to be relatively minor. Once the excava-
tion was filled with soil to the appropriate depth, the sod/
topsoil layer was replaced. There was approximately 20 cm
of soil above the top of the divergence control tube. Little
sign of disturbance (e.g., subsidence or dead vegetation)
was evident at the surface during the 2.5-year period after
installation.
[
45] The Mortandad canyon results are summarized in
Table 3 and are reported in Table 5 for all twelve fluxmeters.
The fluxmeter results indicated a transient nature of drain-
age in these types of environments. Even the fluxmeter with
the most drainage (MCF9) frequently had many days with no
drainage at all. Example plots from fluxmeters in different
areas of the canyon floor are shown in Figure 12. There is a
wide range of drainage values and drainage/p recipitation
ratios (Table 5).
[
46] As noted earl ier, the objective of the fl uxmeter
installations in Mortandad Canyon at Los Alamos was to
examine the role of the terraces/floodplains in the canyon
floor for generating deep drainage/potential recharge. The
fluxmeter results clearly show that in most case s, deep
drainage events (>1 m depth) do occur. Only one of the
twelve fluxmeters (MCF2) did not register any drainage
over the approximately 2.5-year period. In this case, evapo-
transpiration may be greater than the precipitation inputs
creating upward fluxes that prevent deep drainage . MCF2 is
located in the lower canyon where runoff events are not as
frequent as the middle canyon locations [LANL, 2006].
[
47] Drainage fluxes in the channel are typically higher
and probably occur more frequently than most terrace
locations. For example, fluxmeters MCF1 and MCF2 are
adjacent to each other in the lower (and driest) pin˜on-
juniper part of the canyon. MCF1, which is located in the
channel, registered three small drainage events while
MCF2, which is located on the north terrace, did not register
any drainage. MCF7 is in a middle canyon channel location
(ponderosa pine) and has the highest drainage of any
fluxmeter except MCF9. Despite the importance of chan-
nels, generation of deep drainage and potential recharge
from te rraces in semiarid canyons and arroyos should not be
dismissed because, as these results demonstrate, terraces can
generate substantial deep drainage and typically occupy a
much larger portion of the canyon floor than channels.
[
48] Probably one of the most striking sets of results for
Mortandad canyon are those from fluxmeters MCF6,
MCF7, MCF9, and MCF10, all of which registered sub-
stantial amounts of drainage with drainage/precipitati on
ratios of 0.5 or higher (Table 5 and Figure 12). MCF7
and MCF9 even had drainage/precipitation ratios that
exceeded 1.0. The MCF7 result is not surprising since it
is in a channel location that has much more frequent flow
events than MCF1 (the other channel location). The area
where the terrace fluxmeters (i.e., MCF6, MCF9, and
MCF10) were installed was subject to at least three events
which flooded portions of the middle canyon. Most of these
events were flash floods generated by summer storms. One
event resulted from snowmelt following a large amount of
snowfall (see the responses of MCF9 and MCF10 for
February/March 2007 in Figure 12). The flood events can
be seen quite clearly by the large jumps in cumulative
W04420 GEE ET AL.: PASSIVE WICK FLUXMETERS
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drainage in Figure 10. These results clearly show that floods
from large and intense summer storms or snowmelt can be
major drivers of substantial deep drainage in terrace/flood-
plain areas in semiarid canyons.
[
49] Fluxmeters MCF4, MCF5, MCF8, and MCF11 were
not subject to flooding and showed much lower drainage /
precipitation ratios. On the other hand, MCF3 and MCF12
had intermediate drainage/precipitation ratios. These two
fluxmeters were in low areas and the results reinforce the
importance of topographic effects on drain age even at
relatively small spatial scales. MCF3 w as in a small
depression in the lower canyon and had substantially more
drainage than the next closest fluxmeter MCF5. MCF12
was installed in a sediment trap (installed to prevent
contaminated sediments from mig rating down canyon).
Unfortunately, the flash flood in 2005 went over the top
of the data logger and most of the results were lost. It is
likely that drainage at this location was substantially higher
than is reported here.
4. Conclusions
[50] The lysimeter studies in the United States at the
Hanford site and in Germany at the Falkenberg site showed
good agreement for paired fluxmeters tested against large
lysimeters in coarse-textured soils. The lysimeter tests were
run successfully for multiple years, suggesting that passive
wick units are both accurate and durable for use in long-
term studies with coarse-textured soils at these locations.
Results from tests in Tonga and New Zealand show that in
well-drained fine soils, convergent flow can occur but may
be controlled by modifying the fluxmeter design (shorter
control tube or shorte r wick) to improve the collection
efficiency of the meter. The New Zealand studies reveled
moderate to large spatial variability that may be quantified
Figure 12. Cumulative drainage and precipitation from selected passive wick fluxmeters at the Los
Alamos canyon site.
Table 5. Fluxmeter Results for Mortandad Canyon, Los Alamos, New Mexico
Fluxmeter Surface Soil Measurement Period Precipitation (mm) Fluxmeter Drainage (mm) Drain/Precip
a
MCF1
b
Coarse sand 25 May 2005 to 3 Dec 2007 914.1 44.2 0.05
MCF2 Fine to coarse sand 13 May 2005 to 3 Dec 2007 914.1 0.0 0
MCF3 Coarse sand 13 May 2005 to 3 Dec 2007 914.1 131.3 0.14
MCF4 Fine to coarse sand 13 May 2005 to 1 Sep 2007 783.1 4.7 0.01
MCF5 Silty loam topsoil, coarse sand 13 May 2005 to 10 Nov 2007 837.1 1.2 0.001
MCF6 Silty loam topsoil, coarse sand 13 May 2005 to 11 Nov 2007 837.1 428.7 0.51
MCF7
b
Fine to coarse sand 26 May 2005 to 4 Dec 2007 914.1 1799.9 2.0
MCF8 Fine to coarse sand 11 May 2005 to 14 Aug 2007 757.2 32.1 0.04
MCF9 Silty loam topsoil, coarse sand 11 May 2005 to 4 Dec 2007 914.1 4147.6 4.5
MCF10 Silty loam topsoil, coarse sand 11 May 2005 to 4 Dec 2007 914.1 658.1 0.72
MCF11 Silty loam topsoil, coarse sand 11 May 2005 to 4 Dec 2007 914.1 65.4 0.07
MCF12
c
Silty loam topsoil, fine to coarse sand 13 May 2005 to 6 Dec 2007 914.1 258.3 0.28
a
Drain/Precip is the ratio of total drainage divided by total precipitation.
b
In-channel location.
c
MCF12 drainage is likely underestimated; logger was damaged in flood.
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by use of multiple passive wick flux meters. The New
Zealand studies also showed that intact soil cores may be
used successfully to estimate ranges of drainage from
poorly drained soil. The results from the Oregon site show
that both slope and soil type are important in documenting
drainage flux. Lateral flow can dominate on sloping lands
where subsurface soils are compacted and this can cause
downslope areas to receive excess drainage waters. The
results from the Mortandad canyon study at Los Alamos
demonstrate that fluxmeters can be used effectively to
understand the impacts of spatial variability on drainage
particularly in terraced land and floodplains in a semiarid
canyon with ephemeral flow. The relatively low cost of
passive wick fluxmeters is an advantage in these kinds of
studies where measurements in multiple areas are needed
to properly capture spatial variability in drainage fluxes.
Experimental results demonstrate that the canyon terraces do
exhibit transient deep drainage, and only one out of twelve
fluxmeters registered no drainage at all. Spatial variability in
drainage was substantial and drainage/precipitation ratios
ranged from 0 to 4.5. Floods were especially important in
some areas for generating high drainage/precipitation ratios.
Topographic depressions were also areas where local drain-
age was relatively high.
[
51] Passive wick fluxmeters provide a simple and cost-
effective way to quantify drainage where such data have
been difficult to obtain in the past. Optimizing the perfor-
mance of these devices depends on the soil type and the
climatic conditions. The good agreement over multiple
years, between drainage obtained with passive wick flux-
meters and drainage measured with adjacent large lysime-
ters, or estimated via simple water balances, is encouraging
and supports the use of passive wick fluxmeters for long-
term drainage studies, particularly in coarser soils. In finer
textured soils, passive wick fluxmeters perform best under
higher drainage fluxes (e.g., >100 mm/a). Fine soils tend to
drain at lower pressure heads (more negative) than coarse
soils, and this limits the operational range of wick lysim-
eters. In cases where there are fine soils that are highly
aggregated or otherwise contain macropores, passive wick
fluxmeters work much like they do in coarser (sandy) soils,
but are subject to uncertainties that occur with preferential
flow conditions that move ponded surface waters rapidly
downward through root channels and worm holes, etc.,
giving rise to highly variable drainage. Multiple fluxmeters
are needed in such cases to help capture and assess the
degree of spatial variability. Rainfall intensity and duration
must be considered in designing fluxmeters for humid sites
where localized ponding may cause convergent flow result-
ing in an overestimate of drainage rates. Proper matching of
wick length and control tube height to the soil pressure
conditions expected during typical draina ge events will
improve the performance of the passive wick units, although
standard configurations (e.g., 60-cm wicks and 60-cm
control t ubes) appear to work well for a number of appli-
cations, particularly in coarser-textured soils. An additional
advantage of the passive wick fluxmeters described here is
that drai nage water can also be sampled f or c hemi cal
characterization in a straightforward way [e.g., see Gee et
al., 2003]. Time series of coupled chemistry and drainage
data collected from a single instrument makes passive wick
fluxmeters useful tools for examining nutrient and contam-
inant t ransport problems. In addition, we are currently
evaluating the suitability of fluxmeters for sampling stable
isotopes (e.g., d
2
H and d
18
O), which would broaden the
applicability of this type of instrumentation.
[52] Acknowledgments. This work was performed as part of the
Hanford Remediation Closure project for the Richland Operations Office
of the U.S. Department of Energy under contract DE-AC06-76RL01830.
We acknowledge our collaborators, A. Anandacoomaraswamy and his staff
at the Tea Research Institute, Talawakele, Sri Lanka, and the Environmental
Group, Hort Research Group, Palmerston North, New Zealand. We also
acknowledge the generous support from two commercial water fluxmeter
suppliers, Decagon Devices, Pullman, Washington, United States www.
decagon.com), and Sledge Sales, Dayton, Oregon, United States http://
sledgesales.com), who provided test instruments and have been willing to
modify equipment as designs and applications have evolved. Funding for
the Los Alamos study was provided by the Los Alamos National Labora-
tory Environmental Restoration Project, and we thank Danny Katzman for
his support. We also wish t o acknowledge Marvin Gard, Bob Gray, and
Tracy Schofield for their assistance in the field. Funding for the Oregon
study was provided by the U.S. Department of Energy Office of Legacy
Management.
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