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H.S. Öztürk, G. Çaycı Ankara University, Faculty of Agriculture, Department of Soil Science, Ankara, Turkey Abstract The development and continuing refinement of new techniques have enhanced our ability to measure and monitor the storage and movement of soil water in-situ. However there is no single approach that has the best overall performance for a range of soil, crop and landscape conditions. Time domain reflectometry (TDR), frequency domain reflectometry (FDR) and Capacitance probe (CP), which based on dielectric properties of soil and provide point measurement, are suitable for automatic, precise, rapid, and reliable measurement of soil water. Ground-penetrating radar (GPR), on the other hand, offers a fast and non-destructive way for estimating dielectric constant and suitable for mapping of large areas. It is the aim of this paper to define decision making processes for assessing the characteristics of technology in relation to project objectives, the properties of the soil(s) of interest, and to compare advantages and disadvantages in these soil water measuring techniques. Introduction Soil water is essential for plant growth and is the vehicle for solute transport, including nutrients and soil contaminants. Accurate measurement of soil water is crucial for the better management of irrigation water and rainfall capture. Crop yields are generally more closely related to soil water availability than to any other soil and meteorological variable. Therefore the effective use of soil water requires frequent and accurate measurements and the technique should be rapid, reliable, simple, cost effective and non-destructive. On the other hand, soil water is a highly dynamic entity, exhibiting substantial variation in both time and space. This is particularly true near the soil surface, and in the presence of active plant roots (Or and Wraith, 2000). Continuous monitoring of soil water content can be a valuable part of agricultural, environmental and ecological research. Since most of the research project are conducted on multiple sites, the accessibility of sites (including limitation of labor and electric power supply) is the most important consideration when selecting an automated system for measuring soil water content (Veldkamp and O'Brien, 2000). Several new sensors and measurement methods are based on combinations of capacitive, reflective and frequency-shift principles, all of which are governed by the soil dielectric properties. In past years, a plenty of researchers have studied on the dielectric constant property of soil as the basis to estimate the soil water content. The two major techniques that make use of this property are the capacitance probes (CP) and time domain reflectometry (TDR). Other promising technologies being used or developing are frequency domain reflectomretry (FDR) and grand penetrating radar (GPR). The objective of this paper was to give information about new water measurement techniques, to bring up the advantages and disadvantages of these techniques and to guide to those choosing a water measurement technology with a point by point comparison between the technologies presented here. Classical Methods for Soil Water Content Measurement : Thermogravimetry, electrical resistant and the neutron scattering methods have been extensively used on soil water content determination in the past. However, all these techniques did not meet to the increasing demand of our present needs any more. Termogravimetric method is labor and time intensive, time delay required for drying, destructive although it gives precision measurements, direct and inexpensive (Or and Wraith, 2000). The electric resistance method measures the electric resistivity of soils with changes in water content (Gardner, 1986). To monitor the soil water changes in soil, neutron probe (NP) has been extensively used for a long time. The instrument known as a neutron moisture meter consists of a probe and a scaler to monitor the flux of slow neutrons. When the probe lowered into an access tube, fast neutrons are emitted radially into soil where they collide with H nuclei, which have similar mass to neutrons, cause a significant loss of kinetic energy and slow down the fast neutrons. As a result of repeated collision, the speed of fast neutrons diminishes, they are called thermalized or slow neutrons. The flux of slow neutrons is measured by detector. The average loss of neutrons kinetic energy is proportional to the amount of H nuclei in the surrounding soil. Advantages of this method include the ability to repeatedly measure volumetric water content at the same locations, averaging of the measured water content over a substantial soil volume, and ability to measure soil water content at multiple depths and locations using the same equipment. Limitations or disadvantages include radiation hazard and attendant licensing requirements, relatively poor spatial resolution, unsuitability for near-surface measurements, and the soil specific calibration requirement (Or and Wraith, 2000). Working Principles of New Techniques : TDR is an indirect method of determining soil water content. It makes use of the fact that the dielectric constant or permittivity of water is much higher than that of the other soil constitutes. The method involves measuring the propagation velocity of an electromagnetic pulse traveling along a parallel metallic probes (rods) embedded in the soil. This measurement later converted to the volumetric moisture content of soil by various models (Topp et al., 1980, Dalton et al., 1984, Dalton and van Genuchten, 1986). The permittivity,  ; is a complex quantity, but for soil water content measurements the imaginary part can generally be neglected. This part of complex permittivity represents energy absorption by the soil as a result of dielectric losses and ionic conduction. The permittivity is then equal to the real part and magnitude. Thus for vacuum and air =1, for water  81, while for most mineral soil components 3 to 7 (Zegelin et al., 1992). As a result, the permittivity of moist soil varies strongly with water content and can be used to determine soil water content.
FDR sensor was also developed for continuous measurement of soil water content. This system uses the dielectric properties of water but in a different approach than TDR (Bilskie, 1997). As TDR measures the apparent dielectric permittivity of soil, changes in the permittivity can be attributed to changes in soil water content. The FDR sensor, on the other hand, sends an electromagnetic wave along its probes and measures the frequency of the reflected wave, which varies with water content. Each FDR sensor contains an Application Specific Integrated Circuit (ASIC) which measures real and imaginary part of the complex dielectric permittivity simultaneously by the sensor rods and the soil at the single frequency of 20 MHz. The ASIC increases the accuracy of the measurements and eliminates influences of lengths of cables, quality of cables, connectors, and switches, making multiplexing easier and cheaper (Dirksen and Hilhorst, 1995). A capacitance probe (CP) consists of an electron pair separated by a plastic dielectric. The upper and lower electrodes and the plastic separator are in the shape of a cylinder that fits closely inside a plastic access tube. A resonant LC ( L: inductance, C: capacitance) circuit in the probe includes the ensemble of the soil outside the access tube itself, plus the air space between the probe and access tube, as one of the capacitive elements. Changes in the resonant frequency of the circuit depend on changes in the capacitances of the soil-access tube system. Capacitance of a simple two electrode plate capacitor, C,:  Equation 1
Where 0 is the permittivity of free space (8.9 x 10-12 F/m), Ka is the system apparent dielectric constant, a is the overlapping area (m2) of the plates, and d is the thickness (m) of the dielectric separate the plates.
GPR offers a fast and nondestructive way for estimating the soil dielectric constant. Ground-penetrating radar measurements are based on transmission or reflection of an electromagnetic wave in the studied medium. Wave propagation velocity depends on the dielectric constant of the medium and its spatial variations; wave velocity varies from 30 cm/ns in air to 6 to 15 cm/ns in soils. GPR emits electromagnetic (EM) microwaves, and measures dielectric constant. GPR uses a free wave that propagates and spreads in the soil, where it will reflect off interfaces with different dielectric constants. can be found from
 Equation 2
Where C is the speed of light and n is the velocity of the EM wave that can be calculated for GPR, the EM wave speed can be determined with the common midpoint method or from the travel time to a layer of known depth with a distinct .
Comparison of New Techniques : Both old and new soil moisture measurement techniques have some advantages over the other techniques for certain conditions. The TDR method for soil water content measurement is widely applicable and is used for automated data collection. However, obtaining precision and accuracy is very much dependent on wave form interpretation methods used in software (Evett, 2000). Most of them are based on the relationship between volumetric soil water content and dielectric constant (permittivity) of soils ( Topp-Davis-Annan Equation or Universal Equation). But this equation is not adequate for all soils. Dirksen and Dasberg (1993) reported that this equation can be valid for the soils with low clay contents (specific surface) and typical bulk densities ( rb= 1,35-1,5 g cm-3 ). Zegelin et al. (1992) revealed that the use of universal equation gives water balance to within ± 10 % of soil water content. However the Maxwell- De Loor's mixing model for the four components (solid phase, tightly bound water, free water and air ) can account for both factors with average values of the volume fraction and dielectric constant of the tightly bound water. When using the TDR, it is often of great importance to obtain high depth resolution with minimal disturbance of the soil and to be able to measure close to the soil surface. Shorting the probe size increases the accuracy of TDR in measuring the small measured volume of soil (Nissen et al. 1998; Amato and Ritchie, 1995). Superior accuracy using the right calibration equation (within 1 or 2% of volumetric water content) (Roth et al., 1990), excellent spatial and temporal resolution and simple to obtain continuous soil water measurement through automation and multiplexing (Baker and Almaras, 1990) are strengths of the TDR. On the other hand limitation of the TDR method include relatively high equipment expense, limited applicability in soils high in swelling clays and organic matter (Zegelin et al., 1992), and under highly saline condition due to signal attenuation ( Roth et al., 1992). Moreover some clays having high surface area and surface charges and high soil moisture content (Zegelin et al., 1992) weakens the TDR signal and limits usefulness of the method. A recently commercialized device with several advantages is called a FDR. Dirksen and Hilhorst (1995) revealed that the greater complexity of processes occurring at the lower FD frequency results in greater sensitivity for FD than for TDR, but the larger differences between soils also present a greater need for soil specific calibrations. They also noted that when calibrated, the new type FD sensor appears capable of measuring q with at least equal accuracy as TDR, while offering operational simplicity and financial advantages. Surface area, surface charge density, composition of counterions decrease the permittivity at high frequencies but increase it at the 20 MHz frequency of FD sensor. At low frequencies, the soil particles behave as conducting particles without surface effects, whereas at high frequencies the soil particles behave as dielectric inclusions without surface effects. Hilhorst and Dirksen (1995) reported that ionic conductivity can be measured more easily and more accurately with the FD sensor and than with existing time domain sensors. TDR is suited mostly on the studies on stratification detection, whereas FD sensor has superior features from an operational point of view, additionally it has low cost, and is robust and reliable. FDR has several advantages over TDR: interpretation of data is direct; it has low power consumption, it is inexpensive for multiple site measurements; there is no need of an expensive cable tester; its probes can be buried for a long time, because they are designed to withstand harsh environmental conditions, bound water sensibility is as for free water ( for TDR this is less than for free water), and its operation is simple. There are, however, also disadvantages of FDR: accuracy and resolution decreases with decreasing water content, stratification detection is average and it has not been widely used yet. On the other hand all TDR and FDR require special calibration for high clay and organic matter content soils. The propagation of electromagnetic waves is also affected by electrical conductivity and temperature. High clay content has a similar effect on calibration, but the magnitude is dependent on the clay type. The temperature dependence of the FDR sensor various with water content and can be easily corrected for (Campbell Scientific, 1998). Since its low cost and easy-to-use, FDR technique are promising for practical use. Because of these properties FDR is being applied on irrigation scheduling. Laboski et al. (2001) and Kukangu et al. (1999) successfully used this technique for irrigation practices. Capacitance sensors are another means of characterizing the soil dielectric constant. Their design depends on their expected applications. In particular, the electrode geometry has a critical influence on the extension of the probed region. Moreover, the soil may not always be viewed as a medium of statistically uniform dielectric constant (de Rosny et al., 2001). Mead et al. (1998) showed that the device is likely also affected by temperature. For the CP electrodes, the surface area of electrodes is well known but the degree to which the torus of electric force line permeates the soil is not. Thus, it seems that any term equivalent to d (Eq. 1) is particularly poorly defined in this soil-access tube since soil, with all its variability in bulk density and water content, becomes the dielectric in the capacitive system and the shape of the field may be influenced by soil heterogeneity including any gaps between the soil and tube wall induced by tube installation. Installation and calibration for this type capacitance probe gain an importance as described by Bell et al. (1987). The capacitance probe is an attractive device for monitoring soil moisture automatically. However, its sphere of influence is rather small (a few cubic centimeters only) (Chanzy et al., 1998). Their results showed that the calibrations differ significantly from one probe to another. Once calibrated, the capacitance probe provided accurate soil water measurements, but it is advisable to have at least two replicate probes. On the other hand, Evett and Steiner (1995) found poor results with CP gauges and attributed these results to non uniformity of their soils studied and considerable small measurement volume. They concluded that the CP gauge has limited precision and is unacceptable for routine soil water content measurements under their conditions and, however, NP provided acceptable precision. Tomer and Anderson (1995) compared the results of soil water contents with CP and NP and data obtained by TDR. They found that the CP gave greater soil water estimates than the NP when the data grouped according to date. However, when data grouped according to depth, between probe differences changed with measurement. CP measurements were greater than NP measurements at shallow depths. They concluded that CP has several advantages in soil water measurement. Measurement time of the CP is less than for the NP, hazards and expenses incurred with radiation are eliminated, and has good depth resolution. On the other hand CP has several disadvantages that are related to the small soil volume and the nature of the dielectric response that is measured. Therefore, users of CP should investigate the effects of salts, bulk density, and texture. For dry and coarse textured soils changes in water content are difficult to detect with the CP (i.e., less than 10 to 12 percent). More recently, GPR has been employed to follow the wetting front movement beside to monitor changes in soil moisture content (Vellidis et al., 1990). GPR is also suitable method for monitoring moisture content changes in the vadose zone and permit relatively large measurement scales, appropriate for hydrological models of unsaturated processes (Binley et al., 2001). GPR is a near-surface geophysical technique that can provide high resolution images of the dielectric properties of the top few tens of meters of the earth (Knight, 2001; van Dam and Schlager, 2000). Weiler et al., (1998) reported that GPR, which employees an unguided EM wave, shows great promise in the future for nondestructive soil water sensing. They claimed some advantages of GPR over TDR are that it can measure larger volumes of soil than the TDR and can be utilized without disturbing the soil. Disadvantages include that automatic measurements are not possible because every instrument might have to be calibrated first and that it is prone to failure in soils with high clay contents and high salinity contents. In another study, digital GPR for soil moisture determination and mapping of soil water content were evaluated by Chanzy et al. (1996). In this research two modes of operation were considered: the ground mode and the airborne. A strong correlation between the GPR data and the soil water content was observed in both the ground and airborne modes of operation. In the ground mode, soil moisture error after calibration was found lower than 0.03 m3/m3. However in the airborne mode, soil moisture estimation were less accurate (0.046 m3/ m3 ). Additionally, it was asserted that this method has a great potential for mapping soil moisture and is efficient on most natural surface as vegetation and surface microtopography have only a small effect on the reflection of low-frequency pulses. Huisman et al., (2001) evaluated GPR performance, TDR and gravimetric soil water content measurements. The results showed that the calibration equations between GPR and aggregated gravimetrical soil water content were similar to those obtained for TDR, suggesting that available TDR calibrations (e.g. Topp's equation) can be used for GPR. Conclusion The restrictive use of neutron probe, the rapid advancement and the decreasing cost of the non-nuclear methods in resent years, brought about to compare these methodologies. This also defines decision-making process for assessing the characteristics of technology in relation to project objectives. Soil water measurements encounter particular problems related to the physics of the method used. Great effort has been devoted in the last decades to the development of new soil water-content sensors based on the capacitance technique or working with TDR or FDR. Each soil water sensing method has strengths and weakness. A strength in one application may be a weakness in another. All of the methods have their own specific field of application. However they complement each other in some aspects such as sensitivity at low water content. To select the right method, the user must have a good understanding of how its qualities fit the requirements of the project. Finally we conclude whichever sensor one chooses, time must be invested to become proficient with them, and to be cautioned to find out their site-specific behaviour and plant compatibility. References . Amato, M. and J.T. Ritchie. 1995. Small spatial scale soil water content measurement with time-domain reflectometry. Soil Sci. 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