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A simple and flexible field-tested device for housing water monitoring sensors at point discharges

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The Water Monitoring Enclosure (WME) provides a simple and flexible housing for many types of sensors for continuous measurements of water parameters (physical, chemical, or biological) and provides the opportunity of representative sampling for external analyses. The WME ensures a minimum internal water level and this ensures that the internal monitoring equipment remains submerged even when there is no flow into the enclosure. The limited diameter of the inflow pipe and water volume in the WME buffers the flow velocity from dramatic changes. The device ensures that the sediment entering the enclosure from the inflow will be conveyed through the enclosure with minimal sediment accumulation. The device is powered purely from natural hydraulic forces, so it requires no power source, and requires little additional maintenance beyond periodic cleaning. If desired, the WME can also measure discharge entering the device through additional modifications. Water samples were taken throughout the year to validate the effectiveness of the WME. The comparisons of the influent water to the water in the WME for all parameters were below the laboratory analysis standard error or below the limit of quantification, indicating that the water in the WME is representative of the influent water.
Content may be subject to copyright.
A simple and exible eld-tested device for housing water
monitoring sensors at point discharges
Michael Exner-Kittridge, Richard Niederreiter, Alexander Eder
and Matthias Zessner
ABSTRACT
The Water Monitoring Enclosure (WME) provides a simple and exible housing for many types of
sensors for continuous measurements of water parameters (physical, chemical, or biological) and
provides the opportunity of representative sampling for external analyses. The WME ensures a
minimum internal water level and this ensures that the internal monitoring equipment remains
submerged even when there is no ow into the enclosure. The limited diameter of the inow pipe
and water volume in the WME buffers the ow velocity from dramatic changes. The device
ensures that the sediment entering the enclosure from the inow will be conveyed through the
enclosure with minimal sediment accumulation. The device is powered purely from natural hydraulic
forces, so it requires no power source, and requires little additional maintenance beyond periodic
cleaning. If desired, the WME can also measure discharge entering the device through additional
modications. Water samples were taken throughout the year to validate the effectiveness of
the WME. The comparisons of the inuent water to the water in the WME for all parameters were
below the laboratory analysis standard error or below the limit of quantication, indicating that
the water in the WME is representative of the inuent water.
Michael Exner-Kittridge (corresponding author)
Vienna University of Technology,
Centre for Water Resource Systems,
Karlsplatz 13/222, A-1040,
Vienna,
Austria
E-mail: mgkittridge@gmail.com
Richard Niederreiter
UWITEC, Weißensteinstraße 305310,
Mondsee,
Austria
Alexander Eder
Federal Agency for Water Management,
Institute for Land and Water Management
Research,
Pollnbergstraße 1,
3252, Petzenkirchen,
Austria
Matthias Zessner
Vienna University of Technology,
Institute of Water Quality,
Resources and Waste Management,
Karlsplatz 13/222, A-1040,
Vienna,
Austria
Key words
|
environmental monitoring, sensor housing, water monitoring, water quality
INTRODUCTION
Measurements and samples of water parameters are a funda-
mental aspect of all water-related research and monitoring
aside from purely theoretical research. As technology
improves and scientic questions plunge deeper into the
ner scales of time and space, water measurements have
been migrating from grab measurements and samples
taken personally at a eld site to permanently or temporarily
installed instrumentation continuously collecting data. This
type of instrumentation has clear advantages. Researchers
can acquire data at a high frequency and during periods or
at locations that might be difcult to capture (i.e. short dur-
ation rainfall events, distant or temporarily inaccessible eld
sites, etc.). Unfortunately, continuous monitoring equipment
also tends to be much more complex and subsequently more
maintenance-prone than simple grab measurements or
samples.
Direct laboratory analysis is still the m ost accurate
method to analyze water chemistry as the equipment is
regularly maintained, calibrated, and validated to ensure
a specic level of data quality. Researchers who only
require a couple o f dozen samples for periodic eld tests
have used automatic samplers to take samples remotely
at a high temporal frequency (Tan et al. ; Burns
et al. ; Roser et al.  ; Ensign & Paerl ). But
for those who require a high temporal resolution in
addition to longer monitori ng programs s panning months
or years, the costs for both the laboratory analyses and
labor become prohibitive.
Researchers have recognized that if long-duration high
temporal resolution measurements of water chemistry are
to be performed then the laboratory would need to be
mostly divorced from the actual measurement process. Cur-
rently, there are two overarching methods to remotely
collect high temporal resolution water chemistry using tem-
porary or permanent installations. The rst is typically
known as on-line measurement devices. These devices
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doi: 10.2166/wst.2013.655
effectively miniaturize and automate aspects of the labora-
tory analysis (i.e. ltration, addition of chemical additives,
etc.) at a xed location directly at the water body for
sampling. The second is to use an in situ sensor as a proxy
for the measurement of the chemical compound. The
former method has been used extensively in wastewater
monitoring (Legnerová et al. ; Carrasco et al. ; Sten-
holm et al. ). The ability of an automated on-line device
to draw in a sample and prepare the sample through
ltration or chemical additives is useful when the sample
may have high concentrations of many toxic substances.
The primary trade-off of such a highly complex device
with automated pumping and ltration systems is
that it requires a great deal of maintenance to upkeep the
device.
In situ sensors have become increasingly popular within
the last couple decades for both wastewater and non-waste-
water applications (Newman  ; Kaelin et al. ; Kim
et al. ; Kestel et al. ). Hydrologists have been
using similar devices for many decades to measure physical
water parameters (i.e. water level, temperature, electrical
conductivity, etc.). The advantages of in situ sensors include
a very small size relative to the on-line devices, simpler con-
struction and operation, lower cost, and temporal resolution
that can be ner than on-line devices. The primary disadvan-
tage is a lower accuracy and higher detection limit to the on-
line devices.
Both the on-line devices and the in situ sensors have
physical measurement constraints that must be taken into
account before installation to e nsure proper measurement
quality. The constra ints become especially problematic
when meas uring in natural environments with low dis-
charges (<5 l /s). Both types of devices require a water
depth of at least 2030 cm depe nding on the type of
measurement dev ice. The water could either be a free-
owing stream or an intermediate pool of water. Fo r sen-
sor s with polymer membranes, the water level must not
drop below the measurement device as the sensors must
always stay wet. Additionally for in situ sensors, large
ranges in the water velocity can signicantly affect elec-
trochemical signals. If parameters associated with
suspended solids are to be measured (i.e. turbidity, phos-
phate, etc.), then resuspension during storm ow events
should be greatly limited to ensure that the sampling is
representative of the inuent water. Suspended solids
sampling is generally more difcult to be representat ive
of the inuent water than dissolved solids sampl ing.
Harmel et al.() found that, while sampling uncer-
tainty for dissolved solids was ±25%, the uncertainty for
suspended sol ids was over 50% . Finally, continuously
deposited sediments into the measurement pool could
reduce the opti mal water depth and in the worst case
bury the measurement device.
Given the above constraints for the installation con-
ditions for water measurement devices, researchers
traditionally have had two general options when installing
equipment in natural environments. The rst is to simply
excavate a pool directly below the inuent water and
place the measurement devices (or pump tubing) directly
into the pool. The main drawback of the pool is that it
requires frequent excavation of the sediment deposited
into the pool. Due to the reduction in the ow velocity
from the inuent water to the pool, sediment will inherently
deposit into the pool and not be naturally transmitted
through the system. Additionally, the ow velocity in the
pool could vary dramatically depending on the volatility of
the inuent water. The second option is also to excavate a
pool, albeit a smaller one to that of the rst option, and
use a tube with a pump to draw up the water into an external
container where the measurement devices are housed. The
pumpcontainer option is advantageous in that the exca-
vated pool can be much smaller than for the rst option,
and subsequently the sediment deposition is reduced. The
pump container option would ensure both a minimum
water level and a consistent ow velocity for the measure-
ment devices. The key disadvantage for the pump
container option is the pump itself. Additional mechanical
moving parts like a pumping system increase the likelihood
of malfunction and breakdown especially during winter
months where freezing can be an issue. An ideal option
would have the relative simplicity of the rst option, the con-
sistent ow velocity of the pumpcontainer option, and the
ability to continuously transmit sediment through the
measurement system.
We have designed, built, and tested a measurement
system with all of the above constraints. The Water Moni-
toring Enclosure (WME) provides a simple and exible
housing for sensors for continuous measurements of
many types of water parameters (physical, chemical, or bio-
logical) and provides the opportunity of representative
sampling for external analyses (e.g. remote mini-labora-
tories or automatic samplers). The WME ensures a
minimum internal water level and this ensures that the
internal monitoring equipment remains submerged even
when there is no ow into the enclosure. The limited diam-
eter of the inow pipe and water volume in the WME
buffers the ow velocity from dramatic changes. The
device ensures that the sediment entering the enclosure
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from the inow will be conveyed through the enclosure
with minimal sediment accumulation. The device is pow-
ered purely from natural hydraulic forces, so it requires
no power source, and requires little additional maintenance
beyond periodic cleaning. If desired, the WME can also
measure discharge entering the device through additional
modications.
TEST SITE
The test site chosen for the WME was the Hydrologic Open
Air Laboratory (HOAL) catchment located in Petzen-
kirchen in Lower Austria, approximately 100 km west of
Vienna. The catchment has an area of 67 hectares and the
land cover is characterized as 90% agriculture, 5% imperme-
able surface, and 3% forest. There are 12 point discharges
that drain into the catchment stream. These include seven
subsurface tile drains, two springs, and four surface tribu-
taries. Baseow can range from 0.00 l/s at some of the
subsurface drains and surface tributaries to 5 l/s at the catch-
ment outlet. Average baseow at the subsurface drains and
surface tributaries is 0.10.2 l/s with a maximum measured
discharge of approximately 13 l/s. In total, four WMEs
were installed within the catchment.
DESIGN AND CONSTRUCTION
Figure 1 illustrates the individual components of the WME.
There are ve primary components: (1) center shaft, (2)
plunger, (3) bladder, (4) outer enclosure, and (5) outow
siphon.
Figure 2 represents the fully constructed WME. The
WME is almost entirely composed of polyvinyl chloride
(PVC). The only exception is the plunger rod, which is com-
posed of stainless steel. This type of PVC was primarily used
because of its low corrodibility and ultraviolet resistivity.
The inow piping to the WME is standard 50 mm diameter
PVC attached directly to the WME. The overow should be
located directly opposite the inow with a 50 mm diameter
pipe. The outow opening at the bottom of the center shaft
is 4045 mm diameter. A 75 mm diameter PVC pipe is sub-
sequently attached to the bottom of the center shaft, which
is ultimately reduced to 50 mm once it connects to the
siphon. The plunger is set within the center shaft with the
bladder set around the center shaft. Both the plunger and
Figure 1
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The individual components of the WME and associated dimensions: (1) center shaft, (2) plunger, (3) bladder, (4) outer enclosure, and (5) outow siphon.
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the bladder should be of a diameter that would allow them
to move freely within the center shaft with a changing water
level. An adjustable screw xed to the upper end of the plun-
ger and set atop of the bladder ensures that when the
bladder moves vertically the plunger will also move with
it. The adjustable screw on the plunger rod can be raised
or lowered to modify the minimum and maximum water
level within the WME. The plunger should have a exible
rubber seal on the bottom end to facilitate a watertight
seal when in the closed position. The total height of the
WME without the center shaft should be determined by
the instrumentation to be placed inside the WME. Our
WME had a wall height of 30 cm and a total height to the
bottom center of the WME (excluding the center shaft) of
35 cm. An inverted siphon should be placed at the outow
piping of the WME and should not rise above the bottom
of the center shaft where the plunger would be in the
closed position. The overow should be connected to the
outow piping directly after the siphon using a T joint
PVC pipe as shown in Figure 2.
FUNCTION
The WME is designed to provide four key functions: (1)
maintain a minimum water level to ensure that the measure-
ment sensors do not become dry, (2) allow sediment to be
continuously transmitted through the WME, (3) limit the
ow velocity range, and (4) be simple and require little
maintenance.
The 50 mm diameter inow restricts the amount of ow
into the WME that can be readily discharged at the bottom
of a 4045 mm diameter outlet (Figure 1). The inow restric-
tion removes the need for a large overow in case of severe
clogging of the outow and limits the magnitude of the ow
velocity in the WME. The inowing water is directed per-
pendicularly into the WME to allow for proper circulation
and mixing within the WME. The sloped bottom funnel
ensures that any sediment deposition will be gradually trans-
ported down towards the WME outlet during ushing and
ultimately removed.
When a maximum water level threshold is reached
within the WME, the bladder quickly lifts the plunger to
allow a high-velocity ow pulse through the outow pipe.
Following the initial opening of the plunger, the water
level gradually drops since the outow discharge is greater
than the inow discharge. Once the water level reaches a
minimum threshold, the bladder lowers and the plunger
closes the outow. The mechanism is intended to always
fully open regardless of the inow rate to ensure proper sedi-
ment conveyance. In some instances with a long outow
pipe, the closing of the plunger to the outow may bounce
several times before completely closing, owing to natural
water hammer.
The inverted siphon shape was congured to retain a
volume and level of water directly below the outow, but
not as to exceed the height of the plunger inside the WME
as this can cause poor sealing of the plunger. The closed
volume of water in the siphon ensures that, when the
water level in the WME reaches the maximum threshold,
and the plunger is subsequently lifted by the bladder, the
water exiting the outow does not exert a counter force on
the plunger potentially pulling the plunger back to a
closed position. Without the siphon, the bladder and plun-
ger mechanism would not open fully when the initial
maximum threshold is reached, and consequently a steady
state would be reached where the inow would equal the
outow. Due to the high velocities in the siphon compared
with the inow and within the WME, no sediment accumu-
lation occurs in the siphon.
Discharge measurements
When water is regularly entering the WME, the plunger
opens and closes at a regular frequency relative to the
incoming ow. With a consistent ushing frequency from
the incoming ow, measuring the time between the ushings
allows for the estimation of discharge from the WME. The
mechanism functions similarly to a tipping-bucket rain
Figure 2
|
Fully assembled and installed WME with foundation poles. The holes in the lid
of the WME are for the placement of the measurement sensors.
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gauge and can be installed with similar instrumentation.
Q ¼
t
2
t
1
V
(1)
where Q is the discharge, t
1
is the time of the initial opening
of the plunger, t
2
is the time when the plunger closes, and V
is the difference between the volume of water at the maxi-
mum water level and the minimum water level in the
WME. In this situation, V is not equivalent to the volume
of ushed water as the ushed water includes the inow
water during the ushing in addition to the V. For practical
purposes, it may not be possible to measure both an opening
time and a closing time, which could require two separate
devices logging two separate time stamps. Measuring the
time once per opening is more feasible, but a relationship
must be derived between the times of the sequential open-
ings of the plunger to the time between the opening and
the closing of the plunger.
FUNCTIONAL ASSESSMENT
Several tests were performed to verify the effectiveness of
the WME. The tests included (1) a verication that the
water chemistry in the WME is representative of the inuent
water and (2) a verication of the regularity of the ushings
for use in estimating discharge.
Chemical assessment
For the chemical assessment, one sample was collected at
the inuent water (directly before the intake of the WME)
and one sample was collected from within the WME at
four separate times over a 1-year period. Table 1 summarizes
the results from the chemical analysis of the samples. The
sample representativeness error was estimated with the fol-
lowing equation:
SRE ¼
C
a1
C
b1
1

þ
C
a2
C
b2
1

þ ...
n
(2)
where SRE is the sample representativeness error, C
a
is the
concentration of the inuent water, C
b
is the concentration
of the water in the WME, and n is the total number of
sample sets. In our comparison, we had four sample sets.
In addition to the four sample sets that were taken over
the course of a year, another set of 25 samples were taken on
1 day at the same WME to test the laboratory analytical
error. These samples were taken in quick succession and
with great diligence to ensure that minimal error could be
attributed to the sample collection and sample transpor-
tation. Our term laboratory analytical error includes both
sample preparation and analytical error. Only three chemi-
cal parameters (phosphate, nitrate, and chloride) were
analyzed due to cost limitations. The laboratory analytical
error was calculated by the coefcient of variation
(standard deviation divided by the mean) of all 25 samples.
The results are shown at the bottom of Table 1 as lab analyti-
cal error.
The majority of the parameters had neither a consistent
bias nor an error greater than 2% of the original measure-
ments. Although several parameters (e.g. dissolved organic
carbon (DOC), total organic carbon (TOC), phosphate,
ammonium, and total phosphorus) had larger errors, these
parameters were near or below the limit of quantication
of the laboratory analytical techniques. Phosphate was
close to the quantication limit of the laboratory analysis,
while nitrate and chloride were well above the quanti-
cation limit. Nitrate and chloride with a concentration
well above the detection limit had errors of approximately
0.01 and 0.05 respectively, while phosphate with a concen-
tration near the detection limit had an error of
approximately 0.30. Indeed, the error found between the
inuent water and the WME is well under the error for
phosphate and at or below the error for nitrate and chloride.
DOC and TOC had errors above 3%, and we were unfortu-
nately not able to derive laboratory-specic analytical errors.
According to the DIN/EN 1484 standards for the laboratory
analysis of DOC and TOC (Committee EH/3/2 ), the
analytical error can be on average 12 mg/l. As the average
differences for the sample sets for DOC and TOC were 0.13
and 0.35 mg/l respectively, the SRE can be attributed to the
analytical error. During a normal sampling campaign,
sample collection, sample transportation, sample prep-
aration, and sample analysis all add error. Harmel et al.
() found that under typical conditions the cumulative
probable uncertainty was 448% for sample collection, 2
16% for sample preservation/storage, and 521% for labora-
tory analysis. For this reason, it is surprising that our sample
comparisons between the inuent water and the WME had
only a couple percent error for most of the parameters.
Discharge assessment
The data for the assessment of the consistency of the ush-
ing frequency of the WME during constant discharge were
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Table 1
|
Chemical comparisons between the inuent water and the WME taken at four times throughout the year
Location Date
Dissoved
organic
carbon
(mg/L)
Total
organic
carbon
(mg/L)
Phosphate
(mg/l)
Ammonium
(mg/l)
Nitrate
(mg/l)
Total
phosphorus
(mg/l)
Total
nitrogen
(m g/l)
Suspended
solids (mg/l)
Hydrogen
carbonate
(mg/L)
Chloride
(mg/l)
Sulfate
(mg/l) pH
Electrical
Conductivity
(μ s/cm)
Inuent
water
2011-01-14 2.1 3.2 0.031 0.012 9.32 0.106 9.84 19.3 301 19.2 26.0 7.31 636
WME 2011-01-14 2.2 2.7 0.026 0.012 9.41 0.089 9.85 22.0 302 18.4 25.6 7.35 634
Inuent
water
2011-02-23 2.1 2.1 0.008 0.010 13.79 0.015 13.70 0 325 23.7 39.8 7.64 770
WME 2011-02-23 2.4 2.4 0.007 0.020 13.94 0.020 14.17 0 331 23.8 39.4 7.63 770
Inuent
water
2011-05-09 0.4 0.5 0.011 0.011 13.90 0.027 13.95 1 335 27.7 45.8 7.53 783
WME 2011-05-09 0.4 0.9 0.011 0.011 13.90 0.024 14.18 1 336 27.4 44.2 7.46 781
Inuent
water
2012-01-11 1.0 1.3 0.007 0.017 10.85 0.021 11.14 0.0 356 24.4 35.7 7.67 780
WME 2012-01-11 0.9 1.1 0.006 0.021 10.92 0.018 11.33 0.5 347 23.8 35.1 7.56 772
Lab limit of
quantication (mg/l)
0.3 0.3 0.014 0.029 0.10 0.070 1.5
SRE 0.07 0.23 0.13 0.17 0.01 0.18 0.02 0.03 0.01 0.02 0.02 0.01 0.00
Lab analytical error 0.30 0.01 0.05
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collected at 10 separate times throughout the year and had a
coefcient of variation of 0.01. The error in the WME esti-
mation of discharge compared to the manual
measurements of discharge during the previously mentioned
10 measurement times throughout the year was 0.05. The
error was estimated using Equation (2), but replacing con-
centrations with discharges. The ows assessed ranged
from 0.04 to 0.5 l/s. Beyond approximately 0.7 l/s the plun-
ger remains opened continuously and therefore cannot be
used to estimate discharge. If there is a need to measure dis-
charge with the WME, then a slight magnetic force between
the plunger and the outow pipe may be required to ensure
a proper closing.
DESIGN LIMITATIONS
During the operation of the WME, we found two functional
limitations and issues. The rst occurred at a site with
very low discharge (<0.04 l/s) combined with a heavy
ne sediment load (up to 100 mg/l of silt). In this situation,
the WME did not ush frequently enough to continuously
remove all of the sediment transported into the WME. The
maximum sediment accumulation that we found was 4cm
of sediment deposited over a 2-week period.
The second issue was the accumulation of precipitated
calcium carbonate within the WME. Normal agricultural
practice in Austria (and many parts of the world) includes
periodic spreading of lime on the agricultural elds to
ensure that the soil does not become acidic. As with most
chemicals that are spread onto agricultural elds, the cal-
cium eventually makes its way through the various
pathways and into the main water bodies. During our rst
8 months of operation, we had only minimal accumulation
of calcium carbonate, which caused minimal functional
issues for the WME. The rst major rain storm after the
farmers applied the calcium bicarbonate caused signicant
accumulations of calcium carbonate at nearly all of the
WME stations. The primary issue with the build-up of cal-
cium carbonate is that the ushing mechanism of the
plunger within the center shaft seizes from the shrinking
diameter in the center shaft and the additional friction.
The seizing occurred on approximately a third of the
devices, and the others had some degree of problems open-
ing properly from an incomplete plunger seal.
Both issues of the sediment accumulation at very low
ows and calcium carbonate cementation are situational
issues and should be dealt with as such. Periodic cleaning
of the WME is necessary to ensure optimal performance,
and the frequency of cleanings will be unique for each
installed WME due to the local environmental conditions.
During the period when calcium carbonate was accumulat-
ing rapidly, our WMEs were cleaned once every 2 weeks.
During the remainder of the year, the WMEs would only
need to be cleaned once every couple of months or in
some cases not at all.
CONCLUSIONS
The WME has been shown to provide a simple, exible, and
effective housing for many types of water measurement
devices. The WME can function in many environments
that may be remote and without electricity for long periods
of time at nearly any range of ow. The WME ensures a
minimum internal water level and this ensures that the
internal monitoring equipment remains submerged even
when there is no ow into the enclosure. The limited diam-
eter of the inow pipe and water volume in the WME
buffers the ow velocity from dramatic changes. The
device ensures that the sediment entering the enclosure
from the inow will be conveyed through the enclosure
with minimal sediment accumulation. The device is pow-
ered purely from natural hydraulic forces, so it requires no
power source, and requires little additional maintenance
beyond periodic cleaning.
The functional assessments have shown that the WME
has a minimal effect on the chemistry of the water, and
with the addition of a small magnet the WME can also
measure discharge accurately up to 0.5 l/s. In certain
environments, the WME must be cleaned regularly to
ensure the proper functioning of the few moving parts
within the WME. We cannot emphasize enough the impor-
tance of regular maintenance for any water monitoring
device regardless of how simple and basic the device may be.
ACKNOWLEDGEMENTS
We would like to thank the Austrian Science Foundation for
funding our work as part of the Vienna Doctoral
Programme on Water Resource Systems (DK Plus W1219-
N22). We would like to thank Jan Podrouzek for his
frequent eld assistance and tireless efforts installing the
WMEs. We would also like to thank Peter Haas and Ernis
Saracevic for their assistance and input during the labora-
tory testing of the WME.
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1033 M. Exner-Kittridge et al .
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A simple and exible device for housing water monitoring sensors Water Science & Technology
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... Three eddy-correlation stations were set up to understand the spatial distribution of land-atmosphere interactions. Faecal indicators were monitored to test alternative measurement methods and understand the dynamics of faecal contamination, and water quality characteristics were monitored at a number of locations to understand nutrient fluxes (Exner-Kittridge et al., 2013). ...
... While the high sediment concentrations in the HOAL facilitated the sediment process analyses, they turned out to be a challenge for monitoring the water quality parameters, as the stilling wells in which sensors are usually placed tended to silt up quickly. A new device was developed, termed the Water Monitoring Enclosure (WME), which allows in situ monitoring of water quality parameters for highly dynamic, sediment-laden streams (Exner-Kittridge et al., 2013). The WME ensures a minimum internal water level which keeps the monitoring equipment submerged even when there is no flow into the enclosure. ...
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Hydrological observatories bear a lot of resemblance to the more traditional research catchment concept, but tend to differ in providing more long-term facilities that transcend the lifetime of individual projects, are more strongly geared towards performing interdisciplinary research, and are often designed as networks to assist in performing collaborative science. This paper illustrates how the experimental and monitoring set-up of an observatory, the 66 ha Hydrological Open Air Laboratory (HOAL) in Petzenkirchen, Lower Austria, has been established in a way that allows meaningful hypothesis testing. The overarching science questions guided site selection, identification of dissertation topics and the base monitoring. The specific hypotheses guided the dedicated monitoring and sampling, individual experiments, and repeated experiments with controlled boundary conditions. The purpose of the HOAL is to advance the understanding of water-related flow and transport processes involving sediments, nutrients and microbes in small catchments. The HOAL catchment is ideally suited for this purpose, because it features a range of different runoff generation processes (surface runoff, springs, tile drains, wetlands), the nutrient inputs are known, and it is convenient from a logistic point of view as all instruments can be connected to the power grid and a high-speed glassfibre local area network (LAN). The multitude of runoff generation mechanisms in the catchment provides a genuine laboratory where hypotheses of flow and transport can be tested, either by controlled experiments or by contrasting sub-regions of different characteristics. This diversity also ensures that the HOAL is representative of a range of catchments around the world, and the specific process findings from the HOAL are applicable to a variety of agricultural catchment settings. The HOAL is operated jointly by the Vienna University of Technology and the Federal Agency for Water Management and takes advantage of the Vienna Doctoral Programme on Water Resource Systems funded by the Austrian Science Funds. The paper presents the science strategy of the set-up of the observatory, discusses the implementation of the HOAL, gives examples of the hypothesis testing and summarises the lessons learned. The paper concludes with an outlook on future developments.
... Twelve point discharges contribute to the discharge of the stream. These include tile drains, springs and surface tributaries (Exner-Kittridge et al., [33]). The mean annual precipitation is 823 mm/yr . ...
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