Comparison of soil moisture sensors between neutron probe, Diviner 2000 and TDR under tomato crops

Transcription

1 Symposium no. 59 Paper no Presentation: oral Comparison of soil moisture sensors between neutron probe, Diviner 2000 and TDR under tomato crops HENG L.K. (1), CAYCI G. (2), KUTUK C. (2), ARRILLAGA J.L. (1) and MOUTONNET P. (3) (1) Soil Science Unit, FAO/IAEA Agriculture and Biotechnology Laboratory, Seibersdorf, Austria (2) Soil Science Department, Faculty of Agriculture, Ankara University, Diskapi, Ankara, Turkey (3) International Atomic Energy Agency, Joint FAO/IAEA Division, Vienna, Austria# # now 89 Chemin du Four de la Peste, F Pertuis, France Abstract Accurate, precise, fast and ease of measuring soil moisture as well as the ability to measure with depth are the characteristics that are desirable in routine large scale soil water monitoring. In this study the soil moisture neutron probe (SMNP), Diviner2000, a capacitance probe and TDR were compared in a field tomato experiment carried out at the FAO/IAEA Agriculture and Biotechnology Laboratory at Seibersdorf, Austria. The experiment consists of two irrigation treatments: furrow versus drip and three nitrogen levels (0, 100 kg N/ha, 200 kg N/ha), with each treatment replicated three times, giving a total of eighteen plots. The size of the plot was 3.4 m x 5 m. The soil is a clay loam, with high content of gravel. One SMNP aluminium access tube and one Diviner PVC access tube were installed in each plot using the slurry method, 30 cm on each side of the tomato plants. In addition, TDR probes were installed vertically to monitor soil moisture in the top 20 cm. Readings were taken regularly over the growth season from June to September. Calibration of SMNP and Diviner 2000 was carried out using gravimetric method. For SMNP, a coefficient of determination r2 of 0.96 was obtained; separate calibration curves were necessary for the top 15 cm soil depth and below. For Diviner2000, the calibration curve was found similar to that given by the manufacturer, hence factory calibration curve was used. No calibration was done on the TDR. The soil water storage (SWS) was calculated for the top 20 cm and 60 cm, respectively.tremendous scatter resulting in higher standard error was observed in the Diviner2000 measurement for both the 20 and 60 cm depths compared to SMNP, due probably to the small soil volume measured. However, the errors was less under high nitrogen treatment (200 kg N/ha), probably because higher root water extraction giving more even soil moisture. This result was also observed in the top 20 cm with the TDR probes.implications of these sensors for irrigation scheduling and the importance of placement relative to the crop are discussed in the paper. Keywords: soil water storage, soil moisture neutron probe, calibration and comparison of sensors

2 Introduction Fast and accurate monitoring of soil moisture plus the ability to do depth measurement is vital in this age of water scarcity. There is tremendous pressure and challenge to produce more out of less water, at the same time protecting and reducing risks to the environment. Some of the desirable attributes of the technique include being accurate, rapid, reliable, simple and cost-effective (Charlesworth, 2000). Over the last decade, many new developments of soil moisture sensors have been evolved, especially those based on the frequency domain reflectometry (FDR) capacitance technique, due to the rapid developments in the micro-electronic industry.this resulted in many relatively cheap and small sensors being manufactured, giving much more options to the traditional neutron scattering technique, which was the most commonly used method since its development in the 1950s (Gardner and Kirkham, 1952; Van Bavel et al., 1956; Gardner, 1986). Although the soil moisture neutron probe (SMNP) has been shown to be rather robust and suited to large-scale field measurements (Scotter et al., 1979; Moutonnet et al., 1988; Heng et al., 2001), its radioactive source requires licensing during transportation and storage, training of users on safety regulations, thus limit many uses such as remote unattended monitoring. Some of the new FDR capacitance sensors include EnviroSCAN, Diviner2000, CProbe,Gopher, Buddy, Aquaterr, ThetaProbe and Netafilm data collector, to name a few (Charlesworth, 2000; Williams, 2002). The method utilize the high dielectric constant of water compare to soil and air to determine water content of the soil. A pair of electrodes or electrical plates (which can be parallel spikes or circular metal rings) was used as the capacitor. When activated, the soil-water-air matrix around the PVC tube forms the dielectric of a capacitor and completes an oscillating circuit. Changes in the resonant frequency of the circuit depend on changes in the capacitance of the soilaccess tube system. The theory and application of these sensors has been discussed (Dean et al., 1987; Paltineanu and Starr, 1997; Fares and Alva, 1999; Fares and Alva, 2000). In the capacitance method, the probe can be inserted directly into the soil or into a PVC access tube installed in the field for profile measurement. While the new sensors claim to be accurate with minimal skill to operate, costeffective,and many have logging capability, their performance under different soil and cropping systems is only slowly being tested; few papers described comparisons of these methods with the traditional neutron-probe technique. However, it is well known that this category of sensors in general have a small sphere of influence and are very sensitive to small air gap around the tubes during installation, cracks and macropores created by root activities, as well as positional changes in orientation within the tubes. Because of this, good sensor-tube-soil contact for reliable estimation of soil moisture is extremely critical (Evett and Steiner, 1995; Charlesworth, 2000). In this study, we compared the performance of the neutron thermalization with the FDR and time-domain reflectometry (TDR) methods of measuring soil water. The TDR method is also based on the theory that water has a much higher dielectric constant than soil. The method measures the velocity of propagation of a high-frequency signal along a transmission line or wave-guides (steel probes) buried in the soil. The time and speed of travel of the reflected signal from the end of the probe varies with the dielectric of the soil, which is related to the water content of the soil. The theory was described in detail in Topp et al., (1980). The TDR is one method where its universal calibration curve

3 elating the dielectric constant and soil moisture is satisfactory for many mineral soils (Topp et al., 1980; Dalton, 1992; Zegelin et al., 1992). However, one limitation of the method is that portable profile measurement of soil moisture is in general not possible. On the other hand, the neutron method of measuring soil water content uses the principle of neutron thermalization (Greacen, 1981; Gardner, 1986). During operation, high-energy neutrons are emitted from the radioactive source (usually a mixture of Americium 241 and Beryllium). Elastic collisions of the fast neutrons with elements in the soil produce slow neutrons (thermalized) and the process is most rapid when neutrons collide with hydrogen nuclei because of the similar masses. The number of thermal neutrons detected is a measure of the concentration of the hydrogen nuclei and therefore water content in the soil around the probe. The theory of operation and field calibration of SMNP was also described recently by Hignett and Evett (2002). Materials and Methods For the experiment, the Troxler 4301 Model SMNP (Troxler Electronic Laboratories, Research Triangle Park, NC) was compared with Sentek s Diviner2000 capacitance sensor (Sentek Environmental Technologies, Kent Town, South Australia) and the Trase Systems TDR (SoilMoisture Equipment, 801 S. Kellogg Ave., Goleta, CA). The Diviner2000 is a hand-held, portable soil moisture-monitoring device consisting of a portable display/logger unit, connected by cable to an automatic depthsensing probe. During measurement the probe (with the sensor at the lower end of the probe) is inserted into an access tube, a high frequency electrical field (>100 MHz) is created around the sensor extends through the access tube allowed the moisture to be determined. The technology of Sentek has been descried in detail in Buss (1993) and Paltineanu and Starr (1997). The Troxler 4301 model has a 10 mici (0.37 GBq) Am 241/Be neutron source, double encapsulated in a stainless steel housing. All three devices were compared in a tomato field experiment carried out at the FAO/IAEA Agriculture and Biotechnology Laboratory at Seibersdorf, Austria, some 40 km southeast of Vienna. The experiment consists of two irrigation treatments: furrow versus drip irrigation. It was part of a bigger experiment where the nitrogen fertilizer use efficiency at various nitrogen levels (0, 100 kg N/ha, 200 kg N/ha) was studied using 15 N (urea 5%-atom excess) labeled fertilizer. Each treatment was replicated three times, giving a total of eighteen plots. The size of the plot was 3.4 m x 5 m. Each plot has five rows and 9 plants per row. Tomato plants (Lycopersicon lycopersicum L. Karsten ex Farwell) were planted at 50 cm spacing within rows and 70 cm between rows; they were irrigated using either furrow or drip. The furrows were approximately 40 cm wide between rows and the drippers have flow rate of 4 litres h -1 and were placed adjacent to each plant. The soil is classified as a Seibersdorf soil, classified as Typic Eutrocrepts with coarse clay loam texture, with considerable amount of gravel especially below 50 cm. The topography of the site is flat and the average rainfall in the region is approximately 600 mm. The basic physical and chemical properties of the topsoil are given in Table

4 Table 1 Basic physical and chemical properties of Seibersdorf soil. Clay Silt Coarse silt Fine sand Coarse sand F.C. PWP BD (g cm -3 ) OM CEC (cmol kg -1 ) ph (H 2 O) PWP is Permanent Wilting Point Installation of access tubes One SMNP aluminium access tube (4.3 cm OD) and one Diviner2000 PVC access tube (5.1 cm ID and 5.6 cm OD) were installed to 60 cm depth in each plot using the slurry method, 30 cm on each side of the tomato plants. The presence of the gravel made installation of access tubes a difficult task, because of that the Sentek Preferred Method for Diviner2000 was not used. Deeper installation depths were also not possible because of the high gravel content. Nevertheless, care was exercised to minimize gap and soil disturbance created during access tube installation. TDR probes 20 cm long, were installed permanently in vertical position to monitor soil moisture in the top 20 cm (this was extended to deeper depths between and cm in subsequent experiments). The SMNP readings were taken at depths of 15, 30 and 45 cm twice weekly over the growing season of tomato from June to September. Readings from Diviner2000 were taken at 10, 20, 30, 40 and 50 cm depths. In addition, tensiometers were installed at both 15 and 30 cm depths in all plots to monitor the soil water potential. Reading for all three devices was taken at the same time. Both the SMNP and Diviner2000 were calibrated gravimetrically during the course of the experiment. Three soil samples were taken close to the access tubes at depths 15, 30 and 45 cm for the SMNP and at 10 cm intervals for the Diviner2000. The volumetric water content of the soil was determined by multiplying the gravimetric water content with the measured bulk density. Results and Discussion Calibration Figure 1 shows the calibration curves for both the SMNP and Diviner2000 on Seibersdorf soil. Separate calibrations were needed for the top 15 cm soil and those below (15-45 cm) for the SMNP. An r2 value of with root mean square errors of 2.5% (v/v) was obtained for both depths. Calibration on Diviner2000 between scaled frequencies showed rather similar to the non-linear factory calibration, with an r2 of In this study, the factory calibration curve for Diviner2000 was used for subsequent measurement. The daily rainfall and evapotranspiration between June-August for the tomatogrowing season is given in Figure 2. The early part of the crop season in June was rather dry, because of that irrigation water was applied to all plots prior to planting. Subsequent irrigation was given based on soil water potential as read from tensiometers installed at both 15 and 30 cm depths

5 Figure 1 Calibration curve for SMNP (a) and Diviner2000 (b). Comparison of the top 20 cm soil moisture in both the drip and furrow irrigated treatments between TDR, SMNP and Diviner 2000 is shown in Figure 3 for the control (N0) and 200 kg N ha -1 treatments for both furrow and drip irrigation plots. Data on the 100 kg N ha-1 treatment were not presented as the trends were similar to those presented below. Figure 3 shows that soil water storage (SWS) in the top 20 cm is closer between the TDR and SMNP methods whereas that of Diviner2000 deviates significantly from the other two sensors except in the drip control plots. The associated standard error of the mean was also largest in Diviner2000 (not shown). The greater scatter exhibits in Diviner2000 was due to its small volume of sphere of influence (10 cm) and also to its sensitivity to gaps around the sensors introduced during access tubes installation or due to the heterogeneity of the soil. This means that to obtain the same level of precision as the SMNP and TDR more tubes are needed in the case of Diviner2000. Figure 2 Rainfall and potential evapotranspiration distribution during the tomatogrowing season

6 Figure 3 Soil water storage (SWS) in the top 20 cm obtained using soil moisture neutron probe (..), TDR (...) and Diviner2000 (.x.) for furrow control (a), furrow N200 (b), drip control (c) and drip N200 (d). The total soil water storage to 60 cm depth for both the SMNP and Diviner2000 is given in Figure 4. Again much bigger errors are associated with the Diviner2000, especially in the control plots (N0) for both irrigation treatments. The error was in general less in the N200 plots for both sensors; probably due to the bigger water demand under high nitrogen rates helped homogenized soil moisture in the root zones. However, the two methods differ significantly in the SWS especially in the N200 drip treatment, with Diviner2000 giving as much as 50 mm more SWS. The corresponding soil water potential for the N200 drip treatment showed rather low values especially at 30 cm depth (Figure 5). Based on this, it is most probably that the Diviner2000 overestimated the SWS

7 Figure 4 The soil water storage (SWS) measured using soil moisture neutron probe (SMNP) and Diviner2000 capacitance sensor for furrow control (a) and furrow N200 (b), drip control (c) and drip N200 (d) plots. Figure 5 The soil water potential at 15 and 30 cm depths of drip N200 plots

8 Conclusions Comparison of soil moisture using the soil moisture neutron probe, time-domain reflectometry and capacitance method was carried out under tomato crop grown under different irrigation methods. The results showed that with soil of high gravel content, soil moisture obtained by the SMNP and the TDR methods was less variable, probably because of their ability to integrate bigger volume of soil and are more tolerance to the heterogeneity of the soil. The Diviner2000 is not so suitable in this situation, not only did it give a much higher soil moisture compared to the other two methods, the error associated was also bigger. This is die to its sensitivity to gap and to small changes in soil moisture, the later property can be an advantage if one is interested in the wet-front of irrigation water. The larger error means that to obtain the same level of accuracy, more tubes (and measuring sites) are needed for the Diviner2000. There are many criteria one take when choosing a product for soil moisture measurement. While the SMNP is reliable, and the technology matured, the lack of radiation regulation of many countries prohibits its use. On the other hand, the lightweight and ease of operation of the Diviner2000 is advantageous. The TDR in general performances well under most mineral soil without the need for calibration, however, it is costly in comparison. Inconclusion, whichever sensor one chooses, time must be invested to become proficient with them, and to be cautioned to find out their site-specific behavior and crop and soil compatibility. References Buss, P The use of capacitance based measurements of real time soil water profile dynamics for irrigation scheduling. Sentek Environmental Innovations. Sentek Pty. Ltd., South Australia. Charlesworth, P Soil Water Monitoring. CSIRO Land and Water. August. p Dalton, F.N Development of time-domain reflectometry for measuring soil water content and bulk soil electrical conductivity. In G.C. Topp, W.D. Reynolds and R.E. Green (eds.). Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice. SSSA Spec. Publ. Soil Sci. Soc. Amer., Madison, WI, USA. 30: Dean, T.J., J.P. Bell and A.J.B. Baty Soil moisture measurement by an improved capacitance technique: part I. sensor design and performance. J. Hydrol.93: Evett, S.R. and J.L. Steiner Precision of neutron scattering and capacitance type soil water content gauges from field calibration. Soil Sc. Soc. of Am. J. 59: Fares, A. and A.K. Alva Estimation of citrus evapotranspiration by soil water mass balance. Soil Sci. 164: Fares, A. and Alva, A.K Soil water components based on capacitance probes in a sandy soil. Soil Sci. Soc. Am. J. 64: Gardner, W.H Water content. In A. Klute (ed.). Methods of Soil Analysis, Part 1. 2 nd. Am. Soc. Agron., Soil Sci. Soc. Am., Madison, WI, USA. Gardner, W. and D. Kirkham Determination of soil moisture by neutron scattering. Soil Sci. 73: Greacen, E.L Soil Water Assessment by the Neutron Method. CSIRO, East Melbourne, Victoria, Australia. 140 p

9 Heng, L.K., R.E. White, K.R. Helyar, R. Fisher and D. Chen Seasonal differences in the soil water balance under perennial and annual pastures on an acid Spodosol in southeastern Australia. European J. Soil Sci. 52: Hignett, C. and S.R. Evett Neutron Thermalization. In Methods of Soil Analysis, Part 1. Physical and Minereralogical Methods, 3 rd. Agronomy Monograph Number 9. Moutonnet, P. Pluyette, N. El-Mourabit and P. Couchat Measuring the spatial variability of soil hydraulic conductivity using an automatic neutron moisture gauge. Soil Sci. Soc. of Am. J. 52: Paltineanu, I.C. and J.L. Starr Real-time soil water dynamics using multisensor capacitance probes: laboratory calibration. Soil Sci. Soc. Am. J. 61: Scotter, D.R., B.E.Clothier and M.A. Turner The soil water balance in a fragiaqualf and its effect on pasture growth in Central New Zealand. Aust. J. Soil Resear. 17: Topp, G.C., J.L. Davis and A.P. Annan Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resour. Res. 16: Van Bavel, C.H.M., N. Underwood and R.W. Swanson Soil moisture measurement by neutron moderation. Soil Sci. 82: Williams, D Soil water monitoring: choosing the right device. Agfact, February,NSW Agriculture. Zegelin, S.J., I., White and G.F. Russell A critique of the time domain reflectometry technique for determining field soil-water content. In G.C. Topp, W.D. Reynolds and R.E. Green (eds.). Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice. Soil Sci. Soc. Am., Madison, WI