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The following article will guide you about how to measure soil moisture by obtaining direct and indirect methods.
Soil moisture content is, normally, given as a dimensionless ratio of two masses or two volumes. When soil moisture content, given as dimensionless ratio, is multiplied by 100, the value becomes a percentage on mass or volume basis. Determination of soil moisture on volume basis involves finding mass basis figures first.
Once mass basis figures are found, volume basis figures are determined using bulk density. The amount of water in soil can also be given as a depth as if it were accumulated in a layer. A depth of water is typically used in irrigation. Specification of a depth of accumulated water is usually accompanied by a modifier such as “in the rooting zone.”
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Numerous methods of obtaining soil moisture are available and include direct, indirect and remote soil moisture measurement as given below:
Direct measurements of soil-water content involve removing water from a soil sample by evaporation, leaching or chemical reaction. Soil moisture content is calculated from the mass of water removed and the mass of the dry soil. Indirect methods involve measurement of some property of the soil that is affected by soil-water content. Indirect methods can also measure a property of some object placed in the soil. The object placed in the soil is normally a porous absorber which comes to water equilibrium with the soil.
Remote measurements include both non-contact methods and measurement from a great distance. Remote sensing of soil moisture depends on the measurement of electromagnetic energy that has either been reflected or emitted from the soil surface. The variation in intensity of electromagnetic radiation with soil moisture depends on the dielectric properties (index of refraction), soil temperature or a combination of both.
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The property that is important depends on the wavelength region that is being considered. Soil moisture measurements from a great distance normally involve satellite systems measuring the spectral reflectance of the soil surface.
A. Direct Methods (Gravimetric Methods):
Gravimetric methods are widely used for soil moisture measurement. They are simple and inexpensive.
1. Oven Drying:
It is the most widely used of all gravimetric methods. The oven dry method is the standard for the calibration of all other soil moisture determination techniques.
Drying the moist soil to a constant weight in a drying oven is controlled at 110° ± 5°C. Samples should be dried for at least 24 h. Weight of soil remaining after oven drying is used as the weight of soil solids. Moisture content expressed as a per cent is equal to the weight of water divided by weight of soil solids all times 100.
Volumetric water content (%) can be calculated if soil bulk density (BD) is known.
Soil moisture content can be expressed in depth of water (cm) per unit depth of soil.
Depth of water per unit soil depth = Volumetric water content x Soil depth.
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Merits:
1. Oven drying is considered as the standard method for obtaining soil moisture content
2. Very simple equipment requirement
3. No specific site calibrations are required
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4. Inexpensive compared to other methods
5. No health risks associated with the method.
Drawbacks:
1. Sampling is very tedious and time consuming
2. Time to dry sample is approximately 24 h, which will drastically reduce sample rate
3. Obtaining representative soil moisture values in a heterogeneous soil profile is difficult
4. If sampling is required over long periods, it is very destructive to the site
5. It is difficult to determine moisture content at specific depths.
2. Alcohol Burning Method:
Soil moisture from the sample is evaporated by adding alcohol and igniting. Provided the sample is not too large, the result can be obtained in less than 10 minutes. About 1.0 ml of alcohol per g of soil sample at FC and 0.5 ml at PWP is adequate for evaporating the soil moisture. This method is not recommended for soils with high organic matter.
3. Hot Air Drying:
Hot air around 110°C is passed on a screen with weighed samples of moist soil. Hot air vaporises the moisture and sends it out. Soil samples must be pulverised for using this method. It needs relatively expensive equipment.
4. Gypsum Sorption Plugs:
Gypsum plugs placed in soil comes into equilibrium with surrounding soil moisture. They are removed and weighed to determine soil moisture content. It is necessary to calibrate the weight of porous cup with soil moisture content for different soils.
5. Infrared Balance:
It gives fairly reliable moisture estimates in about 5 minutes. It consists of a 250 Watt infrared lamp, sensitive torsion balance and autotransformer, all housed in an aluminum cabinet. The radiation emitted by infrared lamp quickly vaporise the soil moisture. The instrument is directly calibrated in per cent moisture.
6. Feel and Appearance Method:
Though not a method of soil moisture measurement, it is often discussed under direct methods of soil moisture measurement. This method is actually recommended to farmers, as a rough guide, for scheduling crop irrigation. Soil moisture regimes (saturated soil, moist soil, dry soil) can be judged by manipulating the soil with hand and fingers.
In this method, soil samples obtained from representative depths in the crop root zone are observed for colour, plasticity and cohesiveness (by squeezing and rolling between fingers). Soil balls formed by squeezing and thread like structures formed by rolling between the fingers are observed for irrigation scheduling. With experience, accuracy is + or – 5 to 10 per cent available soil moisture.
B. Indirect Methods:
Indirect methods, usually, measures the volumetric soil moisture content. These methods are relatively quick and accurate. However, they require technical skill to operate and are relatively expensive.
1. Tensiometric Methods:
Tensiometer is also called irrometer since they are used in irrigation scheduling. Tensiometers provide a direct measure of tenacity (tension) with which water is held by soil. Tensiometric methods estimate the soil-water matric potential that includes both adsorption and capillary effects of the soil.
Tensiometric instruments have a porous material in contact with the soil, through which water can move (Fig 4.15). Water is drawn out of the porous medium in a dry soil and from the soil into the medium in a wet soil.
The working principle of tensiometer is that when a sealed water-filled tube is placed in contact with the soil through a permeable and saturated porous material, water (inside the tube) comes into equilibrium with the soil solution (it is at the same pressure potential as the water held in the soil matrix). Hence, the soil-water matric potential is equivalent to the vacuum or suction created inside the tube.
Tensiometer consists of a sealed water-filled plastic tube with a ceramic cup at one end and a negative pressure gauge at the other. Typically the measurement range is 0 to 0.80 bar. The vacuum gauge is graduated to indicate tension values up to one atm and is divided in to 50 divisions each of 0.2 atm value. Tensiometer works satisfactory up to 0.85 bars.
Vacuum gauge is calibrated in centibar or hundredths of one “bar”. Bar is an international unit of pressure in the metric system and is equivalent to 14.5 psi or 0.987 atm. One centibar is also equal to 1 kPa (kiloPascal). A reading of zero corresponds to a completely saturated condition, regardless of the type soil.
A reading of 85 indicates a very dry condition for sandy soils or sensitive crops. This also is the functional upper limit for tensiometer readings. A tension higher than 85 cb will cause the water column inside the tube to break rendering it nonfunctional.
Placement of Tensiometer:
Install one tensiometer at a depth of 30 cm and another at 60 cm below the ground surface (root zone). Shallow (30 cm) tensiometer can normally be pushed gently straight into the soil to the required depth. The 60 cm deep tensiometer can be installed using a probe or screw auger. Depth of the hole should be about 1/4 to 1 inch less than the actual depth for the porous tip (Fig 4.16).
Pour 1/4 cup of water into the hole to moisten the soil at the bottom. Insert the tensiometer and gently push it down to the desired depth, usually one-half the effective root zone depth. To ensure good contact between the soil and the porous tip, push gently the tip into the undisturbed soil just below the depth created by the probe.
After the probe has been installed, the soil and porous tip usually reach equilibrium within 24 h and the instrument is then ready to use. The correct depth can be marked on the auger to ensure the correct depth of installation.
When to Take the Readings?
Tensiometer readings should be taken early in the morning to avoid any fluctuations caused by heating up of the tensiometer and the water inside. If the tensiometer has been installed after overnight soaking, reading can be taken 30 minutes after installation. If more than 2 cm air gap is present at the top of the water column below the reservoir, then it should be refilled from the reservoir.
After several refills the reservoir will need topping up with more water. The drier the soil the more often this refilling will be required but remember that water will be drawn back into the tensiometer after irrigation has wet the soil. Write down the readings on the gauge for both the shallow and deep tensiometers.
What do the Readings Mean?
The vacuum gauge readings show the relative suction energy (and thus wetness) of the soil. A high reading on the gauge is caused by a dry soil which has a high suction. Tensiometer gauges have a scale reading from 0 to 100 centi-bars (cb). A tensiometer can operate effectively within a range of 0 to 85 cb.
Meaning of tensiometer readings are given in Table 4.3:
Sandy soils are irrigated at lower suction because the soil-water suction increases rapidly due of their low water holding capacity. As a thumb rule, apply 1 mm of water to reduce the tensiometer reading by 1 cb. For example, when the reading on the 30 cm deep tensiometer reads 40 cb apply 40 mm of water to recharge the root zone. One mm of irrigation equates to 1.0 1 m-2 or 10,000 1 ha-1.
If the reading on the deep tensiometer falls below 10 cb after irrigation, too much water has been applied and the soil is saturated. However, if after irrigation the reading on either tensiometer does not fall to 10 cb, not enough water has been applied. In most soils, allow about 24 h after irrigation for water to penetrate to the depth of the ceramic cup on the deep tensiometer before checking the readings.
When to Irrigate?
Readings from the 30 cm deep tensiometer are used to indicate when irrigation is necessary (Table 4.4).
The following graph (Fig 4.17) is an example of how to interpret tensiometer readings:
A: Crop using water rapidly from the entire root zone as shown by the rapid changes in the readings from both tensiometers. Reading on 30 cm deep tensiometer (50 cb) indicated that irrigation was needed.
B: 50 mm of irrigation water applied which brought both tensiometers back to zero. 60 cm deep tensiometer remained on zero for 24 h and both tensiometers had readings below 10 cb for several days indicating too much water had been applied and resulted in saturated soil conditions in the root zone.
C: Crop using water steadily from the entire root zone and 10 mm of rainfall resulted in 10 cb drop in upper tensiometer but no change in the deep tensiometer reading.
D: Steady water use by crop from the entire root zone with soil becoming drier than recommended irrigation start-up reading. Twenty mm of irrigation applied which reduced the 30 cm deep tensiometer reading by 20 cb but had no effect on the 60 cm deep tensiometer i.e. not enough water had been applied to wet the entire root zone to optimum water content.
E: Soil continuing to dry out. Another irrigation of 30 mm applied which rewetted the entire root zone and returned both tensiometers to 10 cb.
Advantages:
1. Direct reading
2. Up to 10.2 cm measurement sphere radius
3. Continuous reading possible when using pressure transducer
4. Well-suited for high frequency sampling or irrigation schedules
5. Minimal skill required for maintenance
6. Not affected by soil salinity, because salts can move freely in and out across the porous ceramic cup
7. Inexpensive.
Drawbacks:
1. Limited soil suction range (< 1 bar)
2. Relatively slow response time
3. Requires intimate contact with soil around the ceramic cup for consistent readings and to avoid frequent discharge (breaking of water column inside)
4. Especially in swelling or coarse soils, the ceramic cup can lose contact with soil, thus requiring reinstallation
5. Requires frequent maintenance (refilling) to keep the tube full of water, especially in hot dry weather.
Pressure Membrane and Pressure Plate Technique:
Osmotic potential is, usually, measured by extracting solution from the soil and determining its total potential either by vapour pressure measurements or by freezing point depression. In a bulk solution, the matric potential is zero, so that the only remaining component of the potential is osmotic. Often, the osmotic potential of a soil solution is inferred from its electrical conductivity.
To overcome this difficulty Richards (1947) developed pressure membrane apparatus for increasing the pressure on soil rather than decreasing the pressure of water in the tensiometer cup. Laboratory measurements of soil moisture potential are usually made with pressure membrane and pressure (suction) plate equipment (Fig 4.18).
It consists of ceramic pressure plates or membranes of high air entry values contained in airtight metallic chambers strong enough to withstand high pressure (15 bars or more). The apparatus enables development of soil moisture characteristic curves in the higher range of matric potential (> 1 bar), which is not possible on suction plates.
The porous plates are first saturated and then the soil samples are placed on these plates. Soil samples are saturated with water and transferred to the metallic chambers. The chamber is closed with special wrenches to tighten the nuts and bolts with required torque for sealing it.
Pressure is applied from a compressor and maintained at desired level. It should be ensured that there is no leakage from the chamber. Water starts to flow out from saturated soil samples through outlet and continues to trickle till equilibrium against the applied pressure is achieved.
Soil samples are taken out and oven dried to constant weight for determining moisture content on weight basis. Moisture content is determined against pressure values varying from – 0.1 to – 15 bars. The pairs of pressure and moisture content data so obtained are used to construct soil moisture characteristic curve.
2. Electrical or Electromagnetic Methods:
Gypsum blocks (porous/electrical resistance blocks):
Gypsum blocks use the principle of resistivity measured indirectly from a material in equilibrium with the soil. When these blocks reach equilibrium with the surrounding soil, their electric or thermal properties are often regarded as an index of soil-water content.
Gypsum blocks (Fig 4.19) are the most widely used for moisture measurement and are available in a variety of porous materials ranging from nylon cloth and fiberglass to casting plasters. Each block is calibrated in soil and soil density typical of the site the block is to be used in.
Water content of a porous block coming to water equilibrium with a soil depends on the energy status of the water rather than the water content of the soil. Soils with fine pores will contain more water than soil with coarse pores at equal matric potential. Resistance blocks read low resistance (400 to 600 ohms) at field capacity and high resistance (50,000 to 75,000 ohms) at permanent wilting point.
Hence, if porous blocks are to provide an indirect measure of soil-water content, calibration is necessary. Since the equilibrium is a matric potential and not a water content equilibrium, associated soil-water content must be obtained from a calibration curve.
Advantages:
1. Easy to install
2. No maintenance needed
3. Simple and inexpensive
4. Gives quick readings
5. Ideal for irrigation scheduling of fine textured soils where significant portion of ASM is at less than -100 kPa (-15 to -1 bar)
6. Suited to regulated deficit irrigation.
Drawbacks:
1. Block cannot be used for measurements around saturation (0 to 0.3 bar)
2. Block properties change with time, because of clay deposition and gypsum dissolution
3. It does not work well in sandy soils, where majority of ASM is above -100 kPa.
4. Not suitable for swelling soils
5. Blocks may dissolve over time.
3. Granular Matrix Sensors (GMS):
The sensor consists of electrodes embedded in a granular quartz material, surrounded by a synthetic membrane and a protective stainless steel mesh. Inside, gypsum is used to buffer against salinity effects. This kind of porous medium allows for measuring in wetter soil conditions and lasts longer than the gypsum blocks.
However, even with good sensor-soil contact, GMS have rewetting problems after they have been dried to very dry levels. This is because of the reduced ability of water films to re-enter the coarse medium Of the GMS from a fine soil. The GMS material allows for measurements closer to saturation. Measurement range is 0.10 to 2.0 bar (Fig 4.20). Calibration is necessary for soil moisture measurement.
Advantages:
1. Reduces the problems inherent to gypsum blocks (loss of contact with the soil by dissolving and inconsistent pore size distribution)
2. No maintenance needed
3. Simple and inexpensive
4. Salinity effects buffered up to 6 dS m-1
5. Suited to regulated deficit irrigation.
Drawbacks:
1. Slow reaction time. It does not work well in sandy soils, where water drains more quickly than the instrument can equilibrate
2. Not suitable for swelling soils
3. If the soil becomes too dry, the sensor must be pulled out, re-saturated and installed again.
4. Dielectric Methods:
Dielectric soil moisture sensors determine soil moisture by measuring the dielectric constant of soil. This constant is a measure of the ability of a dielectric material to establish an electrical field and is highly dependent on soil moisture. The constant of a dry soil is between 3 and 5, about one for air and is about 80 for water.
Thus, changes in soil moisture content change the dielectric constant of the soil. Calibration equations have been developed that correlate the soil moisture content and the dielectric constant. The most common dielectric methods are capacitance sensors and time domain reflectometry (TDR) sensors.
A number of researchers have made use of the soil dielectric constant to indirectly determine soil-water content. Topp et al (1980) did extensive calibrations (regressions) on a variety of soils at differing water contents to develop an empirical relationship between soil’s dielectric constant and soil-water content.
The relationship developed from this work is commonly referred to as the Topp equation:
θ = – 5.3 x 10-2 + 2.92 x 10-2 K – 5.5 x 10-4 K2 + 4.3 x 10-6 K3
where, K is the measured dielectric constant of soil.
Dielectric methods tend to be affected by high electrical conductivity conditions (salinity) and the presence of large organic fraction in soils. Development of unique calibration relationships for a particular soil can increase precision and accuracy.
Time Domain Reflectometry (TDR) requires a pulse generator, a sampling oscilloscope, such as the one in standard cable testers and a probe connected to a metallic cable (Fig 4.21). During a TDR measurement, an electromagnetic step pulse generated by a cable tester travels down a coaxial cable and along the metal rods of a probe embedded in the soil.
Reflections of the pulse occur at both the beginning and the end of the rods and a waveform showing both these reflections is displayed on the cable tester. The time that passes between the beginning reflection and the end reflection can be determined from the recorded waveform and depends on the velocity of the electromagnetic pulse following the relationship:
where, v = Pulse velocity (m s-1)
t = Time between the reflections (s)
L = Length of the probe needles (m).
The pulse velocity depends on the velocity of light in free space, c (m s-1) and K of the medium between (and around) the probe needles.
Combining above two equations, we can write an expression for calculating the dielectric permittivity of the soil:
Distance between the beginning and ending reflections shown on the display is equal to ct/2.
Size of the TDR probe can be varied in order to achieve a variety of different size sampling volumes. The equipment required to conduct TDR measurements, specifically the cable tester, can be quite expensive and analysis of the waveforms remains challenging. Software has also been developed to improve interpretation of waveforms.
Advantages:
1. Accurate
2. Soil specific calibration is usually not required
3. Easily expanded by multiplexing
4. Wide variety of probe configurations
5. Minimal soil disturbance
6. Relatively insensitive to normal salinity levels
7. Can provide simultaneous measurements of soil electrical conductivity.
Drawbacks:
1. Relatively expensive equipment due to complex electronics
2. Potentially limited applicability under highly saline conditions or in highly conductive heavy clay soils
3. Soil specific calibration required for soils having large amounts of bound water (those with high organic matter content, volcanic soils etc.)
4. Relatively small sensing volume (3.05 cm) radius around length of waveguides.
Capacitance sensors or capacitance probe (Fig 4.22) consists of two electrodes separated by a material called the dielectric, a material that does not readily conduct an electrical current. Normally, cylindrical shaped electrodes are used. Inserting the sensor into soil results in soil becoming part of the dielectric.
An oscillator applies a frequency between 50 and 150 Mhz to the electrodes, which in turn, results in a resonant frequency, the magnitude of which depends on the dielectric constant of the soil. The larger the moisture content, the smaller the resonance frequency. A calibration equation relating the dielectric constant and soil moisture content is necessary.
Advantages:
1. Accurate after soil specific calibration
2. Can read in high salinity levels, where TDR fails
3. Better resolution than TDR (avoids the noise that is implied in the waveform analysis performed by TDRs)
4. Can be connected to conventional loggers (DC output signal)
5. Flexibility in probe design (more than TDR)
6. Some devices are relatively inexpensive compared to TDR due to use of low frequency standard circuitry.
Drawbacks:
1. The sensing sphere of influence is relatively small (4.1 cm)
2. For reliable measurements, it is extremely critical to have good contact between the sensor (or tube) and soil
3. Careful installation is necessary to avoid air gaps
4. Tends to have larger sensitivity to temperature, bulk density, clay content and air gaps than TDR
5. Needs soil specific calibration.
Other, relatively recent, dielectric methods include amplitude domain reflectometry (ADR), phase transmission (PT), time domain transmission (TDT) and ground penetrating radar (GPR).
Amplitude domain reflectometry (ADR) working principle is that when an electromagnetic wave (energy) travelling along a transmission line (TL) reaches a section with different impedance (which has two components: electrical conductivity and dielectric constant), part of the energy transmitted is reflected back into the transmitter.
The reflected wave interacts with the incident wave producing a voltage standing wave along the TL, i.e. change of wave amplitude along the length of the TL. If the soil/probe combination is the cause for the impedance change in the TL, measuring the amplitude difference will give the impedance of the probe. Influence of the soil electrical conductivity is minimised by choosing a signal frequency, so that the soil-water content can be estimated from the soil/probe impedance.
Impedance sensors use an oscillator to generate a sinusoidal signal (electromagnetic wave at a fixed frequency, e.g 100 MHz) that is applied to a coaxial TL that extends into the soil through an array of parallel metal rods, the outer of which forms an electrical shield around the central signal rod (Fig 4.23). This rod arrangement acts as an additional section of the TL, having impedance that depends on the dielectric constant of the soil between the rods.
Advantages:
1. Accurate with soil specific calibration
2. Minimal soil disturbance
3. Can be connected to conventional loggers (DC output signal)
4. Inexpensive due to standard circuitry
5. Not affected by temperature
6. In situ estimation of soil bulk density possible.
Drawbacks:
1. Soil specific calibration recommended for reliable measurements
2. Measurement affected by air gaps, stones or channeling water directly onto the probe rods
3. Small sensing volume (4.42 ml).
5. Phase Transmission (Virrib):
Phase transmission (virrib) working principle is that after having travelled a fixed distance, a sinusoidal wave will show a phase shift relative to the phase at the origin. This phase shift depends on the length of travel along the TL, the frequency and the velocity of propagation. Since velocity of propagation is related to soil moisture content, for a fixed frequency and length of travel soil-water content can be determined by this phase shift.
The probe uses a particular waveguide design (two open concentric metal rings), so that phase measuring electronics can be applied at the beginning and ending of the waveguides (Fig 4.24).
Advantages:
1. Accurate with soil specific calibration
2. Large sensing soil volume (15.2 to 18.9 l)
3. Can be connected to conventional loggers (DC output signal)
4. Relatively inexpensive.
Drawbacks:
1. Significant soil disturbance during installation due to concentric rings sensor configuration
2. Requires soil specific calibration
3. Sensitive to salinity levels >3 dS m-1
4. Reduced precision, because the generated pulse gets distorted during transmission
5. Needs to be permanently installed in the field.
6. Ground Penetrating Radar (GPR):
Ground penetrating radar (GPR) technique is based on the same principle as TDR, but does not require direct contact between the sensor and the soil. When mounted on a vehicle or trolley close to the soil surface, it has the potential of providing rapid, non-disturbing, soil moisture measurements over relatively large areas, whereas TDR is better for detailed measurements over small areas.
Although, it has been applied successfully to many field situations, GPR has not been widely used because the methodology and instrumentation are still only in the research and development phase. It is however likely that small, compact and inexpensive GPR systems will be available in the near future for routine field studies.
7. Radiation Methods:
Neutron scattering:
This method consists of americium and beryllium (241Am/9Be) as neutron source and boron trifluoride (BF3) gas or lithium trifluoride (LiF3) as detector (Fig 4.25).
The working principle is that the fast neutrons are emitted from a decaying radioactive source (241Am/9Be) and when they collide with particles having the same mass as a neutron (protons, H+), they slow down dramatically, building a “cloud” of “thermalised” (slowed- down) neutrons. Since water is the main source of hydrogen in most soils, the density of slowed-down neutrons formed around the probe is nearly proportional to the volume fraction of water present in the soil.
Neutron probe is a long and narrow cylinder, containing a source and detector. Measurements are made by introducing the probe into an access tube (previously installed into the soil).
If the soil is dry, the cloud of neutrons will be less dense and extend further from the probe. If the soil is wet, the neutron cloud will be denser and extend a shorter distance.
The density of the neutron cloud is measured by the detector and the resultant signals are processed electronically and displayed as a number on the front panel of the gauge. The gauge reading is an index of volumetric soil moisture content (%).
It is possible to determine soil moisture at different depths by hanging the probe in the tube at different depths. Soil moisture is obtained from the device based on a linear calibration between the count rate of slowed-down neutrons at the field (read from the probe) and the soil moisture content obtained from nearby field samples.
Conditions met while placement of access tubes:
1. The site should be representative of the field to be monitored
2. No void spaces should exist between the access tube and soil
3. Mixing of soils adjacent to access tubes should be avoided during placement
4. No vertical flow of water should occur along the access tube from the surface
5. The top of the tube must be plugged to exclude water either from irrigation or rainfall
6. Permanent tubes are buried below plow depth and covered with soil. The soil cover is removed when measurements are to be made
7. When tubes are to be monitored more often, they should be installed in the crop row after seedling emergence with the top extending above ground.
Advantages:
1. Robust and accurate
2. Inexpensive per location (a large number of measurements can be made at different points with the same instrument)
3. One probe allows for measuring at different soil depths
4. Large soil sensing volume (10.2 to 40.6 cm) radius, depending on moisture content
5. Not affected by salinity or air gaps
6. Stable soil-specific calibration.
Drawbacks:
1. Safety hazard, since it implies working with radiation
2. Requires soil specific calibration
3. Heavy, cumbersome instrument
4. Takes relatively long time for each reading
5. Readings close to the soil surface are difficult and not accurate
6. Manual readings; cannot be automated due to hazard.
Gamma-Ray Attenuation:
Gamma ray attenuation method is a radioactive technique that can be used to determine soil moisture content within 1 to 2 cm soil layer.
The amount, a beam of monoenergetic gamma rays is attenuated or reduced in intensity in soil depends upon the soil’s constituent elements and the density of the soil column. Gamma ray attenuation assumes that scattering and absorption of gamma rays is related to the density of matter in their path.
Gamma ray attenuation also assumes that the specific gravity of a soil remains relatively constant as the wet density changes with moisture content. Changes in wet density are measured by the gamma transmission technique and the moisture content determined from this density change. Simply, if soil constituents and bulk density without water remain constant, changes in gamma ray attenuation represent changes in water content.
Basic equipment includes a gamma ray source surrounded by a collimator, a detector with a collimator and a scalar. Cesium-137, which emits gamma rays at 0.662 MeV and Americium at 0.060 MeV are well suited for water content measurements.
Gamma rays may be collimated to a narrow beam, which permits a representative reading to be obtained at any position in the soil. Precision of gamma ray methods for measuring water content varies with several soil properties. Bulk density measurements vary with thickness and density of the soil column, the absorption characteristics of the soil and the size of the gamma count in a moist and dry soil column.
Since attenuation of gamma rays is independent of the state of the water in the material tested, the measurement of attenuation is unaffected by the transition of liquid water to ice. Therefore, use of gamma attenuation has an advantage in that measurements of dry bulk density and total water content (including ice) can be made simultaneously.
Merits:
1. Temporal soil moisture changes can be easily monitored
2. Sampling time is relatively short, 10 seconds
3. Measurement is nondestructive.
Drawbacks:
1. Extreme care must be taken to ensure that radioactive source in not a health hazard
2. Equipment can be relatively expensive
3. Large variations in moisture content can occur in highly stratified soil and limit spatial resolution
4. Inability to measure in situ water condition when the soil is freezing, thawing or frozen
5. Field instrumentation is costly and difficult to use.
8. Thermal Methods:
As thermal inertia of a porous medium depends on moisture content, soil surface temperature can be used as an indication of moisture content.
The amplitude of diurnal range of soil surface temperature is a function of both internal and external factors. Internal factors are thermal conductivity and heat capacity or thermal inertia. External factors are primarily meteorological: solar radiation, air temperature, relative humidity, cloudiness and wind.
Combined effect of these external factors is that of the driving function of the diurnal variation of surface temperature. Thermal inertia is an indication of the soil’s resistance to this driving force. Since both heat capacity and thermal conductivity of a soil increase with an increase of soil moisture, the resulting thermal inertia increases.
Numerous tests show that for particular soils, the diurnal range of surface temperature is a good measure of the moisture content in the surface (0-4 cm) layer. In addition, for a given set of diurnal meteorological conditions, there is wide range of temperature amplitude- water content relationships for different soils.
However, when soil-water content was transformed into pressure potential, a single relationship was found for the different soil types investigated. This is the basis for expressing moisture values as a per cent of field capacity, where field capacity is the moisture content of the -1/3 bar pressure potential.
Heat Dissipation:
Thermal conductivity of water produces heat dissipation, so that a dry material will heat up faster than a wet one. In other words, the heat flow in a porous material is proportional to its water content.
A thermal heat probe consists of a porous block containing a heat source and an accurate temperature sensor (Fig 4.26). The block temperature is measured before and after the heater is powered for a few seconds. Thereby, block moisture is obtained from the temperature variation.
Since the porous block, placed in contact with the soil, is equilibrated with the soil-water, its characteristic curve will give the soil-water potential. Hence, the sensor must be provided with the calibrated relationship between the measured change in temperature and soil-water potential. Measurement range: 0.1-30 bar (less accurate for 10 to 30 bar range).
Advantages:
1. Wide measurement range
2. No maintenance required
3. Up to 10.2 cm measurement cylinder radius
4. Continuous reading possible
5. Not affected by salinity because measurements are based on thermal conductivity.
Drawbacks:
1. Needs sophisticated controller/logger to control heating and measurement operations
2. Slow reaction time. It does not work well in sandy soils, where water drains more quickly than the instrument can equilibrate
3. Fairly large power consumption for frequent readings.
Heat Capacity Sensors:
Under vapour equilibrium conditions, water potential of a porous material is directly related to the vapour pressure of the air surrounding the porous medium. This means that the soil- water potential is determined by measuring the RH of a chamber inside a porous cup equilibrated with the soil solution.
Heat capacity sensors (heat pulse probe) consist of a ceramic shield or screen building an air chamber, where a thermocouple is located (Fig 4.27). Temperature response (increase) from a small heat input is used to calculate heat capacity. Heat capacity increases linearly with volumetric water content.
The screen type is recommended for high salinity environments. RH in the air chamber is calculated from the wet bulb Vs dry bulb temperature difference. Measurement range: 0.5 to 30 bar (less accurate for 10 to 30 bar range).
Advantages:
1. High sensitivity
2. Scientifically rigorous readings (except in wetter soil conditions)
3. Suitable where typical moisture conditions are very dry.
Drawbacks:
1. Not recommended at shallow soil depths, due to high susceptibility to thermal gradient
2. Small sensing volume
3. Very slow reaction time, because reaching vapour equilibrium takes time
4. Low accuracy in the wet range
5. Specialised equipment is required for the sensor’s excitation and reading.
Soil psychrometer:
Its working principle is that under vapour equilibrium conditions, water potential of a porous material is directly related to the vapour pressure of the air surrounding the porous medium. This means that the soil-water potential is determined by measuring the RH of a chamber inside a porous cup equilibrated with the soil solution.
A soil psychrometer (Fig 4.28) consists of a ceramic shield or screen building an air chamber, where a thermocouple is located. The screen type is recommended for high salinity environments. RH in the air chamber is calculated from the wet bulb Vs dry bulb temperature difference. Measurement range: 0.5 to 30 bar (less accurate for 10 to 30 bar range).
9. Remote Sensing:
Infrared or electromagnetic properties of soil can be used for predicting soil moisture content. Backscattering from an extended target, such as a soil medium, is characterised by the target’s scattering coefficient. Scattering coefficient represents the link between the target properties and the scatterometer responses.
For a given set of sensor parameters (wavelength, polarisation and incidence angle about 0), the scattering coefficient of bare soil is a function of the soil moisture, surface roughness and dielectric properties. Dielectric properties depend on the soil’s moisture content.
Presence of a vegetative canopy over the soil surface reduces the sensitivity of the radar backscatter to soil moisture by attenuating the signal as it travels through the canopy down to the soil and back and contributing a backscatter component of its own. The effect of vegetative cover on the radar response to soil moisture is to reduce the sensitivity by about 40 per cent compared with responses from bare soil when the two responses are compared as a function of field capacity in the top 5 cm.
Limiting factor is the ability of the system to measure soil moisture in the top 5 to 10 cm layer only. The sampling depth for active microwave sensors also is limited to the surface few centimeters of the soil for some wavelengths.
Remote sensing with microwave offers rapid data collection over large areas on a repetitive basis. Several questions still need to be answered concerning the dependence of sensor observation on soil moisture and other parameters, like vegetation. Major problems related to remote sensing seem to be the spatial resolution, depth of penetration and cost.
Merits:
1. Relatively quick method
2. No calibrations are required
3. No health risk involved with the systems.
Drawbacks:
1. Not as accurate as direct methods and limited to the top layer of the soil surface
2. Very complex equipment normally involving satellites
3. Very expensive method involving the use of satellite systems in most cases.
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