ChapterPDF Available

Figures

A) Schematic diagram of an infiltrometer. I) Main reservoir (Mariotte), II) rubber stoppers, III) bubbling tubing, IV) water outlet, V) stop valve, VI) metallic ring, VII) purge and measurement of hydraulic load, VIII) stand base, IX) constant hydraulic load, X) insertion depth, XI) pressure sensor access to air chamber, XII) data logger, XII) pressure sensor access to the water column. B) Classical infiltration data results showing the transient and steady state phases of the infiltration process. acknowledge the need for instrumentation and data recording devices (infiltrometer and permeameter) to automate the data acquisition, minimize human errors and reduce the time spent in taking measurements (Amezketa-Lizarraga et al., 2002; Johnson et al., 2005). Therefore many devices have been reported and patented since the 1940s (Bull, 1949) to try to automate the data acquisition process. Automated devices rely on data recording units (data loggers). However the main constraint is the local availability of such equipment, followed by cost. In many cases researchers implement their own devices without automation, such as a double ring infiltrometer (Carlon-Allende, 2006). Automating an infiltrometer requires accurate measurement of the change of height of the water column over time, as the water is allowed to exit the container. Some of the methods used for measuring the column height are the use of paired infrared sensors in a plastic cylinder (Wilson et al., 2000), float valve system with meter spool ring infiltrometer (Amezketa-Lizárraga et al., 2002), Time domain reflectometry (TDR) infrared detectors and float sensor or pressure sensors (Ankeny et al., 1988). The use of pressure transducers is probably the most common choice because of low-cost, simplicity, easy implementation and reliability. Overman et al. (1968) reported the application of pressure transducers since the mid 60s, to implement a variable load laboratory infiltrometer, designed specifically for low-permeability materials. Constanz & Murphy (1987) generated a system that could measure the height of a column of water from pressure changes in a Mariotte reservoir and thus infer the infiltration data. Their instrument used Transamerica CEC 4-312 pressure transducers, with pressure range ± 12.5 psi. The automated device allowed rapid data acquisition with minimal supervision. Ankeny et al. (1988) reported that the use of one transducer produced measurement errors due to bubbling inside the container and adapted the design of Constanz & Murphy (1987) to a tension infiltrometer (disc) with two PX-136
… 
EL-USB-3 Voltage data logger from Lascar Electronics. A) Physical dimensions and B) typical connection for logging sensor signals. Reproduced with permission. Copyright 2011 Lascar Electronics. All Rights Reserved from Lascar Electronics. The EL-USB-3 is a stand-alone data logger powered internally by a 3.6 Volts battery, capable of taking 32,510 readings in the range of 0-30 Volts with 50 mV resolution. The signals are applied to the data logger through a detachable cap, so that it can be removed from the instrumentation electronics for programming and data transfer without disconnecting wires. The data logger includes a USB interface for setting the data acquisition sampling rate from 1 second to 12 hours (1 second, 10 seconds, 1 minute, 5 minutes, 30 minutes, 1 hour, 6 hours and 12 hours) and also for transferring the results to a host PC. The operation can be assessed by observing the activity of two LEDs, red and green, which are included (Fig. 4). Once the test has concluded, the measured data can be transferred to a host PC for off-line analysis through the USB interface using the software included. Thus the EL-USB-3 includes all the necessary components shown in Fig. 2C corresponding to the digitizing section of the instrumentation scheme proposed. Using a commercial data logger reduces instrumentation development time. Nevertheless, the signal conditioning section must consider the operating characteristics of the data logger to maximize the measurement resolution. That is the voltage corresponding to the maximum height of the water column (100 cm) must be +30V.
… 
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12
Instrumentation for Measurement of
Laboratory and In-Situ Soil Hydraulic
Conductivity Properties
Jose Antonio Gutierrez Gnecchi et al.
*
1
Instituto Tecnológico de Morelia, Departamento de Ingeniería Electrónica
México
1. Introduction
Measurement of soil hydraulic conductivity properties is very important for soil
characterization, modelling of water transport and waste contaminant migration through
soil, management of soil organic matter and management of water resources. Moreover,
measurement of hydraulic properties is also important for developing strategies to increase
crop productivity, and 3-D modelling of water migration properties to predict groundwater
and aquifer recharge. Amongst the most common methods, used in laboratory and field test
trials to determine the properties of water propagation through soil, are measurement of
hydraulic conductivity and wetting front detection. However since the hydraulic
conductivity properties vary considerably from region to region (and even for the same
region and type of soil) numerous and diverse methods are continuously reported that fit
particular needs. Despite the large number of methods and apparatus reported, and
commercially available instruments for measuring the dynamics of water propagation
through the soil, it is still necessary to continue developing new and improved
instrumentation systems to increase the quality and quantity of reliable information and
reduce systematic errors. In addition, commercial instruments may only be available from
foreign distributors. Thus the use of imported technology, with little or no technical support
locally, and the added import tax costs result prohibitive for the average producer and
precludes the use of electronics instrumentation by producers without a technical
background. Since 77% of the water in Mexico is used in agriculture, the availability is
scarce in many wide areas, and the water usage efficiency is low, the situation becomes
more critical due to the demand for increased productivity. Undoubtedly, research and
development activities in higher education institutions should have scientific, technological,
social and economical impact in the surroundings. This chapter presents the results of the
cooperation between ITM-Electronics Engineering Department (Spanish: Instituto Tecnológico
de Morelia), INIRENA-Research Centre for Natural Resources Studies (Spanish: Instituto
*
Alberto Gómez-Tagle (Jr)
2
, Philippe Lobit
3
, Adriana Téllez Anguiano
1
, Arturo Méndez Patiño
1
,
Gerardo Marx Chávez Campos
1
and Fernando Landeros Paramo
1
1
Instituto Tecnológico de Morelia, Departamento de Ingeniería Electrónica, México
2
Instituto de Investigaciones Sobre Los Recursos Naturales, Laboratorio de Suelos, Michoacán, México.
3
Instituto De Investigaciones Agropecuarias y Forestales, México
Hydraulic Conductivity – Issues, Determination and Applications
226
Nacional de Investigación Sobre Los Recursos Naturales) and IIAF (Spanish: Instituto de
Investigaciones Agropecuarias y Forestales) to develop instrumentation for measuring some of
the properties that govern the dynamics of water propagation through soil.
1.1 Water usage in Mexico
Water resources in Mexico are considered essential for national security. Urban, industrial
and agricultural conservation of the environment, economic and social development depend
on the rational management of water resources. In Mexico, the surface dedicated to
agriculture is approximately 21 million hectare (abbreviation ha) (10.5% of the national
territory) of which 6.46 million ha are irrigated zones and 14.5 million ha are rainfed zones.
Most of the fresh water resources are dedicated to agriculture, where the 77% is allocated for
consumptive use (Table 1).
USE
ORIGIN
TOTAL
VOLUMEN
PERCENTAGE
OF
EXTRACTION
SUPERFICIAL SUBTERRANEAN
Agriculture
1
40.7 20.5 61.2 76.8
Public Water
Supply
2
4.2 7.0 11.2 14.0
Self sustained
industry
3
1.6 1.6 3.3 4.1
Thermoelectric 3.6 0.4 4.1 5.1
TOTAL 50.2 29.5 79.8 100.0
1 km³ = 1 000 hm³ = 1 thousand of millions of m³.
Data correspond to volume allocated through to December 31 of 2008
1
Includes agriculture, livestock, aquaculture, and other, according to the REPDA-CNA (Public Rights
Register of Water- National Water Commission) classification. Includes 1.30 km³ of water
corresponding to irrigation districts pending registration.
2
Includes urban public and domestic uses according to the REPDA-CNA classification.
3
Includes industrial agro industrial, commerce and services according to the REPDA-CNA
classification.
Source: National Water Commission (CONAGUA: http://www.cna.gob.mx).
Table 1. Consumptive use of water in Mexico according to the source of origin. (Thousands
of millions of cubic metres, km
3
)
The Free Trade Agreement of North America and the globalization of markets and the
economy, impose more demands on Mexican producers to increase efficiency and quality of
agricultural production, optimizing the use of resources in a sustainable manner. Now it is
necessary to produce more, with better quality and lower costs to meet local demand,
compete with imported agricultural products and eventually to produce products that meet
the quality standards that exist in international markets (weight, size, color and texture). As
part of Mexico’s National Water Program 2007-2010 (Mexican National Water Commission,
Spanish: Comision Nacional del Agua [CONAGUA], 2008) it is proposed that the use of
technology for irrigation modernization would increase water productivity by 2.8%
annually, measured in kilograms per cubic meter of water used in irrigation districts, going
from 1.41 in 2006 to 1.66 in 2012, and will result in greater benefit to producers. At the same
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
227
time, it is also proposed that the reduction of energy consumption will lead to achieve a
more efficient use of water. However, it is common that the term "technification" generally
corresponds to hydraulic infrastructure for drainage of surplus water. Although it has been
reported (Mexican National Water Commission, Spanish: Comision Nacional del Agua
[CONAGUA], 2010) that technification of agriculture has increased by 50% nationwide,
compared to 2000, the reports do not specify what level or type of modernization is done,
and generally consists of pumping equipment and/or hydraulic installations for the
evacuation of excess water. To a lesser extent, the use of agro-meteorological stations is also
included as part of efforts to introduce technology to the field. However, it is necessary to
increase the level of modernization of the Mexican countryside in order to achieve precision
agriculture practices at regional and national levels.
1.2 Soil hydraulic conductivity
Soil water infiltration is a process by which water propagates from the soil surface, inwards,
through the porous media. One of the properties that govern the rate of propagation of
water through the soil is hydraulic conductivity, which in turn, depends on a number of
factors such as soil content and texture (Das Gupta et al., 2006), vegetation root hardness
(Rachman et al., 2004; Seobi et al., 2005), soil preparation (Park & Smucker, 2005), chemical
content (Schwartz & Evett, 2003), soil temperature and weather conditions (Prunty & Bell,
2005; Chunye et al., 2003), stability and continuity of the porous system (Soracco, 2003),
including macro (Mbagwu, 1995), meso (Bodinayake et al., 2004) and microporosity (Eynard
et al., 2004). Amognst the methods reported for studying the hydraulic properties of soils,
the infiltrometer and permeameter are probably the most commonly used devices in field
(Angulo-Jaramillo et al., 2000) and laboratory tests (Johnson et al. , 2005) respectively. Other
methods used for characterizing soil hydraulic properties reported are heat-pulse soil water
flux density measurements (Kluitenberg, 2001), electromagnetic measurements (Dudley et
al., 2003; Seyfried & Murdock, 2004), radiation-based measurements (Simpson, 2006)
image analysis (Gimmi & Ursino, 2004) and multimodal instruments (Pedro Vaz et al.,
2001; Schwartz & Evett, 2003) that permit measurement of several variables
simultaneously. The infiltrometer is a very popular instrument among researchers (Fig.
1A), because knowledge of soil hydraulic properties is a key factor in understanding their
impact on hydrological processes such as infiltration (Esteves et al., 2005) the superficial
flow and aquifer recharge. Basic infiltrometers are relatively simple devices, which
essentially consist of a reservoir (fitted with a graduated scale), a metallic ring (single or
double) partially inserted into the soil, and a stop valve. A test is conducted by allowing
the liquid to exit the container, either directly or through a pipe into the ring, measuring
the rate of water infiltration while maintaining a small positive pressure on the fluid. The
infiltration process consists of two main parts: the transient and steady state (Fig. 1B). The
transient state occurs from the beginning of the experiment up to the time when a
constant rate of water infiltration is attained.
Once the soil sample is saturated with water, the constant pressure maintains a constant
infiltration rate. Hydraulic conductivity can then be calculated using the entire data set
(Wu1 method) (Wu & Pan, 1997) or the data corresponding to the steady state phase (Wu2
method) (Wu & Pan 1999) by measuring the slope of the resulting curve. However,
recording the infiltration process data from direct, visual measurements is a highly
demanding task, both, in time and economic resources; data has to be recorded in time
intervals between 1 to 5 minutes in elapsed times ranging from 0.5 to 4 hours. Many authors
Hydraulic Conductivity – Issues, Determination and Applications
228
Fig. 1. A) Schematic diagram of an infiltrometer. I) Main reservoir (Mariotte), II) rubber
stoppers, III) bubbling tubing, IV) water outlet, V) stop valve, VI) metallic ring, VII) purge
and measurement of hydraulic load, VIII) stand base, IX) constant hydraulic load, X)
insertion depth, XI) pressure sensor access to air chamber, XII) data logger, XII) pressure
sensor access to the water column. B) Classical infiltration data results showing the transient
and steady state phases of the infiltration process.
acknowledge the need for instrumentation and data recording devices (infiltrometer and
permeameter) to automate the data acquisition, minimize human errors and reduce the time
spent in taking measurements (Amezketa-Lizarraga et al., 2002; Johnson et al., 2005). Therefore
many devices have been reported and patented since the 1940s (Bull, 1949) to try to automate
the data acquisition process. Automated devices rely on data recording units (data loggers).
However the main constraint is the local availability of such equipment, followed by cost. In
many cases researchers implement their own devices without automation, such as a double
ring infiltrometer (Carlon-Allende, 2006). Automating an infiltrometer requires accurate
measurement of the change of height of the water column over time, as the water is allowed to
exit the container. Some of the methods used for measuring the column height are the use of
paired infrared sensors in a plastic cylinder (Wilson et al., 2000), float valve system with meter
spool ring infiltrometer (Amezketa-Lizárraga et al., 2002), Time domain reflectometry (TDR)
infrared detectors and float sensor or pressure sensors (Ankeny et al., 1988). The use of
pressure transducers is probably the most common choice because of low-cost, simplicity, easy
implementation and reliability. Overman et al. (1968) reported the application of pressure
transducers since the mid 60s, to implement a variable load laboratory infiltrometer, designed
specifically for low-permeability materials. Constanz & Murphy (1987) generated a system
that could measure the height of a column of water from pressure changes in a Mariotte
reservoir and thus infer the infiltration data. Their instrument used Transamerica CEC 4-312
pressure transducers, with pressure range ± 12.5 psi. The automated device allowed rapid data
acquisition with minimal supervision. Ankeny et al. (1988) reported that the use of one
transducer produced measurement errors due to bubbling inside the container and adapted
the design of Constanz & Murphy (1987) to a tension infiltrometer (disc) with two PX-136
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
229
transducers with measurement range 0-5 PSI (Omega Engineering, Stanford, CT) and a 21X
Campbell data logger (Campbell Scientific, Inc.). The resulting scheme using two transducers
required precise timing, but minimized the variability generated by bubbling and reduced the
standard deviation from 6.2 mm (single transducer) to 2.2 mm. Prieksat et al. (1992) used the
two-transducer design in a single ring infiltrometer, to register data from multiple locations
simultaneously, facilitating the characterization process. Casey and Derby (2002) used a
differential pressure sensor (PX26-001DV, Omega Engineering, Stanford, CT) and evaluated
the device in the field, achieving 0.05 mm standard deviation. The authors noted that the
improvement in resolution might not change significantly the estimation of soil hydraulic
properties, but could be useful when data are processed as exponential relations methods such
as Ankeny (1992) or Reynolds and Elrick, (1991). Johnson et al. (2005) constructed six
laboratory variable load permeameters, using pressure sensors PX236 (Omega Engineering)
and perspex tubes, to work with undisturbed samples, using a data logger programmed to
record readings at regular intervals. The comparison with the manual method showed no
significant differences for texture analysis. Špongrová (2006), designed, built and tested a fully
automated tension infiltrometer, that included both the measurement of water level and the
control of the voltages applied, using a Honeywell differential pressure transducer with range
0 to 5 PSI (0 to 34.4 kPa), connected to a Campbell 21X datalogger (Campbell Scientific Inc.)
and a laptop. The results showed that the equipment reduced the monitoring time, increasing
the number of test trails per day. Although there are several commercial devices, such as
manual or automated tension infiltrometers they generally depend on external data logger units.
2. Case study 1: Automated infiltrometer using a commercial data logger
One of the preferred methods for measurement the height changes of the water column
involves the use of pressure transducers. Therefore, it is necessary to implement an
instrumentation and data acquisition system that can be used to gather information of the
infiltration process for off-line signal processing. Fig. 2 shows the classic data acquisition
scheme used. The instrumentation scheme consists of a pressure transducer, a signal
conditioning and amplifier stage, and a digitizing unit with data storage capabilities for
transferring the measurements to a host computer. To allow some level of autonomy, and
ease of use, it is necessary to use low-power, versatile analogue and digital devices.
Fig. 2. Block diagram of the instrumentation and data acquisition system used for
automating soil water infiltration measurements. A) Pressure sensor, B) instrumentation
amplifier, C) data logger including analogue to digital converter, internal memory, interface
circuitry and power supply for transferring data to D) a PC. E) Power supply circuit for the
analogue electronics section.
Hydraulic Conductivity – Issues, Determination and Applications
230
Fortunately, the advances in electronics technology over the last two decades have resulted
in a number of components that can be obtained from local and international distributors to
build the analogue and signal conditioning circuitry. As to the digitizing section, a number
of data logger units are commercially available with impressive operating characteristics.
The case study presented in this section is based on the choice of a low-cost data acquisition
unit.
2.1 Data logger
One particular low-cost, simple-to-use device is the EL-USB-3 data logger from Lascar
Electronics (Fig. 3).
Fig. 3. EL-USB-3 Voltage data logger from Lascar Electronics. A) Physical dimensions and B)
typical connection for logging sensor signals. Reproduced with permission. Copyright 2011
Lascar Electronics. All Rights Reserved from Lascar Electronics.
The EL-USB-3 is a stand-alone data logger powered internally by a 3.6 Volts battery, capable
of taking 32,510 readings in the range of 0-30 Volts with 50 mV resolution. The signals are
applied to the data logger through a detachable cap, so that it can be removed from the
instrumentation electronics for programming and data transfer without disconnecting
wires. The data logger includes a USB interface for setting the data acquisition sampling rate
from 1 second to 12 hours (1 second, 10 seconds, 1 minute, 5 minutes, 30 minutes, 1 hour, 6
hours and 12 hours) and also for transferring the results to a host PC. The operation can be
assessed by observing the activity of two LEDs, red and green, which are included (Fig. 4).
Once the test has concluded, the measured data can be transferred to a host PC for off-line
analysis through the USB interface using the software included. Thus the EL-USB-3 includes
all the necessary components shown in Fig. 2C corresponding to the digitizing section of
the instrumentation scheme proposed. Using a commercial data logger reduces
instrumentation development time. Nevertheless, the signal conditioning section must
consider the operating characteristics of the data logger to maximize the measurement
resolution. That is the voltage corresponding to the maximum height of the water column
(100 cm) must be +30V.
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
231
Fig. 4. Data logger USB connector and operation assessment depending on the LED activity.
Reproduced with permission. Copyright 2011 Lascar Electronics. All Rights Reserved
2.2 Power supply
The first step in developing the instrumentation circuitry consists of obtaining a little over
+30V from a +9V battery, because interfacing with the data logger requires that the analogue
instrumentation operate with a voltage slightly over 30V. The circuit must be small and
must consume very little current from the battery. Fig. 5 shows the block diagram of the
power supply.
Fig. 5. Block diagram of the power supply. A) The battery feeds a linear voltage regulator. B)
the output from the regulator is increased to (over) +30 V to power up the instrumentation
amplifier.
The battery feeds a low dropout, adjustable linear regulator (TPS7101 from Texas
Instruments©) which provides the regulated supply voltage to the DC/DC converter (Fig. 6).
Hydraulic Conductivity – Issues, Determination and Applications
232
Fig. 6. A) Description of the TPS7101 (linear low dropout regulator) terminals. B)
Identification of the index area (terminal 1). C) Regulator diagram. Copyright Texas
Instruments©. All Rights Reserved.
The TPS7101 output voltage is adjusted to +8.3V by trimming R1 which is a 1 MegaOhm, 20
turns, trimming potentiometer. R2 is a fixed value ¼ Watt precision, metal film resistor. C8
is a tantalum capacitor, and is used for filtering the output and provides stability to the
voltage regulator. The TPS7101's output feeds the DC/DC converter formed by the two
ICL7660 integrated circuits from Intersil © (U2 and U3) (Fig. 7B). The ICL7660 is a low-
power monolithic CMOS power supply that can be configured easily to double the input
voltage and also to provide complimentary negative voltage, requiring a minimal amount of
non-critical passive components. In this application, U2 is configured to perform two
operations: U2 inverts the input voltage and also doubles the positive input. The circuit
uses low forward-voltage-drop Schottky diodes to reduce the effect of the voltage drop
across the circuit. Another feature of the ICL7660 is that it can be cascaded to increase the
differential voltage. Thus, U3 is configured to double the negative voltage obtained from U2.
In effect, the array of U2 and U3 increases the regulated voltage from the TPS7101 to
approximately 31 Volts, enough to provide the energy for the instrumentation amplifier and
pressure sensor.
Fig. 7. A) Pinout of the ICL7660 CMOS converter. B) circuit diagram used to increase the
TPS7101’s output to ~+31V. Reproduced with permission. Copyright 2011 Intersil Americas
Inc. All Rights Reserved
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
233
Typical current consumption values for the ICL 7660 are 80 µA (ICL7660A) which makes it
suitable for this and other battery powered applications. In order to maintain the high
efficiency of the CMOS voltage converters it is necessary to reduce the current consumption;
therefore the analogue instrumentation must also be a low-power circuit.
2.3 Pressure transducer
The water reservoir is built using an 80 – 100 cm perspex pipe with rubber stoppers on each
end. The pressure at the bottom, when the container is full (100 cm H
2
O @ 4
o
C) is 9.806 kPa.
Therefore, it is necessary to use a differential pressure transducer with 10 kPa measurement
range (Fig. 8).
Fig. 8. MPX2010 DP differential pressure transducer A) physical model and B) connections.
Reproduced with permission. Copyright Freescale Semiconductor, Inc. 2004 - 2011. All
Rights Reserved.
The MPX2010DP differential pressure transducer from Freescale Semiconductor© is a
temperature-compensated piezoresistive pressure sensor which provides a very accurate
and linear output voltage proportional to the applied pressure in the range of 0 – 10 kPa.
The recommended voltage supply is 10V, and the error and linearity figures are specified
for 10V. However, the MPX2010DP is a ratiometric device; that is the maximum output
voltage depends on the reference voltage which means that a different supply may be used
so long as the reference voltage is very stable.
2.3.1 Pressure sensor reference voltage
Recalling that the MPX201DP pressure transducer is a ratiometric device, it is necessary to
provide a highly-stable voltage reference signal to achieve correct operation, regardless of
voltage and temperature variations.
It is common to find circuits that suggest the use of the voltage supply line to power up the
pressure transducer (Fig. 9A). Unless the pressure transducer includes an internal voltage
reference supply (i. e. it is a voltage compensated device), it is necessary to use a dedicated
voltage reference circuit. (Fig. 9B). In terms of temperature variations, voltage reference
circuits are specified in ppm/
o
C (parts per million per degree centigrade). Consider a
reference circuit specified to change at a rate of 100ppm/
o
C. If the circuit output value is
10V @ 20
o
C, exposing the integrated circuit to a 50
o
C would change the output from 10V to
10.03 Volts ensuring the correct operation of the transducer. Some devices may also be
specified to 10ppm/
o
C increasing the stability of the overall instrumentation circuit.
Hydraulic Conductivity – Issues, Determination and Applications
234
Fig. 9. A) Incorrect voltage supply for a bridge-type sensor. B) The correct use of bridge-type
sensor involves the use of a reference voltage circuit.
2.4 Instrumentation amplifier
Instrumentation amplifiers are a type of differential amplifier with high input impedance
and adjustable gain that constitute essential building blocks in analogue electronics. One of
the classical configurations of instrumentation amplifiers uses three operational amplifiers
to form a two-stage amplifying circuit (Figure 10).
Fig. 10. A) A commercially available quad op-amp can be used to build B) a general-purpose
instrumentation amplifier.
The differential input signal feeds the first amplifying stage formed by op-amp 1 and op-
amp 2. The output between both op-amp 1 and op-amp 2 (Va and Vb respectively) is
differential weighted version of the input voltage. The gain of stage one, G1 (1):
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
235
21
11
2
R
G
R
=+
(1)
can be adjusted using a single trimming potentiomenter, R2. The differential output from
stage one, enters a third operational amplifier (op-amp 3) configured as subtracting
amplifier with gain, G2, (2):
4
2
3
R
G
R
=
(2)
The overall output is then a single-ended version of the differential input voltage. The
overall gain of the instrumentation amplifier circuit is the multiplication of both amplifying
stages given by (3):
21 4
12 1
23
TOTAL
RR
GGG
RR

==+


(3)
The importance and usefulness of instrumentation amplifiers have resulted in multiple
versatile commercial integrated circuits, with impressive operating characteristics, that
allow gain adjustment using a single variable resistor and/or digital signals. One particular
integrated circuit that is suitable for portable applications is the INA125 from Texas
Instruments© (Fig. 11).
Fig. 11. A) Texas Instruments INA125 instrumentation amplifier pinout, B) typical
application circuit and C) gain selection table. Copyright © 2009, Texas Instruments
Incorporated. All Rights Reserved.
The INA125 also incorporates a selectable voltage reference circuit, which will be used to
supply the reference voltage to the pressure transducer (Fig. 12). The INA125 is a low power
Hydraulic Conductivity – Issues, Determination and Applications
236
device (quiescent current 460 µA) and can operate over a wide range of voltages from a
single power supply (2.7V to 36V) or dual supply (±1.35V to ±18V), which makes it suitable
for battery powered applications.
Fig. 12. Circuit diagram of the instrumentation amplifier and voltage reference circuit for the
pressure sensor.
The voltage reference value can be adjusted using the jumper J5, to provide 10V, 5V, 2.5 V or
1.24V.
2.5 Complete instrumentation circuit
The complete circuit is shown in Figure 13.
The design includes an on-off switch and connectors to calibrate and monitor the output
voltage using a multimeter.
2.6 Ring and reservoir
The size of both the reservoir and the ring may differ depending on the type of soil to be
analysed. In addition, the hydraulic conductivity properties of soil vary throughout the test
field, and thus a large metallic ring may be used to investigate a large area as much as possible.
Analysis of sandy soils may require a larger reservoir compared to clay type soils, because
coarse materials have higher hydraulic conductivity and require a larger amount of water to
reach the saturated steady state, compared to fine particle soils. In laboratory test trials, it is of
little concern the handling of a large reservoir, a heavy metallic ring, computers and electronics
instrumentation, and there is tap water available nearby. However, carrying all the necessary
materials in field tests may be a difficult task. Therefore the size of the reservoir and metallic
ring is a compromise. The infiltrometer described here uses a 1 metre long, 6.35 cm diameter
perpex pipe; the ring is made of an iron pipe (8.0 cm long and 8.8 cm diameter) .
2.7 Infiltrometer assembly
Fig. 14 shows the infiltrometer design and assembly, including instrumentation circuitry.
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
237
V_OUT
-7.88V
+
C6
10uF@25V
+
C1
10uF@25V
J11
CON4
1
2
3
4
GND_PWR
Switch mode power supply. U2 doubles the input voltage and also provides a
negative voltage with similar magnitude to the input voltage. U3 doubles the
negative voltage from U2. The total output is above + 30 volts, used to power the
instrumentation amplifier.
Low droput
regulator
TPS7101QP (DIP).
The maximum
input voltage +Vin
is 11 Volts. The
output voltage
depends on the
choice of resistors
R1 and R2. Adjust
R1 to obtain +8V to
+ 8.3V
J7
GND
1
REFERENCE OUTPUT
VOLTAGE TO
PRESSURE SENSOR AND
INPUT FROM PRESSURE
SENSOR
GND
J6
VOUT
1
J!4
CON4
1
2
3
4
V+
+VCC
VrefOUT
GND_PWR
J5: SELECT
THE
SENSOR
EXCITATION
VOLTAGE
+
C4
10uF@25V
+9.2V
V-
GND_PWR
+
C8
10uF
J9
GND
1
V+
R2
120K
+Vin
GND_PWR
J10
CON4
1
2
3
4
+8.3V
GND
J5
VREF_SELECT
1
3
5
7
2
4
6
8
J3
SW_A
1
COMP
COMP
U1
SENSOR_PRES_DIFF
1
2
3
4
GND
V+
+VCC
V-
J1
GND_PWR
1
J8
VOUT
1
V-
+VCC
GND_PWR
J2
+V_battery
1
+8.3V
VrefOUT
+
C2
10uF@25V
C7
0.1U
J13
CON4
1
2
3
4
GND_PWR
VrefOUT
+
C5
10uF@25V
+
C3
10uF@25V
-Vin
U3
ICL7660CPA
1
2
3
4 5
6
7
8
NC
CAP+
GND
CAP- VOUT
LV
OSC
+VIN
+8.3V
+Vin
+VCC
GND_PWR
D1
1N5819M/CYL
R4 50K
+Vin
R1
1MEG
+8.3V
D2
1N5819M/CYL
POWER SUPPLY & INSTRUMENTATION AMPLIFIER 01
04
INFILTROMETER ANALOGUE INSTRUMENTATION
11Wednesday, March 23, 2011
Title
Size Document Number Rev
Date: Sheet
of
U2
ICL7660CPA
1
2
3
4 5
6
7
8
NC
CAP+
GND
CAP- VOUT
LV
OSC
+VIN
R5 10
R3
1K
GND_PWR
Battery
+9V or +9.6V
GND_PWR
INSTRUMENTATION
AMPLIFIER
+Vin
-Vin
U1
TPS7101QP
1
2
3
4
8
5
6
7
GND
EN
IN
IN
PG
OUT
OUT
FB
V-
PRESSURE SENSOR
CONNECTIONS
(AT THE BOTTOM
OF THE
MARRIOTE)
-Vin
ON-OFF
SWITCH
1P1T (SW1)
IC4
INA125
1
2
3
12
13
14
15
16
4
6
9
8
7
10
11
5
+V
SLEEP
GND/-VCC
VrefCOM
VrefBG
Vref2.5
Vref5
Vref10
VrefOUT
Vin+
RG_A
RG_B
Vin-
VO
SENSE
IA_ref
GND
V+
GND
GND
+VCC
J4
SW_B
1
GND
+Vin
+VCC
VrefOUT
-7.88V
NO
CONNECT
Fig. 13. Instrumentation circuit for measuring infiltration data using a pressure transducer.
Fig. 14. A) Designed infiltrometer. B) close-up of the pressure sensor assembly. C) The
single- sided circuit is fitted into a small (2.6” X 2.2”) printed circuit board fits into the
plastic enclosure.
Hydraulic Conductivity – Issues, Determination and Applications
238
The pressure sensor is installed at the bottom of the reservoir and soldered into a small
printed circuit board to ensure the correct connectivity between the sensor and the
instrumentation circuitry. The instrumentation circuit is installed, outside the infiltrometer,
inside a small plastic enclosure, as well as the battery and data logger. Once the plastic
enclosure is attached to the infiltrometer, the data logger can be removed from the
electronics without removing any connections. The result is a compact versatile
infiltrometer, which can easily be transported for field tests.
2.8 Test results
The infiltrometer was tested in two different test locations around the Cuitzeo Lake
watershed (19º58' N, 101º08' W): sandy loam (Fig. 15A) and sandy soil (Fig. 15B).
Fig. 15. Comparison of test results using the automated infiltrometer vs visual
measurements for A) sandy loam and B) sandy soils.
The results from the automated measurement data acquisition system are much more
consistent throughout the test, improving the quality of information compared to visual
observations. Table 2 shows a summary of hydraulic conductivity results, using the Wu2
method and data from the steady state region.
Type of
Soil
Measurement
Method
Average Hydraulic
conductivity K
Fs
(mm/hr)
Standard
Deviation
Number of test
trials N
Sandy loam
Automated
infiltrometer
49.65 21.72 7
Visual
Observations
68.31 42.85 7
Sandy soil
Automated
infiltrometer
2282.2 429.98 5
Visual
Observations
1671.82 793.56 5
Table 2. Summary of the hydraulic conductivity results obtained from automated
measurements and visual observations.
The automated infiltrometer can produce more reliable information than that obtained using
visual measurements. For instance the standard deviation obtained from automatic data is
smaller compared to visual observation. The resolution for a 1 metre water column is 1.66
mm approximately. The equipment presented in this case study is considerably easy to
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
239
implement, low cost and can be used for laboratory and field test trials alike. Nevertheless,
it still relies on the use of commercial data loggers. Alternatively, a dedicated data logger
based on an ultra-low power microcontroller can be used to allow reviewing the data on-site
and also to store the results of multiple test trials.
3. Case study 2: Automated infiltrometer using a dedicated data logger
Using a commercial data logger and relatively simple analogue electronics allows rapid
development of test prototypes. On the other hand, since the determination of hydraulic
conductivity infiltration requires performing multiple tests, it is desirable that all the results
are stored in non-volatile memory without having to transfer the results to the host PC,
immediately after each test has been concluded. Higher resolution may also be required for
correct in-situ characterization of different types of soils. Moreover, since measurements are
not taken continuously (i. e. the lowest sampling rate may be 1 second) it may be desirable
to be able to shutdown the analogue circuitry in between measurements to extend battery
life. Fig. 16 shows the schematic diagram of the proposed data acquisition system.
Fig. 16. Schematic diagram of a data acquisition system based on a low power
microcontroller, specially designed for hydraulic infiltration measurements.
3.1 Dedicate data logger operation description
The equipment follows the same design philosophy for case study 1. The measurement
system is based on the MPX2010DP pressure sensor, and the INA125 instrumentation
amplifier is used to provide the reference voltage and measure the differential output from
the transducer. However, the digitizing section is now based on a microcontroller.
3.1.1 Choosing the microcontroller
Several powerful microcontrollers are available from multiple companies that can be used to
perform all the necessary data acquisition and signal processing operations. One
particularly useful family of powerful microcontrollers suitable for low power operation is
Hydraulic Conductivity – Issues, Determination and Applications
240
the MSP430 series from Texas Instruments© (Fig. 17). Case study 2 is based on
MSP430F149IPAG microcontroller from Texas Instruments©. The MSP430 is a 16-bit RISC,
ultra-low-power device with five power-saving modes, two built-in 16-bit timers, a fast 12-
bit A/D converter, two universal serial synchronous/asynchronous communication
interfaces (USART), 48 Input/Output pins, 60 kB of flash memory and 2 kB of RAM, which
permits the implementation of all the functions required to build the data acquisition
system. Initially, it was considered that basic signal processing algorithms (digital filter) are
the main functions to be included. However, a JTAG interface implemented on the
prototype allows in-system programming so that the equipment can be updated, and
further signal processing algorithms can be included in the future, without changing the
hardware.
TXD0
JTAG_RESET
LCD_RS
LCD6
BOTON4
+3.3V
LCD1
ERROR
JTAG1
OPCIONAL
LCD4
+3.3V
OPER_RESET
LCD3
JTAG3
LCD[0..7]
LCD_RS
J7
CON1
1
LCD7
SLEEP_INA
RESET
BOTON0
LCD_EN
RXD0
SALIDA VREF
LCD2
+3.3V
V_INF
GND
GND
LCD4
ENTRADA VREF
JTAG2
OPER_RESET
LCD0
JTAG0
P6_ANA1
BOTON2
BOTON3
GND
LCD[0..7]
Y1
XTA L
+3.3V
LCD5
R2
47K
SLEEP_MAX3233
PROGRAMAR
+3.3V
BOTON5
J6
CON6A
1
3
5
2
4
6
DATA LOGGER 02 DE 04 01
MICROCONTROLADOR CON IN-SYSTEM PROGRAMMING
A
24Tuesday, December 18, 2007
Tit le
Size Document Number Rev
Date: Sheet
of
OPERACION
NORMAL
JTAG[0..3]
R3
5K
VCTRL_LCD
+3.3V
GND
BOTON0
BOTON4
JTAG3
BOTON6
BOTON1
SLEEP_INA
BOTON7
59
4
41
1
8
18
52
11
10
45
2
37
14
56
54
25
23
55
20
58
22
29
40
60
5
28
24
17
21
43
9
36
13
62
44
48
3
57
46
47
51
34
32
39
50
49
63
31
7
30 61
12
6
42
35
33
19
26
27
64
38
53
15
16
P6.0/A0
P6.5/A5
P4.5/TB5
DV_CC
XI N
P1.6/TA1
XT2 OU T
V_REF-/VE_REF-
VE_REF+
P5.1/SIMO1
P6.3/A3
P4.1/TB1
P1.2/TA1
TMS
TDO/TD I
P2.5/ROSC
P2.3/CA0/TA1
TDI
P2.0/ACLK
RST*/NMI
P2.2/CAOUT/TA0
P3.1/SIMO0
P4.4/TB4
P6.1/A1
P6.6/A6
P3.0/STE0
P2.4/CA1/TA2
P1.5/TA0
P2.1/TAINCLK
P4.7/TBCLK
XOUT/TCLK
P4.0/TB0
P1.1/TA0
AV_SS
P5.0/STE1
P5.4/MCLK
P6.4/A4
TCK
P5.2/SOMI1
P5.3/UCLK1
P5.7/TBOUTH
P3.6/UTXD1
P3.4/UTXD0
P4.3/TB3
P5.6/ACLK
P5.5/SMCLK
DV_SS
P3.3/UCLK0
V_REF+
P3.2/SOMI0 P6.2/A2
P1.0/TACLK
P6.7/A7
P4.6/TB6
P3.7/URXD1
P3.5/URXD0
P1.7/TA2
P2.6/ADC12CLK
P2.7/TA0
AV_CC
P4.2/TB2
XT2 I N
P1.3/TA2
P1.4/SMCLK
CONECTOR PARA LCD
BOTON1
GND
BOTON6
V_INF
P6_ANA0
J4
JTAG
1
3
5
7
9
11
13
2
4
6
8
10
12
14
BOTON2
GND
GND
J1
CON4A
1
3
2
4
J3
JUMPER
1
3
5
2
4
6
LCD7
LCD_EN
GND
LCD[0..7]
J2
CON2
1
2
LCD2
C9
10n
LCD6
INTERFASE JTAG
JTAG0
BOTON7
RESET
SLEEP_MAX3233
BOTON5
JTAG_RESET
Extra (LED)
RXD0
BOTON3
P6_ANA2
J5
LCD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
LCD5
ERROR
J8
CON1
1
LCD0
+3.3V
LCD1
C5
100nF
RST/NMI
GND
LCD3
V_REF_1_25
TXD0
JTAG1
JTAG2
C6
100nF
LCD[0..7]
Fig. 17. Schematic diagram of the data logger based on the MSP430F148IPAG.
3.2 Operation of the data logger
The microcontroller interfaces with the user through a keyboard and LCD display, thus
allowing the operation of the device in test fields, and reviewing the measured information in
real time or right after the test has concluded. The microcontroller controls the data acquisition
process, and stores each measurement in non-volatile flash memory. The microcontroller shuts
down the analogue circuit in between samples to save battery power and enters a low-power
mode. Prior to taking each sample, the microcontroller turns on the analogue circuit and waits
100 ms to allow the analogue output to settle and take a stable measurement. The data logger
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
241
unit uses a 9V battery but it can also operate with a supply voltage as low as 4V. The MSP430
itself operates with 3.3V, so the same voltage is used throughout the circuit. The INA125 is
very useful in this case, because it can operate with a voltage as low as 2.7V. The INA125’s
internal reference voltage circuitry requires that the power supply voltage is, at least, 1.25 V
above the desired reference voltage and thus only the +1.24 V reference option can be used.
The MSP430 internal voltage reference is adjusted to 2.5 volts, and the INA125 is adjusted to
output 2.5V when the water column is full. In addition to accuracy, versatility, and
compactness, it is necessary that the equipment can operate in low power mode to increase
battery life. Therefore the MSP430 records data at fixed, programmable intervals, from 1
second, and then 10 seconds steps up to 60 minutes, selected by the user prior to each test. A
real-time clock algorithm is implemented, using Timer A, so that the microcontroller can enter
energy saving mode LMP3 consuming 2μA approximately in between samples. During the
energy saving mode, the microcontroller also turns off the transducer voltage reference source,
instrumentation amplifier and display. 100 miliseconds before each measurement is taken, the
voltage reference source and instrumentation amplifier are activated, allowing the
measurement to settle. The user can select the LCD to remain off while taking measurements.
During operation, the LCD can also be activated temporarily to supervise the measured data,
and then switched off again. The results of each test are stored in the flash memory, starting at
memory block 0x3F. Before each test, the microcontroller detects which memory blocks are
used and starts saving data in the next empty block. Thus, up to 90 tests can be conducted in-
situ. The user can also select which memory block to erase, (i.e. which experiment) instead of
erasing the entire memory, also contributing to saving battery life.
3.3 Electronics instrumentation assembly
Fig. 18A shows the double-sided printed circuit board. The board, keyboard and display
and battery are fitted into a plastic enclosure (Fig 18B, 18C). In a similar manner to case
study one, the pressure transducer is located below the reservoir and the wires carrying the
voltage supply and signals are connected to the data logger.
Fig. 18. A) Data Logger Printed Circuit Board (PCB). B) The PCB, C) keyboard and display
and interface connections are also fitted in the plastic enclosure.
3.4 Transferring data for permanent storage and analysis
A C++ program interface was implemented to allow the user to transfer the data to a host
PC for permanent storage, off-line results visualization and analysis (Fig. 19). Prior to each
Hydraulic Conductivity – Issues, Determination and Applications
242
test the user can set the time, date and sampling rate for the experiment. The test
information is stored at the beginning of each memory block, followed by the column height
measurements. Once the experiment (or several experiments) has been completed, the user
can review the measured data in-situ. Alternatively the user can transfer the results to a host
PC, through the RS232 connection or with RS232-USB adaptors, to allow compatibility with
current PC configuration ports.
Fig. 19. A) The C++ software B) analyses the measurements using the Wu1 and Wu2
methods and C) plots the results.
The software processes the data and allows inspection of each value (Figure 18B). The
program calculates hydraulic conductivity using the WU1 and WU2 methods, thus allowing
result comparison.
3.5 Test results
The infiltrometer was tested in three different test locations around the Cuitzeo Lake
watershed (19º58' N, 101º08' W): clay, loam and sand (Table 3).
Site Wu1 Wu2 Guelph
Clay
Average Kfs 5.497 2.231 2.782
Standard deviation 8.163 3.185 2.584
Number of test trials, N 26 26 13
Loam
Average Kfs 79.551 150.401 95
Standard deviation 63.58 82.86 97.05
Number of test trials, N 36 36 3
Sand
Average Kfs 708.30 963.41 ----
Standard deviation 722.37 758.28 ----
Number of test trials, N 9 11 -----
Table 3. Summary of hydraulic conductivity tests using the automated infiltrometer.
Measurements where also obtained with a Guelph permeamenter, for comparison, except
for sandy soil, because it was not possible to reach the required depth to introduce the 50 x
Instrumentation for Measurement of Laboratory
and In-Situ Soil Hydraulic Conductivity Properties
243
120 mm probe due to sample collapse. A commercial Guelph permeameter is a constant-
head device, which also operates on the Mariotte siphon principle and allows simultaneous
measurement of field saturated hydraulic conductivity, matric flux potential, and soil
sorptivity in the field (Soilmoisture Equipment Corporation, Santa Barbara California, U. S.).
In this work the Guelph permeameter operates as a "benchmark methodology" and is not to
be considered it as the only valid acceptable method. Direct point-by-point comparison of
results using different test methods is not valid since data is taken from different locations.
In addition, the automated infiltrometer presented is limited to conduct tests at the surface,
so Kfs variations at other soil depths is out of reach, in contrast with the Guelph
permeameter, capable of measuring Kfs up to 80 cm depth without any special instruments.
Nevertheless, the device described in this case study allowed the estimation of field
saturated hydraulic conductivity in agreement with the Guelph permeameter in some cases.
4. Conclusion
The automated infiltrometers presented in this work, can produce reliable information
about the infiltration process in-situ, with little supervision. The devices also allow the
acquisition of a large number of measurements compared to visually obtained
information, thus facilitating the calculation of Ks. Case study one shows the use of low-
cost data loggers to automate the measurement process. A considerable simple
instrumentation circuit is necessary to obtain the maximum resolution from the data
logger. If a dedicated device is required, case study 2 shows the use of microcontroller
technology to build the data logger unit. The DAQ units allow sample time adjustment
on-site, which permits the investigation of different types of soils. The automated
infiltrometer offers a ~0.25mm column height measurement resolution improving the
quality of Ks calculations. Both cases present affordable and reliable instrumentation
solutions, that can be built for about $ 100 US dollars without considering development
time investment. Current and future work includes the development of a multi-channel
simultaneous sampling system, so that the test field can be correctly characterized using
multiple infiltrometers located around the test site.
5. Acknowledgment
The authors acknowledge the financial support from Public Education Secretariat–Mexico
(Spanish: SEP-Secretaría de Educación Pública) under grants SEP-DGEST 4328.11-P and
PROMEP ITMOR-CA1 103.5/11/1091 to carry out this work.
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... van Genuchten (1980); Durner (1994); Durner and Flühler (2006)). Hydraulic conductivity can be considered as an indispensable parameter for soil characterization, simulation of water and mass transport in vadose and saturated zone, management of soil organic matter and sustainable development of regional water resources (Gutierrez Gnecchi et al., 2011). ...
... Empirical methods based on systematic data collection are used as to correlate K s with soil properties like particle size, soil texture, pore size and relative effective porosity (Gootman et al., 2020;Gupta et al., 2020;Huang et al., 2019;Hwang et al., 2017;Picciafuoco et al., 2019;Vereecken et al., 2010Vereecken et al., , 1992Wösten et al., 1999). Experimental methods are distinguished in laboratory and field methods (Durner and Flühler, 2006;Gutierrez Gnecchi et al., 2011;Morbidelli et al., 2017). However, the adequacy and cost-effectiveness of these methods can often be a limiting factor in soil-water modeling applications. ...
... However, the adequacy and cost-effectiveness of these methods can often be a limiting factor in soil-water modeling applications. In this concept, although the number of methods and apparatuses used is quite large, it is still a need to elaborate new methodological approaches and instrumentation technologies to enhance the quality and quantity of reliable information and to provide alternative options to address the uncertainty of hydraulic parameters estimations (Gamie and De Smedt, 2018;Gutierrez Gnecchi et al., 2011;Zhang et al., 2007). A physical analogue for soil water movement can be modelled with the Hele-Shaw (Hele-Shaw, 1898) apparatus. ...
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The estimation of saturated hydraulic conductivity is a key parameter for studying the water flow in the unsaturated zone of the soil. In this work, a combined modelling approach of the waterfront movement is elaborated, integrating a physical analogue type device with image analysis. A customized Hele-Shaw apparatus is designed for the wetting process visual inspection, while an optical flow algorithm is used for the numerical calculation of flow velocity and saturated hydraulic conductivity values at a pixel level of the image processing. Saturated hydraulic conductivity values estimated by the proposed method are in close agreement with values found in the literature under similar soil conditions. Also, the proposed method seems to be a useful tool for a comprehensive analysis of the waterfront movement in the vadose zone.
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Permeability is a vital parameter for the design and construction of structures involving ocean engineering. Based on the steady-state heat transfer theory and Darcy's law, a novel in-situ test method for permeability in saturated sandy porous media is introduced in this work. This approach aims to obtain permeability through the inversion of the measured temperatures. Temperatures measuring device with a constant heater was installed in an insulating experimental tank filled with sandy sediments of different permeability. Further, a numerical model based on the Finite Element method was simulated to validate the feasibility of the proposed method and accuracy of the experimental data. Besides, the results obtained by the constant head test were compared with those calculated by the novel in-situ test method, considering different surface temperatures of the heater and different sediments’ permeability. It shows that the permeability obtained by in-situ method are reliable and accurate (the accuracy is within one order of magnitude) in both numerical simulations and experimental tests. The effects of different surface temperatures of the heater and permeability of porous media on permeability calculation results were also discussed. The surface temperature was found that has little influence on permeability. And the proposed method is applicable when the permeability is higher than 10⁻¹² m². The findings can provide some reference to the in-situ measurement of submarine sediments' permeability.
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Water infiltration into soil from a ring infiltrometer is an important issue for many scientific and engineering problems. Due to the complexity of the three-dimensional water flow, no generalized solutions are available to describe the infiltration process from a finite ponding source. This study applied a scaling technique to three-dimensional axisymmetric infiltration from a single-ring infiltrometer to obtain a generalized infiltration solution and evaluated the effects of soil conditions and ring geometry on the scaled infiltration curves. A numerical model based on Richards' equation in the cylindrical coordinates was developed to simulate infiltration processes for three representative soils. By properly choosing length- and time-scale factors, the infiltration curves of the three test soils representing different hydraulic properties were successfully scaled to essentially a single dimensionless infiltration curve. Tests showed that the dimensionless infiltration curve was not very sensitive to applied ponding head, ring diameter, ring insertion depth, and initial soil conditions. Finally a dimensionless infiltration equation in the form of i* = a + b/t(*0.3) was developed. The example showed that the generalized infiltration equation can predict the infiltration curve simulated by Richards' equation very well.
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Saturated hydraulic conductivity is a measure of the ability of a soil to transmit water and is one of the most important soil parameters. New single-ring infiltrometer methods that use a generalized solution to measure the field saturated hydraulic conductivity (K(s)) were developed and tested in this study. The K(s) values can be calculated either from the whole cumulative infiltration curve (Method 1) or from the steady-state part of the cumulative infiltration curve by using a correction factor (Method 2). Numerical evaluation showed that the K(s) values calculated from the simulated infiltration curves of representative soil textural types were in the range of 87 to 130% of the real K(s) values. Field infiltration tests were conducted on an Arlington fine sandy loam (coarse-loamy, mixed, thermic, Haplic Durixeralfs). The geometric means of the K(s) values calculated from the field-measured infiltration curves by Method 1 and Method 2 were not significantly different. The geometric mean of the K(s) calculated from the detached core samples, however, was about twice that of the K(s) calculated from the infiltration curves, which was consistent with earlier findings. Unlike the earlier approaches, Method 1 calculates K(s) values from the whole infiltration curve without assuming a fixed relationship (α = K(s)/φ(m)) between saturated hydraulic conductivity and matric flux potential φ(m).
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A falling head permeameter is described in which pairs of infrared emitters and detectors on a sight tube are used to measure the flow rate associated with the passage of water through a granular solid under the action of a diminishing pressure head. An equation relating pressure head to elapsed time is derived from which permeability may be calculated. In order to verify the accuracy and sensitivity of the instrument, permeability measurements carried out on a graded quartz sand are compared to those obtained by the more conventional constant head measurement. Excellent agreement is obtained between the permeability values obtained using both measurement methods. Experimental results are also reported for the measurement of the permeability of a range of sieved sand fractions. The falling head permeameter described here is particularly suitable for the measurement of the hydraulic conductivity of granular solids such as sands and soils through which a high flow rate may be expected.
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Information on the most important physical properties that influence the saturated hydraulic conductivity (Ks) of soils is useful in modelling water and solute movement during ponded infiltration and in estimating both temporal and spatial variation in Ks. In this study the Ks of 18 sites with different land use histories on a watershed in the Nsukka plains of southeastern Nigeria was determined and related to selected soil physical properties. The purpose was to develop a simple statistical model for estimating Ks from more easily determined properties and to evaluate how close Ks is to Philip's fitted soil water transmissivity (A) and measured steady (final) infiltration rate (Ic) (Philip, 1957). The saturated hydraulic conductivity correlated positively with total porosity (r2 = 0.182) and macroporosity (Pe), defined as pores with equivalent radius > 15 μm (r2 = 0.635) and negatively with bulk density (r2 = 0.533). Mesoporosity (i.e., pores with an equivalent radius of 1.5–15 μm) and microporosity (i.e., pores with an equivalent radius of 0.1–1.5 μm) also correlated negatively with Ks with respective r2 values of 0.275 and 0.100. The best fit model relating Ks to the soil physical properties was Ks = 0.07e0.08(Pe) (r2 = 0.946). With this model the threshold Pe value below which there is a drastic reduction in Ks lies between 15 and 20%. Using an independent data set to validate this model, predicted and measured Ks values were generally in good agreement. This model is valid for Pe values between 1.3 and 41.2% which is the common range in these upland soils. The values of Ic, Ks and A were statistically different (P ≤ 0.001) and varied in the order Ic > Ks > A, showing that the assumption that at long infiltration times these values are all approximately equal does not hold for these soils.
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ABSTRACT,trenches. Tracer studies with tensiometers, piezometers, and suction lysimeters have been used to identify sub- The majority of landscapes, natural or cultivated, are nonlevel. surface flow pathways in a steep watershed by Harr However, specifically designed instruments are not available for esti- (1977), Anderson et al. (1997) and Torres et al. (1998). mation of soil hydraulic properties in sloping landscapes. The objective These methods, however, are time-consuming, tedious of this study is to examine if tension and double-ring infiltrometers are suitable for determination of soil hydraulic properties on sloping to perform under field conditions, and sometimes re- soil surfaces. A field experiment was conducted in adraulic properties on sloping lands. Watson and Lux- moore,(1986) and Wilson and Luxmoore,(1988) used tension infiltrometers in conjunction,with double-ring T ension infiltrometers(Perroux and White, 1988) infiltrometers for measuring infiltration rates (hydraulic and double-ring infiltrometers (Bower, 1986) are use- conductivity) and water-conducting macro- and meso- ful instruments that offer a simple, fast, and convenient porosity in forest watersheds with slopes up to 20%. means of determining soil hydraulic properties based on,Using single-ring infiltrometers, Elliott and Efetha (1999) in situ infiltration measurements at the soil surface. Ten- measured,infiltration rates in conventionally,tilled and sion infiltrometers have proven,useful for characterizing
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Contaminant interactions with soil organic matter (SOM) are cen- tral to understanding the fate and transport of chemicals in soil envi- ronments.Elucidationofsorptionprocesseswillfacilitatetheefficiency of passive remedial methods and improve the accuracy of risk assess- ment models. Early studies in the 1960s identified a relationship be- tween SOM and the sorption of chemicals and laid the foundation for an area of research which is still active today. The onset of analytical instrumentation assisted the characterization of SOM chemical fractions, namely the fulvic acid (FA) and humic acid (HA) fractions. The employment of SOM chemical fractions in contaminant sorption studies has produced many empirical relationships between contaminant sorption behavior and SOM structure. More recently, molecular-leveltechniquessuchasnuclearmagneticresonance(NMR) spectroscopy have been applied to examine specific interactions be- tween contaminants and SOM fractions. These methods enable direct studies and are likely to further improve the fundamental understand- ing of contaminant interactions with SOM in the near future. For instance, NMR techniques should produce mechanistic information that will enable the accurate explanation of sorption phenomena at the macroscopic and landscape level. In addition to SOM chemical struc- ture, researchers must consider the organic matter physical conforma- tion at the soil-water interface because chemical methods provide structural information of the whole sample but do not provide detail about their physical architecture within the soil. This manuscript high- lights studies which have examined contaminant interactions at the macroscopic- and molecular-level and demonstrates the common themes stemming from different levels of investigation.
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Water flow and solute transport through soils are strongly influenced by the spatial arrangement of soil materials with different hydraulic and chemical properties. Knowing the specific or statistical arrangement of these materials is considered as a key toward improved predictions of solute transport. Our aim was to obtain two-dimensional material maps from photographs of exposed profiles. We developed a segmentation and classification procedure and applied it to the images of a very heterogeneous sand tank, which was used for a series of flow and transport experiments. The segmentation was based on thresholds of soil color, estimated from local median gray values, and of soil texture, estimated from local coefficients of variation of gray values. Important steps were the correction of inhomogeneous illumination and reflection, and the incorporation of prior knowledge in filters used to extract the image features and to smooth the results morphologically. We could check and confirm the success of our mapping by comparing the estimated with the designed sand distribution in the tank. The resulting material map was used later as input to model flow and transport through the sand tank. Similar segmentation procedures may be applied to any high-density raster data, including photographs or spectral scans of field profiles.
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Knowledge of soil hydraulic parameters and their spatiotemporal variation is crucial for estimating the water and solute fluxes across the land-atmosphere boundary and within the vadose zone at different scales. The objective of this study was to determine soil hydraulic conductivities (saturated hydraulic conductivity, Ksat, and unsaturated hydraulic conductivity, K(Y)) and their spatial and temporal variations in a clay-dominated biporous Vertisol near College Station, TX, using tension infiltrometers. The study was conducted within a 20- by 16-m plot across several seasons during a 21-mo period (May 2003-January 2005) to investigate the impact of varying disk sizes (measurement support) on K(Y), and the spatial and temporal variations of K(Y) under natural environmental conditions due to pore space evolution. Infiltration occurred in a bimodal fashion consisting of preferential flow (occurring at soil water pressure heads (Y) 52 0.05 to 0 m) and matrix flow (at Y 52 0.2 to 20.1 m). Biological and structural mac- ropores present in the soil resulted in gravity-dominated flow near saturation (Y 52 0.05 to 0 m) for all experiments. The Student's t-test of analysis of variance indicated that hydraulic conductivities were not affected by changes in the infiltration disk sizes. Although the K(Y) values at four different locations within the plot did not show sig- nificant spatial variability, they demonstrated strong temporal vari- ation during the 21-mo period based on the evolution of natural environmental conditions due to seasonal precipitation, root growth and decay, and structural pore space dynamics. Temporal trends of K(Y) indicated that hydraulic conductivities close to saturation were positively correlated with antecedent moisture conditions reflecting liquid cohesion, water films bridging across cracking peds, and the activation of flow in biological and structural macroporosity in the biporous soil system.
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We report the results of a systematic long-term study of infiltration rate (IR) in a large scale effluent recharge plant, showing a significant dependence of the infiltration rate on temperature (T). Water level and T were continuously monitored and recorded in several infiltration basins of an operating wastewater treatment plant (WWTP) during the course of a 4-yr study of basin geochemistry and performance. Infiltration rates were calculated from the slope of linear plots of water level vs. time during the drainage phase. Systematic interseasonal variations of IR were observed and were strongly correlated to water T. Calculations showed that the variation of IR with T was generally 1.5 to 2.5 times larger than that predicted from effluent viscosity changes per se, suggesting the possible involvement of other T-dependent factors. This may have profound effects on the overall efficacy of wastewater reclamation and other water-recharge operations.
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Analytical solutions of the heat equation must be evaluated in order to implement the heat-pulse method for measuring soil water flux density, We developed an improved procedure for evaluating the integrals in these solutions by recognizing that they can be reduced to a single function, W,known as the well function for leaky aquifers, The evaluation procedure was improved further by developing an efficient method to approximate W. This method, which involves summing the first few terms of an infinite series, also provides a simple means of determining the approximation error. For a wide range of input parameters, at most two terms of the series are needed to approximate W with error less than 10(-4) in absolute value. Thus, numerical integration is not required in order to implement the heat-pulse method for measuring soil water flux density. Our results regarding the evaluation of W are relevant to other problems in soil science and groundwater hydrology in which the function W appears.