A New Approach Towards Large Scale Soil Moisture Mapping by Radio Waves
A new approach for obtaining integrated estimates of soil moisture content over larger regions of typically 10–50 km is described. It is based on a known correlation between propagation characteristics of low frequency radio surface waves and surface soil moisture, and provides valuable new benefits especially for meteorological prognostic models and for soil water estimation in agriculture. The paper consists of (1) a description of the theory of radio wave propagation with an extension of the classical theory of Norton (Proceedings of the Institute of Radio Engineers, Vol. 24, 1936), specifically the exploitation of the phase information, (2) demonstration of a method which guarantees the selection of reliable results from a large measurement data set, (3) a presentation of a new low cost measurement device to detect the amplitude and phase changes, and (4) results from initial measurements providing evidence that theoretical calculations are consistent with the measured change of electromagnetic signal properties due to soil moisture change.
KeywordsSoil moisture Radio wave propagation Surface wave Large scale sensing
Large-area integrated soil moisture measurements are required for various applications in the fields of meteorology, surface geology, hydrology, agriculture, and forestry. Advances in theory and measurement methods over the last two decades have resulted in major improvements that enable the sensing of soil moisture both locally and for larger areas. New measurement approaches include local ground based techniques as well as aircraft and satellite technologies. However, there is still a significant gap between small-scale sensors and very large surface airborne and satellite remote sensing systems. The latter provide a very shallow soil penetration, and are not adequate for deriving estimates of water evaporation and retention. Continuous mapping of the surface layer down to 1–2 m on scales between kilometers and hundreds of kilometers has not been achieved so far. This paper deals with new approaches using low frequency radio surface waves, where a correlation between propagation characteristics and surface soil moisture exists. The large scale soil moisture data will provide significant advances, especially for meteorological prognostic models and for soil water estimation in agriculture.
Commonly used in situ sensors measure soil moisture locally in a small measurement volume and exhibit a high spatial variability due to heterogeneity of the soil. Networks of in situ sensors have been established and operated in the frameworks of, for example, the African monsoon multidisciplinary analysis, AMMA, in 2006  and the Convective and Orographically-induced Precipitation Study, COPS, in 2007 . They rely on low-cost soil moisture sensors covering lengths of about 20 cm . Advanced soil moisture sensors with a length of a few tens of meters were developed as well . Additionally, there are satellite based soil moisture sensing systems which cover large areas but usually suffer from coarse temporal availability and shallow penetration depth. Spatial resolution for the satellite systems ranges from tens of meters to tens of kilometers. As an example, the recently launched ESA earth explorer with its microwave imaging radiometer  has a temporal resolution of 3 days and a spatial resolution around 50 km. Therefore, there exists a gap between in situ sensors and remote sensing systems.
New approaches have been made at the authors’ institution in recent years by using the influences of soil moisture on electrical properties exerted on transmitted electromagnetic signals. The so-called FreeLineSensing method has been developed for continuous soil moisture monitoring over large distances using wave propagation along high voltage power lines [6, 7]. The propagation of low frequency (30–300 kHz) signals along these power lines depends on the soil electrical parameters below the lines, such as conductivity and permittivity, which are related to soil moisture. The low frequency signal has to be coupled by hardware devices to the high voltage line both at the transmitter and the receiver site. Typical lengths of the sensing lines are between 5 and 50 km. An improvement of the method was made by replacing the hardware connection of the signal generator and receiver with magnetic or capacitive coupling. Though the FreeLineSensing method is a significant improvement towards large scale soil moisture mapping, the installation and measurement activities require the technical and legal permission by the line owner. In addition, this method is limited to sensing the soil properties directly under the line.
Another approach for large area determination of soil moisture is based on electromagnetic waves generated by lightning . The rise time of propagated electromagnetic fields produced by cloud-to-ground lightning is sensitive to changes of the surface conductivity, which is related to soil moisture. The main limitation of this method is the requirement of vertical lightning, so there may be not sufficient data available.
2 Low Frequency Radio Wave Propagation
The basic idea is demonstrated by a simplified model (Fig. 1). In reality radio waves may propagate in four different ways from a transmitter to a receiver. Usually, the received signals result from multipath propagation. The direct wave travels on the optical path between transmitter and receiver. The second wave is reflected at the ground between transmitter and receiver. The third one is a reflected wave at the ionosphere. The fourth one is the so-called surface wave according to the definition of Sommerfeld . For soil moisture determination only the surface wave, which travels in a limited region at the air/soil interface, is useful. It is very important that the other waves are avoided so that the simplified model with a single surface wave (Fig. 1) is valid. Therefore, transmitter and receiver have to be close to the earth to avoid the ground reflection. Also the direct wave has to be avoided; this can be achieved by keeping a minimum distance of approximately 50 km between transmitter and receiver to make use of the earth’s curvature. The ionosphere reflection can be neglected by choosing a proper measurement time and circumstances. Usually there is strong ionospheric absorption without reflection for the low frequency range during daytime around noon due to the effect of solar radiation.
2.1 Selection of the Surface Wave Only Condition
3 Surface Wave Theories and Simulation
During the time of Sommerfeld and Norton, research focused only on amplitude decay to predict radio range for communication applications. The most comprehensive work on radio wave propagation at low frequencies was presented by Wait . He studied several assumptions such as the earth’s curvature, stratified models and reflections for propagation calculations. In his review paper, he summarized his life work on groundwave propagation. Wait clarified some difficult problem areas, cleared up some confusion on definitions, and listed 123 important papers as references in this field. A recent publication of Green shows further refinement of the theory .
distance between transmitter and receiver
relative soil permittivity
free space permittivity
a loop antenna for the magnetic component of the transmitted wave (oriented transversal to the line of sight to the transmitter for maximal signal reception),
a 20 m long ground antenna with metal electrodes pointing in the direction of the DCF77 transmitter which measures the voltage in the soil (line length chosen for a sufficient signal amplitude),
a GPS receiver and
a storage oscilloscope for data recording.
5 Experimental Results
Dry weather was present for more than 2 months long before the field test period. The initial phase and field strength were used as a reference for dry soil and later changes are considered in relation to these. A rain event on day 3 is associated with marked drops of the amplitude and a phase change of −5°. During a subsequent dry spell of 9 days, the amplitude recovered close to its initial value, which can be attributed to drying soil conditions. The phase angle, however, turns by about −10° which is not easily explainable. In the subsequent 16 days, there are a number of significant rainfalls. The amplitude changes by about −15% and the phase signal consistently changes by approximately an additional −10°. Because of the dry start, we could assume a mean soil conductivity of about 0.005 S/m which changed to about 0.015 S/m between the driest and wettest conditions. Based on the authors’ expertise in this area, and on the experimental results of Grosskopf and Vogt , both conductivities appear reasonable. During the following dry period of almost 3 weeks, phase and amplitude data are mostly missing, but the changes within another rainy period show some correlation. The simple but low cost setup of measurement devices (about EURO 500 excluding storage oscilloscope) is not yet ideal. The rain data representing convective precipitation from only six stations must be considered as a poor proxy of average soil moisture over the 100 km long propagation distance. Nevertheless, the data indicate a basic response of the signal properties to dry and moist surface conditions, which can only be assessed quantitatively by much larger experimental efforts which are planned for the future.
Based on theoretical calculations, it is shown that low-frequency radio waves may have the potential for large area monitoring of soil moisture which is needed for meteorological modeling and for agricultural applications. The surface wave propagation depends on the soil properties and varies both in amplitude and phase. Using existing man-made, low frequency transmitters and low-cost GPS synchronized receivers placed on a 2D grid, soil moisture estimates with resolution between 10 and 100 km can be achieved. A method has been developed to guarantee favorable conditions for reliable measurement results. Initial results from a simplified experimental setup show some correlation between phase and precipitation. Further experiments, including detailed soil moisture measurements on the transect between transmitter and receiver have to be performed. Other factors which affect the measurement have to be investigated as well, especially soil temperature influence [19, 20], conductivity gradients, penetration depth of the radio wave and disturbances of urban areas. For example, penetration depth can be estimated for a given frequency and a nominal soil conductivity on the basis of a skin depth calculation for homogeneous soils. More realistic cases with stratified soils require more sophisticated models.
- 1.Kohler, M., Kalthoff, N., & Kottmeier, C. (2009). The impact of soil moisture modifications on CBL characteristics in West Africa: A case study from the AMMA campaign. Quarterly Journal of the Royal Meteorological Society. 135, doi:10.1002/qj.430.
- 3.Schlaeger, S., Huebner C., & Becker. R. (2005). Simple soil moisture probe for low-cost measurement applications, Proceedings of Sixth International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, (pp. 258–265). Weimar, Germany.Google Scholar
- 4.Hübner, C., Schlaeger, S., Becker, R., Scheuermann, A., Brandelik, A., Schaedel, W., et al. (2005). Advanced measurement methods in time domain reflectometry for soil moisture determination, Electromagnetic Aquametry (pp. 317–347). Berlin: Springer.Google Scholar
- 5.Pinori, S., Crapolicchio, R., & Mecklenburg, S. (2008). Preparing the ESA-SMOS (soil moisture and ocean salinity) mission: Overview of the user data products and data distribution strategy, Microwave Radiometry and Remote Sensing of the Environment, MICRORAD, (11–14 March 2008).Google Scholar
- 6.Brandelik, A., & Hübner C. (2005). Verfahren und eine Vorrichtung zur Bestimmung von Eigenschaften des Erdreichs, German Patent No. 10253772.Google Scholar
- 8.Scheftic, W. D., Cummins, K. L., Krider, E. P., Sternberg, B. K., Goodrich, D., Moran, S., & Scott, R. (2008). Wide-area soil moisture estimation using the propagation of lightning generated low-frequency electromagnetic signals, 20th International Lightning Detection Conference, April 21–23, Tucson, Arizona.Google Scholar
- 10.Piester, D., Bauch, A., Becker, J., Polewka, T., Rost, M., Sibold, D., et al. (2006). PTB’s Time and Frequency Activities in 2006: New DCF77 Electronics, new NNTP servers and calibration activities. Proceedings of 38th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Reston, (5–7 Dec, pp. 37–47, 2007) Virginia, USA.Google Scholar
- 11.Sommerfeld, A. (1909). Über die Ausbreitung der Wellen in der drahtlosen Telegraphie, (vol. 28) Ann. der Physik.Google Scholar
- 12.Howard W. Sams & Co. (1977). ITT Reference Data for Radio Engineers. ISBN: 0-672-21218-8.Google Scholar
- 13.Bullington, K. (1957). Radio propagation fundamentals. Bell System Technical Journal, 36(3), 593–626.Google Scholar
- 14.Zenneck, J. (1907). Über die Fortpflanzung ebener elektromagnetischer Wellen längs einer ebenen Leiterfläche und ihre Beziehung zur drahtlosen Telegraphie, Ann. der Physik.Google Scholar
- 15.Norton, K. A. (1936). The propagation of radio waves over the surface of the earth and the upper atmosphere. In Proceedings of the Institute of Radio Engineers Vol. 24.Google Scholar
- 16.Wait, J. R. (1998). The ancient and modern history of EM ground-wave propagation. IEEE Antennas and Propagation Magazine, (vol. 40).Google Scholar
- 17.Green, E. H. (2007). Derivation of the Norton Surface Wave Using the Compensation Theorem, IEEE Antennas and Propagation Magazine. (47–57, Dec. 2007).Google Scholar
- 18.Grosskopf, J., & Vogt, K. (1941). The measurement of electrical conductivity for a stratified ground. Hochfrequenztechnik und Elektroakustik, 58, 52–57.Google Scholar