IRRIGATION AND DRAINAGE
Irrig. and Drain. 52: 203–217 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ird.095
COMPARISON OF THREE CALIBRATION PROCEDURES FOR TDR SOIL MOISTURE SENSORSy H. QUINONES,1 P. RUELLE2* AND I. NEMETH2 1
2
Irrigation Unit, Instituto Mexicano de Tecnologı´a del Agua (IMTA). Paseo Cuahunahuac Num. 8532, Progreso, Jiutepec, Me´xico Research Unit of Irrigation, CEMAGREF, French Institute of Agricultural and Environmental Engineering Research, Montpellier, France
ABSTRACT The application of time domain reflectometry (TDR) principles to determine soil water content has been widely accepted as an alternative method since Topp et al. (1980) proposed an empirical relationship between the dielectric constant of soil and soil moisture content. However, a wide range of studies have made it clear that the proposed function has only restricted validity. Although the response of the TDR signal shows a steady correlation with soil moisture, it is also affected by others factors related to the physical and chemical nature of soil and the solute characteristics of soil solution. It is therefore necessary to determine a particular calibration for each situation. Three methods are used to calibrate a TDR sensor. First, two well-known methods are compared: noncontinuous wetting on one hand, and continuous wetting by dripping of a soil sample on the other. The third consists of the progressive introduction of a sensor in a soil sample with a known moisture content. The three methods all lead to similar consistent relationships. The methods are validated using field data taking into account spatial variation of measurement at a given date and water inputs and uptakes under an irrigated corn crop during a growing season. Copyright # 2003 John Wiley & Sons, Ltd. key words: TDR; calibration procedure; soil moisture
RE´SUME´ L’utilisation de la re´flectome´trie dans le domaine temporal (RDT) pour de´terminer l’humidite´ du sol est largement admise depuis la publication par Topp et al. (1980) d’une relation empirique entre la constante die´lectrique du sol et la teneur en eau. Cependant de nombreuses e´tudes font apparaıˆtre que la relation propose´e n’a qu’une validite´ restreinte. La relation existant entre les caracte´ristiques du signal TDR (constante die´lectrique, fre´quence) et la teneur en eau du sol est modifie´e par les caracte´ristiques physiques et chimiques du sol et les proprie´te´s de la solution du sol. Dans chaque situaton rencontre´e, il est donc ne´cessaire d’e´tablir une relation d’e´talonnage particulie`re. Trois me´thodes sont mises en œuvre pour e´talonner un capteur TDR. Tout d’abord, deux me´thodes bien connues sont compare´es: d’une part l’humidification discontinue et d’autre part l’humectation continue par un goutteur d’un e´chantillon de sol. La troisie`me repose sur l’introduction progressive d’un capteur dans un e´chantillon d’humidite´ connue. Les trois me´thodes conduisent a` des relations e´quivalentes. Elles sont ensuite valide´es a` partir de mesures au champ prenant en compte la variabilite´ spatiale des mesures a` une date donne´e et les apports et les pre´le`vements sous une culture de maı¨s durant une campagne d’irrigation. Copyright # 2003 John Wiley & Sons, Ltd. mots cle´s: TDR; proce´dure d’e´talonnage; teneur en eau du sol
* Correspondence to: P. Ruelle, CEMAGREF-Gr. de Montpellier, 361 rue J-F Breton, BP 5095, 34033 Montpellier, Cedex 1, France. E-mail:
[email protected] y Comparaison de trois me´thodes d’e´talonnage pour des capteurs TDR de mesure de l’humidite´.
Copyright # 2003 John Wiley & Sons, Ltd.
Received 17 October 2002 Revised 22 January 2003 Accepted 26 February 2003
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INTRODUCTION A wide variety of methods are used to measure or to estimate soil water content, ranging from destructive (gravimetric) to non-destructive methods (gamma radiation probe, neutron probe, porous blocks, etc.). It is to be noted that repetitive sampling in the same places and compulsory sample manipulation often preclude destructive techniques for evapotranspiration determination; as for the non-destructive techniques, the neutron probe has been the most frequently used tool for soil moisture measurement since it was proposed in 1967 by Van Bavel and Stirk. For shorter periods (from a few hours to several days), the neutron probe is questionable mainly because for most soil, moisture variation is slight and moreover when used near the soil surface it is often considered as not perfectly accurate. Over the last few years, the TDR technique has become quite common for measuring soil water content and its kinetics, becoming a valuable and useful tool to support hydrology research. IMKO (1999) describes some of the advantages and disadvantages of all measurements methods. Detailed descriptions of the physical principles of TDR have been given in many papers (for example Topp et al., 1980, 1988; Dalton et al., 1984; Dasberg and Dalton, 1985; Zakri, 1997; Pereira dos Santos, 1997); that is why this document will present only some general principles. The propagation speed of an electromagnetic wave in a vacuum is similar to the light speed (c) [3 108 m s1], but in a given medium, the electric losses and the relaxation phenomenon cause a modification of the wave displacement and its speed (v) can be expressed by c v ¼ pffiffiffiffi ð1Þ K with K standing for the dielectric constant of the medium. The most common polar liquid in nature is water, which has a high dielectric constant (K ffi 80), compared to those of air (1.0005) and dry soil (1.7 g cm3) the TDR signal tends to overvalue soil moisture and if the bulk density is relatively small, (
